key: cord-274802-7ioiwsd8
authors: Varghese, Praveen Mathews; Tsolaki, Anthony G.; Yasmin, Hadida; Shastri, Abhishek; Ferluga, Janez; Vatish, Manu; Madan, Taruna; Kishore, Uday
title: Host-pathogen interaction in COVID-19: Pathogenesis, potential therapeutics and vaccination strategies
date: 2020-08-19
journal: Immunobiology
DOI: 10.1016/j.imbio.2020.152008
sha: 
doc_id: 274802
cord_uid: 7ioiwsd8

Abstract The current coronavirus pandemic, COVID-19, is the third outbreak of disease caused by the coronavirus family, after Severe Acute Respiratory Syndrome and Middle East Respiratory Syndrome. It is an acute infectious disease caused by Severe Acute Respiratory Syndrome Coronavirus 2 Virus (SARS-CoV-2). The severe disease is characterised by acute respiratory distress syndrome, septic shock, metabolic acidosis, coagulation dysfunction, and multiple organ dysfunction syndromes. Currently, no drugs or vaccine exist against the disease and the only course of treatment is symptom management involving mechanical ventilation, immune suppressants, and repurposed drugs. As such the severe form of the disease has a relatively high mortality rate. Last 6 months have seen an explosion of information related to the host receptors, virus transmission, virus structure-function relationships, pathophysiology, co-morbidities, immune response, treatment and most promising vaccines. This review takes a critically comprehensive look at various aspects of host-pathogen interaction in COVID-19. We examine genomic aspects of SARS-CoV-2, modulation of innate and adaptive immunity, complement-triggered microangiopathy, and host transmission modalities. We also examine its pathophysiological impact during pregnancy, in addition to various gaps in our knowledge. The lessons learnt from various clinical trials involving repurposed drugs have been summarised. We also highlight the rationale and likely success of the most promising vaccine candidates.

receptor on enterocytes in the small intestine and is consistent with clinical reports of gastrointestinal symptoms and viral shedding in faeces (34, 35) . This has been further resolved with the comprehensive identification of host cells/tissues expressing both ACE2 and TMPRSS2 ( Figure 2 ). Thus, likely targets for SARS-CoV-2 primarily include secretory goblets of the nasal mucosa, lung type II pneumocytes and absorptive erythrocytes of the small intestine (36) . Of note, this study also showed that the ACE2 receptor is an interferonstimulated gene in SARS-CoV-2 infection in the cells of the human upper nasal epithelium and lung, predominatly mediated by IFN-α2 and IFN-γ (36) . Moreover, bystander cells are subject to interferon-mediated effects (upregulation of ACE2 receptor) rather than SARS-CoV-2 infected cells, suggesting a mechanism of enhanced viral targeting and entry during pathogenesis and a possible avenue for therapeutic intervention (36) .

Analysis of genetic variation in the ACE2 gene has identified single nucleotide polymorphisms (SNPs) that differ in frequency globally among the human population, particularly between males and females (37). Characterising these SNPs more fully with epidemiological and clinical data on COVID-19 will in time shed light on the precise molecular mechanisms of transmission and disease. Furthermore, in the SARS-CoV-2 viral S protein, 27 amino acid substitutions have been described, although these occurred outside the RBD that directly interacts with ACE2 (14) . Of paramount importance is characterising the genetic variation and its consequences in the S protein and its RBD, as this will determine whether the SARS-CoV-2 virus is evolving and is likely to be a seasonal infection with new variants for the human population. Undoubtedly, variation in the S protein and ACE2, the central interface of hostpathogen interaction in COVID-19 will have evolved from natural selection contributing to the pathogenesis of this disease.

proteins and transported into the endoplasmic reticulum (ER). These proteins are processed via the secretory pathway and are transported into the ER-Golgi intermediate compartment (69, 70) , where the full-length viral genomes are packaged with the nucleocapsid N protein, budding from the membrane, and thus forming the enveloped mature virion (71) . The N protein has two domains that can bind the RNA genome, with the aid of nsp3 protein, and attaching it to the RTC, facilitating the packaging of the virus (72) (73) (74) . The viral M protein has three transmembrane domains and is responsible for the majority of protein-protein interactions needed for virus assembly, including membrane curvature and binding the nucleocapsid (75, 76) . Pseudo-virus particles can also only be formed when there is a co-expression of M protein and E protein, indicating the requirement of both these two proteins to form the coronavirus envelope (77) . The viral E protein is also involved in structural shaping of the viral membrane envelope and in inhibiting M protein aggregation, as well as a role in pathogenesis (78) (79) (80) (81) . After the assembly of the mature virions, they are transported in vesicles, where they are released from the infected cell via exocytosis (82) .

Unlike SARS, COVID-19 patients had the highest viral load near presentation, which could account for the fast-spreading nature of this epidemic. In a study involving COVID-19 patients in Hong Kong recorded high viral load on presentation with the onset of symptoms and also when the symptoms are mild (83) . SARS CoV-2 viral RNA load was detected in the deep throat (posterior oropharyngeal) saliva samples for 20 days or even longer. The peak of the viral load correlated positively with age. Viral load in posterior oropharyngeal saliva samples was higher during the first week of symptom onset, which gradually declined. Thus, the location of sample collection and the timing for the onset of symptoms both are important factors to be considered for the detection of SARS CoV-2 positive cases. In the same study, most of the patients showed rising antibody titres 10 days after symptom onset, though the serum antibody levels did not show correlation with clinical severity (83) .

The patient's antibody to SARS-CoV-2 viral nucleocapsid protein using infected cell lysates was identified on the 10th day after symptom onset by western blot (84) . In another study involving 285 patients with COVID-19, all were tested positive for antiviral IgG within 19 days after symptom onset. Both IgG and IgM titres reached a plateau within 6 days after seroconversion (85) . In Wuhan Tongji Hospital, around 60 convalescent patients tested positive for the IgG against the virus, while 13 patients tested negative for IgM, where IgG titre was higher comparatively. Both the antibody titres showed a decrease when tested weeks apart (86) . Thus, titres of SARS-CoV-2 antibodies can reflect the progress of viral infection and can be a vital component to understand the development and prognosis of the disease and similarly timing of antibody seroconversion is also crucial for determining the optimum duration for collecting serum specimens for antibody diagnosis. As previously mentioned, several other studies also confirmed the presence of SARS-CoV-2 nucleic acids in the faecal, urine samples and rectal swabs of COVID-19 patients and thus it becomes essential to ascertain viral load dynamics in such samples too (87) (88) (89) .

Transmission SARS-CoV-2 is transmitted through "respiratory droplets", which are large droplets of virusladen mucus or through close contact with infected individuals (90) (91) (92) (93) . At the same time virus has also been reported to spread via asymptomatic but infected individuals in several countries, including China, Germany, USA, and India (91, (94) (95) (96) (97) . A systematic review and meta-analysis of 172 observational studies with no randomised controlled trials and 44 relevant comparative studies in health-care and non-health-care settings revealed transmission of virus decreased as physical distancing increased to 1 metre or more (98) . Eye protection, N95 or similar respirators in health-care settings and 12-16-layer cotton or surgical masks in the community were found to greatly control the transmission (98) . Studies have also established that the median half-life of the aerosolised virus is ~1.2 hours under lab conditions, similar to the SARS-CoV. However currently, no evidence supports real-world airborne transmission of the virus through aerosols (99) . SARS-CoV-2 was found to remain viable for up to 4 hours on copper surfaces, up to 24 hours on cardboard surfaces, and up to 72 hours on plastic and stainless-steel surfaces. Thus, there exists a possibility of contact transmission to occur, although no confirmed cases of contact transmission have been reported (99) . The virus was also found in the faeces of infected patients showing that the virus can survive and replicate in the digestive system (100) . This suggests that there may be a possibility of an oral-faecal route of transmission, though again no confirmed cases have been reported (101) . The Royal College of Obstetricians and Gynaecologist UK have reported that transmission from mother to baby antenatally or intrapartum is possible although this requires further study for confirmation; there appears to be no evidence supporting vertical transmission to the foetus (102) (103) (104) . Additionally, as reported by WHO and CDC, the virus has not been found to be transmitted by breastfeeding and has not been found in breastmilk of COVID-19 mothers (105, 106) . COVID-19 was found to have low severity and mortality than SARS, but it is highly contagious and affecting comparatively more men than women (94, 107, 108) . The difference in fatality rate between males and females may probably be explained by the fact that as ACE2 is located on the X chromosome. There may be alleles that confer resistance to COVID-19, at the same time, oestrogen and testosterone sex hormones have different immunoregulatory functions that may contribute to protection or severity of the disease (109, 110) . The disease has also been found to disproportionately affect older aged persons and people suffering from social deprivation, diabetes, severe asthma, cardiovascular disease, obesity, haematological malignancy, recent cancer, kidney, liver, neurological or autoimmune conditions (111) . Studies have also reported that members of minority communities such as the black and south Asian populations, are at a higher risk of the disease (111) .

The incubation period of the disease ranges between 2 to 14 days and the median incubation period is approximately 4-5 days before symptom onset (87, 92, 112, 113) . During the onset of the illness, the common symptoms that most patients exhibited were fever and cough. Other symptoms include conjunctivitis, myalgia (muscle pain) or fatigue, headache, dyspnoea (short of breath), chest pain, diarrhoea, nausea, rhinorrhoea (runny nose), vomiting, loss of appetite, abdominal pain, gastrointestinal bleeding, autoimmune haemolytic anaemia, and sometimes haemoptysis (coughing of blood) (108, (114) (115) (116) (117) (118) . Patients have also reported anosmia (loss of smell), dysgeusia (distortion of the sense of taste) (119) (120) (121) (122) . For SARS-CoV-2 asymptomatic patients, anosmia, hyposmia, or dysgeusia are symptoms that were suggested for screening (123) . In addition to these, neurological manifestations such as dizziness, headache, impaired consciousness, acute cerebrovascular disease, ataxia, seizure, nerve pain, skeletal muscular injury manifestations, intracerebral haemorrhage, central nervous system vasculitis, encephalopathy, encephalitis, cranial neuropathies and psychosis were reported predominantly in older people (124) (125) (126) . In paediatric patients, an autoimmune and autoinflammatory disease, Paediatric Inflammatory Multisystem Syndrome (PIMS), also known as Multisystem Inflammatory Syndrome in Children (MIS-C), has been reported to occur after SARS-CoV-2 infection (127) (128) (129) (130) (131) (132) (133) (134) .

Cutaneous manifestations of COVID-19 have also been reported (135) (136) (137) . A case report from Strasbourg, France reported purpuric lesions in the lower extremity (137) . An Italian study reported patients presenting with an erythematous rash, urticaria and chickenpox-like vesicles mainly in the trunk with little or no itching that did not correspond to disease severity (138) . The prolonged use of personal protective equipment and repeated washing have also led to an increase in dermal conditions such as pressure injury, contact dermatitis, itch, pressure urticaria, and exacerbation of pre-existing skin diseases (124) .

The first step of infection is the inhalation of viral particles present in respiratory droplets from an infected host. Once inhaled, the virion enters the nasal cavity of a healthy host and likely binds to goblet and ciliated cells in the nose that express ACE2 (32) . At this time, a limited innate immune response may occur, and the virus replicates and moves further down the respiratory tract via the conducting airways.

As the virions proliferate and spread towards the upper respiratory tract, usually a robust innate immune response is triggered by the detection of the virions by pattern recognition receptors (PRRs) like Toll-Like Receptors, RIG-1, and MDA-5. This may present several symptoms starting from dysphonia (hoarseness), ulceration of the epiglottis and subglottis, and profound oedema and granulations in the subglottis and also in the upper trachea (139) . In a few patients, mild tachypnoea and coarse breath sounds were also observed while the virus is in the upper airway (91) . Furthermore, the detection by PRR leads to the expression of type 1 interferons (IFN) in the early stages of infection, which helps establish an anti-viral state in the cells by producing inflammatory cytokines and chemokines. The SARS-CoV produces an enzyme that adds 2' O-methyl group to viral RNA, which helps it evade detection by MDA-5, thereby delaying the induction of IFN. Studies have established that unlike an early IFN response, a delayed IFN response causes an inability to control viral replication, leading to cellular damage of airway epithelia and the lung parenchyma and an eventual lethal inflammatory cytokine storm (140) (141) (142) .

The SARS-CoV-2 papain-like protease, which is essential to generate the RTC, has been shown to preferentially cleave the ubiquitin-like protein ISG15 from interferon responsive factor 3 (IRF3), attenuating type I interferon responses (143) . The C-terminus of the SARS-CoV-2 non-structural protein 1 was reported to bind to the 40S ribosomal subunit and block the mRNA entry tunnel (144) . This obstruction effectively inhibits the RIG-I-dependent innate immune response (144) . Accordingly, no significant expression of IFN was detected up to 48 hours post-infection with SARS-CoV-2. Only IL-6, which correlates with respiratory failure, Acute Respiratory Distress Syndrome (ARDS), and adverse clinical outcomes were upregulated. Monocyte Chemoattractant Protein-1 (MCP1), C-X-C motif chemokine (CXCL) 1, CXCL5, and CXLC10, were also upregulated 48 h post-infection with SARS-CoV-2 (145) . The suppression of innate immune activation and annihilation of T cells can help explain the mild or even the lack of symptoms in many infected patients. The increased viral replication efficiency in the respiratory tract early on leads to the highly efficient person-to-person transmission of the virus in the community (145) .

The virions further migrate towards the lower respiratory tract and reaches the alveoli where it binds to the type 2 pneumocytes and begins replication. As the type 2 pneumocytes undergo apoptosis after viral release, they secrete inflammatory mediators like CXLC proteins that attract macrophages and neutrophils ( Figure 5 ) (146) . The stimulated macrophages further secrete cytokines such as IL-1β, IL-6 and Tumor Necrotic Factor α (TNF-α). The released cytokines trigger a "cytokine storm", which stimulates the release of vascular endothelial growth factor (VEGF), monocyte chemoattractant protein-1 (MCP-1), IL-8, and additional IL-6, as well as reduced E-cadherin expression on endothelial cells causing vasodilation and increase capillary permeability (147) . This causes the plasma to leak into the interstitial spaces and alveoli, increasing interstitial and alveolar oedema. The increased alveolar oedema decreases the level of surfactant in the alveoli. This causes an increase in the surface tension in the alveoli, which leads to alveolar collapse. Oedema and alveolar collapse may present as multiple peripheral ground-glass opacities in subpleural regions of both lungs, which is observed in many patients (148) . Chest CT scan of patients also revealed bilateral multifocal infiltrates and mediastinal and hilar lymphadenopathy in some patients (91) . These decrease the gas exchange efficiency causing hypoxemia and increased work of breathing presenting as dyspnoea (shortness of breath), culminating in ARDS (149) . Abnormal coagulation parameters, mainly elevated D-dimers seem to be associated with a higher risk of development of ARDS in COVID-19 patients (150) . The aberrant wound healing may even lead to fibrosis than other forms of ARDS (151) . Stimulated neutrophils secrete Reactive Oxygen Species (ROS) and proteases which destroy both infected and uninfected type 1 and type 2 pneumocytes, leading to further reduced gas exchange and alveolar collapse, respectively (152) .

Furthermore, the dead pneumocytes slough off into alveoli filling them up with fluid, protein deposits, cell debris, macrophages, and neutrophils. This causes pulmonary consolidation, which leads to altered gas exchange and causes hypoxemia (153, 154) . The consolidation also leads to productive cough. The hypoxemia can further trigger chemoreceptors that stimulate the Sympathetic Nervous System (SNS) that causes tachycardia (increased heart rate) and tachypnoea (increased respiratory rate) (155, 156) . The Central Nervous System (CNS) is also affected by the high concentrations of IL-1β, IL-6 and TNF-α in the blood, as these cytokines stimulate the hypothalamus to release prostaglandins such as PGE2, which causes an increased body temperature leading to fever (157) .

Studies have also reported elevated levels of myeloperoxidase (MPO)-DNA and citrullinated histone H3 (Cit-H3), which are markers used to detect Neutrophil extracellular traps (NETs), in the serum of COVID-19 patients (158) . Furthermore, control neutrophils treated with COVID-19 patient serum exhibited NETosis (158) . NETs, while protecting the host from invasive pathogens, have been attributed to play a role in many autoimmune and vascular diseases. For example, NETs are known to contribute to ARDS, pathogen-induced acute lung injury, thrombosis and can contribute to further cytokine release lading to the inflammation (158) .

An increased frequency of neutrophils, eosinophils and monocytes was reported in severe COVID-19 positive patients; severe patients showed further increase in neutrophils though their activation status had not altered. There was no significant change in the immature granulocyte frequencies. However, there was an inverse correlation between frequency of immature granulocytes in moderate and severe patients with the duration since the appearance of symptoms. Severe patients exhibited lower percentages of both conventional and plasmacytoid dendritic cells (DC) (159) .

Response Syndrome (SIRS). The spread of the inflammation from the lungs into the circulatory system causes increased capillary permeability within the systemic circulation. This leads to a decrease in blood volume along with increased vasodilation of systemic arteries, leading to decreased peripheral resistance. The decreased blood volume, along with peripheral resistance, causes hypotension (decreased blood pressure), which decreases perfusion to other organs leading to Multisystemic Organ Failure (MOF) (160-162). The cytokine storm has also been shown to trigger autoimmune haemolytic anaemias (AIHA) (with warm or cold antibodies) (117, 163, 164) . Most of the studies report manifestation of AIHA early, during the active phase of COVID-19 (within 4 to 13 days), a timeframe matching that of the cytokine storm (117, 127, 163, 164) . As a result of ARDS, SIRS and MOF, patients suffering from severe SARS-CoV-2 infection exhibit significantly elevated levels of, IL-2, IL-8, IL-17, G-CSF, GM-CSF, MIP-1α, CRP, and Ddimer, in addition to IL-6, IL-1β and TNF-α (145). There are reports suggesting that in addition to the lungs, SARS-CoV-2 infection may induce the multiorgan injury in patients involving brain, heart, liver, kidney, intestine and eyes (165, 166) . COVID-19 associated neurological complications

The neurological pathologies observed in COVID-19 are similar to those observed in previous coronavirus epidemics (167) . Myoclonus and demyelination are reported in a few cases (126, 168, 169) . A study conducted in Wuhan, China involving 214 COVID-19 patients, reported that 78 patients developed neurological manifestations (125) . In another study from Strasbourg, France where effectively 58 patients were recruited for an observational study, reported agitation in 69% of the patients, confusion in 65%, and 67% of the patients had corticospinal tract signs (170) . A systemic review and meta-analysis of literature databases for psychiatric and neuropsychiatric presentations in coronavirus infections reported transient encephalopathies with features of delirium and psychosis (171) . The study also reported cognitive dysexecutive syndrome and delirium with agitation in a few cases (171) . There is also a reported case of autoimmune encephalitis with the typical clinical features of opsoclonus and myoclonus, and another case of autoimmune encephalitis with a radiological imagery showing typical limbic encephalitis (167) . The exact mechanism for encephalopathy may be multifactorial (effect of sepsis, hypoxia, and/or cytokine storm) (172) . A few cases of Guillain-Barré Syndrome (GBS) associated with SARS-CoV-2 have been reported from Italy (173) . However, further epidemiological and mechanistic study is required to confirm the incidents of GBS in COVID-19. The binding of the virus to the ACE-2 receptors on endothelial cells causes extravasation of red blood cells leading to cerebral microbleeds (137, 167) . There have also been reports of severe strokes in COVID-19 patients, but further study is required to determine its association with COVID-19 (167) . Magnetic resonance imaging (MRI) revealed abnormalities such as meningeal enhancement, ischaemic stroke, perfusion changes, microhaemorrhages, medial temporal lobe signal abnormalities similar to that seen in viral or autoimmune encephalitis (170, 174) . Very few cases have been reported where SARS-CoV-2 was detected in CSF and its supportive histopathological features; no reports of the virus in the brain exist yet (167, 175, 176) . Thus, it is important to establish whether the above-described syndromes may be caused due to either direct viral injury, hyperinflammation, vasculopathy and/or coagulopathy, autoantibody production to neuronal antigens, sepsis and hypoxia, or a combination of these (172) .

Out of the first 41 patients diagnosed with COVID-19 in Wuhan, 5 of them had myocardial injury associated with the SARS-CoV-2, which mainly manifested as an increase in highsensitivity cardiac troponin I (115) . The hemogas analysis showed hypoxia; laboratory tests showed elevation of C-reactive protein, transaminases and lactate dehydrogenase, and lymphopenia (177) . Several patients showed abnormal myocardial zymogram, showing high levels of creatine kinase (112) . Because of an excessive inflammation, hypoxia, immobilisation and diffuse intravascular coagulation (DIC), COVID-19 patients may predispose to both venous and arterial thromboembolic disease (87, 115, 178) . It has also been observed that concomitant acute thrombosis of the abdominal aorta and pulmonary embolism induces cardiovascular complications in COVID-19 patients, suggesting an association of hypercoagulable condition with the disease (179) .

COVID-19 patients with abnormal liver function were also documented, where patients had Alanine Aminotransferase (ALT) or Aspartate Aminotransferase (AST), Bilirubin, Acute Phase Recants (APR) like CRP, Fibrinogen and IL-6 above the normal range (112, 180) . Sepsis, hypovolaemia, and nephrotoxins were found to be important contributors to kidney damage in COVID-19 patients. Cardiorenal syndrome, particularly right ventricular failure, might lead to kidney congestion and acute kidney injury in COVID-19 patients (112) . Symptoms such as olfactory and gustatory dysfunctions were also found to be related to COVID-19 (181) . SARS-CoV-2, facilitated by TMPRSS2 and TMPRSS4, was found to infect and reproduce in ACE-2 + mature enterocytes (100) . However, the virions released into the intestinal lumen were inactivated by stimulated human colonic fluid and no infectious virions were recovered in stool samples, in spite of the presence of viral RNA in stools. This study thus established the intestine as a site of viral replication and its effect on local and systemic illness and overall COVID-19 progression (100).

As in the case respiratory infections by Respiratory syncytial virus and SARS-CoV, the eyes have been shown to act as a portal of entry for the virus. While there have been no reports of SARS-CoV-2 transmission in humans via ocular tissues, further studies are required to exclude the eyes as a source of infection and as a portal of entry. Moderate conjunctivitis could be the first sign of severe respiratory distress in COVID-19 patients (182) . Studies from China on patients with COVID-19 reported conjunctivitis and other ocular manifestations, such as epiphora, conjunctival congestion, or chemosis in patients with severe COVID-19 (183) (184) (185) (186) . The studies also reported a few patients with positive conjunctival swab for COVID-19 determined by RT-PCR (183, 184, 186) . Similar results were also reported in a study conducted by the National Institute for infectious diseases in Rome, Italy (187) . In addition to the conjunctivitis, the ocular swabs were positive for SARS-CoV-2 even when nasopharyngeal swabs tested negative for the virus. This suggests that the conjunctiva may sustain viral replication for an extended period of time (118) .

There are reports from France, Italy, United Kingdom and the United States of America, suggesting the presentation of autoimmune and auto inflammatory diseases in children, especially in children of African descent, such as paediatric inflammatory multisystemic syndrome (PIMS), also known as, multisystemic inflammatory syndrome in children (MIS-C) (127) (128) (129) (130) (131) (132) (133) (134) . This syndrome includes Kawasaki-like disease, Kawasaki disease shock syndrome, toxic shock syndrome, myocarditis and macrophage activation syndrome (127) (128) (129) (130) (131) (132) (133) (134) . The exact cause for Kawasaki disease remains unknown; however, it is believed that it is caused by an apparent atypical immune response to pathogens in genetically predisposed individuals (188) (189) (190) . Previous studies have implicated the pathogenesis of Kawasaki disease with the infection of certain members of the coronavirus family (191, 192) . The temporal association between the beginning of COVID-19, SARS-CoV-2 infection and the onset of PIMS suggest a causal link (129) . This is further supported by the fact that in most cases, the patients exhibiting PIMS tested positive for IgM or IgG SARS-CoV-2 antibodies (127) (128) (129) (130) (131) (132) (133) (134) . The presence of IgG antibodies clearly indicates a delayed onset of PIMS following SARS-CoV-2 infection (127) (128) (129) (130) (131) (132) (133) (134) . The onset of PIMS occurred 4-5 weeks after acute COVID-19 (193) . The patients presented with fever, diffused skin rashes, rash/oedema of hands and feet, conjunctivitis, dry cracked lips, cervical lymphadenopathy and arthritis. The Kawasaki-like disease caused by COVID-19 exhibited a few differences in both clinical and biochemical features from patients suffering from Kawasaki disease without SARS-CoV-2 infection. Clinically, the patients suffering from COVID-19 associated Kawasaki-like disease were older and the disease occurred in both sexes, unlike the classical Kawasaki disease that occurs in younger male children (193) . The COVID-19 associated Kawasaki-like disease also had a higher incidence of abdominal pain and/or more frequent diarrhoea, meningeal and respiratory involvement, and a strikingly different myocarditis severity and frequency when compared to classical Kawasaki disease (127) (128) (129) (130) (131) (132) (133) (134) . Biochemically the patients exhibited leukopenia with thrombocytopenia, increased ferritin, elevated myocarditis markers and high levels of procalcitonin, CRP and cytokines were observed when compared to classical Kawasaki disease (127) (128) (129) (130) (131) (132) (133) (134) . Nearly 62% patients also showed resistance to the initial treatment with intravenous IVIg infusion, and required a second infusion for successful treatment (128, 132) . While the children exhibited the devastating effects of the cytokine storm associated with COVID-19, such as heart failure, pneumonia, gastrointestinal, neurological and renal manifestations, the paediatric patients in the French study rarely had respiratory manifestations (128) . This suggests a different host immune response in children compared to adults. Treatment for PIMS involves the administration of IL-1 receptor antagonist, IL-6 receptor blockers such as Tocilizumab or Sarilumab, IVIg, and steroids or biologics to control inflammation (128, 132) .

COVID-19 is known to affect older members of the population disproportionately, with adults over the age of 65 years making up to 80% of hospitalization and having a 23-fold greater risk of death (111, 194) . One possible explanation for this could be the increased baseline inflammation, called inflammaging, commonly observed in individuals over the age of 60 (195) . Studies have shown increased baseline serum concentrations of CRP and cytokines such as IL-6 and IL-8 (196) . Inflammaging could be the result of the accumulation of mis-folded proteins, compromised gut barrier, obesity and impaired clearance of dead or dying cells (196, 197) . Senescent nonlymphoid cells have been known to secrete inflammatory cytokines, chemokines, growth factors, and matrix metalloproteinases (195, 198) . This increased baseline inflammation inhibits antigen-specific immunity affecting the efficacy of many vaccines (199) . Studies have shown that treatment with rapamycin, MAPK inhibitor or steroids reduces this excessive inflammation and enhances vaccine efficacy (195, 200, 201) . In case of COVID-19, this baseline inflammation may itself not be detrimental but contributes to the initiation of an inflammatory cascade that ends in the deadly cytokine storm (195) . Furthermore the accumulation of senescent cells in the lungs of older patients could inhibit T cell response, induce NKR ligand expression, which marks the cells for elimination by infiltrating T cells expressing NKRs (195) . As observed in the case of vaccines against other respiratory viruses, inflammaging may reduce the efficacy of COVID-19 vaccinations in this already disproportionately affected group.

As with any infection, both the innate and adaptive arms of the immune system are required to mount a successful defence against a viral incursion. In case of COVID-19, a decrease in the circulating T helper cells (CD4 + cells), Cytotoxic T cells (CD8 + cells), B cells, natural killers cells, lymphocytes, monocytes, eosinophils and basophils has been reported (108, 149, 159, 202) . A retrospective, single-centre study involving 452 patients revealed a significant decrease in the total number of regulatory T cells, memory T cells and suppressor T cells (203) . The study also reported an increase in the percentage of naïve T cells (203) . As naïve T cells help respond to novel pathogens that the immune system has not yet encountered by managing release of cytokines, this may help explain the hyperinflammation (204) . The lower levels of memory T cells reported may also explain the relapses reported in COVID-19 convalescent individuals (204) . Direct infection of THP-1 cells, human peripheral blood monocyte-derived macrophages and dendritic cells by MERS-CoV and infection of T cells and macrophages by SARS-CoV has been reported (6, 205) . Hence, it can be speculated that SARS-CoV-2 may also infect monocytes and macrophages by a mechanism that is yet to be elucidated (204) . Receptors such as CD147 on the surface of T cells and other immune cells may mediate viral entry (206) . The clinical trial with anti-CD147 monoclonal antibody, Meplazumab, showed promising efficacy and safety in COVID-19 patients (207) . However, CD147 did not show a direct interaction with the S protein of SARS-CoV-2 (208) . Similarly, lymphopenia can be attributed to SARS-CoV-2 direct infection and lymphocyte death, destruction of the lymphatic organs, and/or high levels of the programmed cell death protein 1 (PD-1) on CD8 + T cells (which is known to trigger T cell exhaustion) (204, 209, 210) . Lymphocytopenia, neutrophilia and neutrophil-to-lymphocyte ratio are being used as a predictor for the severity of the illness during early stages of infection and a poor outcome in COVID-19 (159, 202, 211, 212) . This further alludes to the hyper-inflammatory nature of COVID-19. Furthermore, COVID-19 patients were reported to have elevated serum levels of highsensitivity C-reactive protein and procalcitonin, whose levels have been associated with high risks of mortality and organ injury (213) . Lower percentage and count of CD3 + , CD4 + , and CD8 + lymphocytes populations serve as a prognostic marker for mortality, organ injury, and severe pneumonia (213) . SARS-CoV exposed as well as a subset of non-exposed people exhibit a cross-reactive T cell repertoire (214) . Studies have also reported the presence of SARS-CoV-2 spike glycoprotein-reactive CD4 + T cells in peripheral blood of a subset of donor who were not infected with SARS-CoV-2 (215, 216) . These S reactive CD4 + T cells were found to primarily react with the C-terminal of the S epitope (216) . This binding preference could be attributed to the presence of overlapping human coronavirus MHC-II epitopes in the C-terminal domain. Hence, these CD4 + T cells are cross reactive clones generated during previous infections with endemic human coronavirus (216) . A Long-term Information and Knowledge for Ageing -Camden' (LINKAGE) sub-study is currently underway to study if pre-existing antibodies and specific T cells contribute to the devastating effect observed in old people (217) .

The B cell response occurs alongside the T helper cell response (~1 week post infection) in COVID-19 patients and helps mount a humoral response via antibodies that would help neutralise the virus (109) . Characterisation of the transcriptome during the recovery stage of the disease revealed significantly lower levels of naive B cells, while plasma B cell levels had increased in peripheral blood mononuclear cells (159, 218) . It was found that a certain subset of patients who contract the disease may not develop long-lasting antibodies against the pathogen; it is possible that these patients may be susceptible to the re-infection (109) .

Immune cell profiling of COVID-19 patients in the recovery stage by single-cell sequencing has identified several new B cell-receptor changes such as IGHV3-23 and IGHV3-7, and isotypes used earlier for vaccine development including IGHV3-15, IGHV3-30, and IGKV3-11 (218) . The strongest pairing frequencies, IGHV3-23-IGHJ4, has been suggested to indicate a monoclonal state associated with SARS-CoV-2 specificity (218) . Antibodies analysed from the serum of COVID-19 patients revealed no cross-reactivity with the S1 subunit of the SARS CoV spike antigen, while some reactivity was observed between the nucleocapsid antigens of SARS-CoV and SARS-CoV-2 (85) . The RBD-specific IgM and IgG antibodies were significantly elevated in the severe and recovered patients (159) . Investigations conducted on COVID-19 recuperating rhesus macaque models, re-infected with SARS-CoV-2, reported no measurable viral spreading, clinical manifestations, or histopathological changes associated with COVID-19 (219) . The study found lower viral loads in nasopharyngeal or anal swabs 5 or 7 days after reinfection, compared to the recorded viral loads 5 or 7 days after the initial infection with SARS-CoV-2 at similar sites. Similarly, increased levels of leukocytes and neutrophils were recorded 14 days after reinfection, compared to the levels measured during the initial infection. Significantly higher specific antibody titres were recorded 14 day post reinfection. There were also increased activation of CD8 + T cells, changes in CD4 + Tcm cells and memory B cells. Thus, increased production of neutralising antibodies protected the primates against COVID-19 re-infection (220, 221) . A study on 149 COVID-19 convalescent individuals revealed that plasma collected after 39 days of symptom manifestation had a variable half-maximal pseudovirus neutralizing titres of less than 1:50 in 33%, below 1:1,000 in 79%, and only 1% showed titres above 1:5,000 (222) . Interestingly, in spite of the low titres reported, antibodies specific to three distinct epitopes on the RBD of the SARS-CoV-2 S protein neutralized at half-maximal inhibitory concentrations as low as single digit ng/ml (222) . Hence, a vaccine that can elicit the production of such highly potent antibodies, or monoclonal antibodies raised against the RBD of the SARS-CoV-2 S protein, may be highly protective. However, studies on SARS-CoV and MERS-CoV revealed that neutralizing antibodies to S protein can potentially augment severe lung injury by exacerbating inflammatory responses (109, (223) (224) (225) . Hence, therapeutic antibodies should be carefully studied to minimise any unwanted pro-inflammatory activity while retaining maximum virus neutralizing capacity.

Additional specific insights on the intracellular life cycle have also been gained from nextgeneration sequencing (NGS) studies on the transcriptome and epi-transcriptome profile of SARS-CoV-2 virus and infected host cell. This fundamental approach has given an insight into the specific molecular dialogue between the pathogen and the host cell. This dialogue is complex. The SARS-CoV-2 transcriptome has been studied in high resolution. It has revealed its highly complex nature, mainly as a result of numerous discontinuous transcription events, revealing canonical and non-canonical RNA transcripts with RNA modifications (16) . In addition to the canonical full-length genome and other 9 sgRNAs, this study also found numerous non-canonical RNA transcripts of unknown ORFs that contained RNA modifications. 41 putative RNA medications were identified at an AAGAA motif. These previously unknown ORFs represent the epi-transcriptome of SARS-CoV-2 and has revealed numerous viral transcripts that may be involved in pathogenesis (16) .

Another study looked at transcriptome profiling in the primary human lung epithelium and compared differences between SARS-CoV-2 and SARS-CoV infection and identified several pathways potentially involved in pathogenesis and gender-specific differences in clinical presentation (226) . Among the genes that were upregulated were a cluster involved in the cytokine-mediated signalling pathways, and in particular, the IL-17 signalling pathway (226) . Specifically, cytokine pathways driven by Nuclear Factor Kappa-light-chain-enhancer of activated B cell (NF-κB), Toll-like receptors (TLRs), Mitogen-Activated Protein Kinase (MAPK), Bone Marrow Stromal Cell Antigen 2 (BST2), IL-32, TNF Alpha Induced Protein 3 (TNFAIP3), TNFAIP3 interacting Protein 1 (TN1P1), Intercellular Adhesion Molecule 1 (ICAM-1), Intercellular Adhesion Molecule 2 (ICAM-2), Matrix Metallopeptidase 9 (MMP9), Baculoviral IAP Repeat Containing 3 (BIRC3), and Rho Family GTPase 1 (RND1), were significantly upregulated during SARS-CoV-2 infection, suggesting a significant role in pathogenesis (226) . Moreover, RELA (NF-κB p65 subunit) seems to be significantly upregulated in SARS-CoV-2 infection, leading to IL-8 involvement (226) . Of note is the expression of oestrogen receptor 1 (ESR1), which was also enhanced under SARS-CoV-2 infection, suggesting sex hormones may be involved in differential expression during viral infection and may have implications for the differences in clinical severity seen between genders (226) . Additionally, over 24 and 48 hour post-infection, CXCL-2 was significantly upregulated in SARS-CoV-2 infection compared to SARS-CoV (226) . A recent study using single-cell RNA-Seq in human, non-human primate and mouse tissues/cells was able to resolve further the host cellular targets for SARS-CoV-2 and their abundance in specific tissue/cell types (36) . The study identified ACE2 and TMPRSS2 co-expressing cells (lung type II pneumocytes, ileal absorptive enterocytes and nasal goblet secretory cells) and also determined that that ACE2 is induced by Interferon-Stimulated Gene, suggesting a possible mechanism for enhanced viral infection (36) .

The clinical pathways of COVID-19 disease severity may also depend on host-specific factors that may contribute to the 'cytokine storm', or Cytokines Release Syndrome (CRS), which is the massive release of pro-inflammatory cytokines including cytokines (IL-1β, IL-2, IL-6, IL-7, IL-8, and TNF-α) and chemokines such as CXCL10 and CCL2 in the lungs (172, 227) . These genomic approaches also shed light on the specific genetic host factors that predispose individuals to this severe clinical presentation. Proteomic and transcriptomic studies on bronchoalveolar lavage (BAL) samples from COVID-19 patients have also revealed considerable insights into the expression of SARS-CoV-2 receptors, co-receptors, immune responses, as well as risk factors for severe disease e.g. age and co-morbidities. Asthma, chronic obstructive pulmonary disease (COPD), hypertension, smoking, obesity, and male gender status were all associated with higher expression of ACE2 and CD147 in BAL, as well as bronchial biopsy and blood from COVID-19 patients (206) . Furthermore, there was a positive correlation between the expression of CD147-related genes in BAL and the age and body mass index (BMI) of COVID-19 patients (206) . In another study on BAL from COVID-19 patients, an association was observed between COVID-19 severity and enhanced levels of certain cytokines, e.g. CCL2/MCP-1, CXCL10/IP-10, CCL3/MIP-1A, and CCL4/MIP1B (228) . This study also found that SARS-CoV-2 triggered apoptosis and the p53 signalling pathway in lymphocytes, probably causing additional lymphopenia in these patients (228) . A comparison of transcriptome profiles between patients with COVID-19 and influenza A virus infection revealed an absence of significant type I interferon/antiviral responses with SARS-CoV-2 infection, with enhanced expression of genes involved in metabolic pathways e.g. haem biosynthesis, oxidative phosphorylation and tryptophan metabolism, suggesting an important role for mitochondria during SARS-CoV-2 infection (229) . Furthermore, a meta-analysis on BAL data from COVID-19 patients also revealed an excess for neutrophils and chemokines (229) .

In meta-transcriptomic sequencing of BAL from 8 COVID-19 patients, the expression of proinflammatory genes, especially chemokines, was significantly elevated in these patients compared to community-acquired pneumonia patients and healthy controls, suggesting hypercytokinemia (230) . It also revealed enhanced dendritic cell and neutrophil activity (230) . In contrast to SARS-CoV, which induces an ineffective interferon response, SARS-CoV-2 was found to strongly initiate expression of numerous interferon stimulated genes, which are thought to significantly contribute to immunopathogenesis (230) . Similarly, an analysis of RNA-seq data sets of BAL from COVID-19 patients identified upregulation of neutrophil, inflammatory genes and chemokines, which may be involved in immunopathology, e.g. TNFR, IL-8, CXCR1, CXCR2, ADAM10, GPR84, MME, ANPEP, and LAP3 (231) .

Chronic co-morbidities for COVID-19 patients include cardiovascular disease, hypertension, diabetes, stroke and malignant tumour (112) . It was also found that parameters such as older age, underlying hypertension, high cytokine levels (IL-6, IL-10, and TNF-α), and high lactate dehydrogenase level were significantly associated with severe COVID-19 during hospital admission (166) . In a study involving 184 ICU patients with COVID-19 pneumonia, all of them showed an incidence of thrombotic complications such as symptomatic acute pulmonary embolism (PE), deep vein thrombosis, ischemic stroke, myocardial infarction or systemic arterial embolism (165) . Approximately, one-third of patients experienced gastrointestinal symptoms. During hospitalization, a substantial proportion of patients presented cardiac injury, liver, and kidney dysfunction, and hyperglycaemia. ICU COVID-19 patients had higher plasma levels of IL-2, IL-7, IL-10, GSCF, IP10, MCP-1, MIP-1, and TNF-α, compared to non-ICU patients. Majority of ICU patients diagnosed with COVID-19 were found to be at highest thrombotic risk (165) . Patients with severe COVID-19 likely developed ARDS and died of respiratory failure. Biopsy samples at autopsy from a patient who died from severe COVID-19 showed bilateral diffuse alveolar damage with cellular fibromyxoid exudates, and mononuclear inflammatory lymphocytes in both lungs (149, 177) . Diffuse alveolar damage with fibrin rich hyaline membranes are pathological results of COVID-19. In a study, 12 COVID-19-infected cancer patients were found to have underlying diseases, such as hypertension, diabetes and chronic obstructive pulmonary disease (232) . Cancer patients with accompanying COVID-19 infection showed deteriorating conditions and poor outcomes, and thus it was recommended to avoid treatments causing immunosuppression (233) .

The complement system is an integral part of the innate immune response. It consists of a group of plasma proteins produced mainly by the liver or membrane proteins expressed on cell surface. These proteins interact in a cascade that leads to the opsonization of pathogens and the induction of inflammatory responses. The complement system comprises of three distinct activation pathways, i.e. Classical, Alternative or Lectin (MBL). The activation of these pathways is based on different molecules present on the pathogen surfaces. The classical pathway is initiated by the binding of C1q to the pathogen surface or antibody complex. The initiation of the alternative pathway is triggered by the binding of a spontaneously activated complement component to pathogen surface. The binding of the MBL to mannose-containing carbohydrates on pathogens triggers the initiation of the lectin pathway. The early events of three pathways eventually converge to generate a protease called, C3 convertase, which is covalently bound to the pathogen. The C3 convertase then cleaves C3, present in plasma, into C3a and C3b. The C3b binds to the pathogen and targets it for destruction by phagocytes. Furthermore, C3b binds with the C3 convertase to form C5 convertase, which produces C5a and C5b. C5b triggers the late events of the complement cascade, which are a series of polymerization reactions where C6, C7, C8 and C9 interact with each other to form the Membrane Attack Complex (MAC). The MAC can damage the membrane of certain pathogens by creating a pore in it. The C5a and C3a produced are important small peptide mediators of inflammation [Reviewed in (234) ].

Studies in C3 -/-(gene-deficient) mice infected with SARS-CoV revealed the presence of C3 activation products such as C3a, C3b, iC3b, C3c, and C3dg 1 day post infection (235) . The C3 deficient mice showed significantly less respiratory dysfunction and lower weight loss as compared to control. The mice also showed significantly lower levels of neutrophils and monocytes compared to the control. Lower IL-6, TNF-α and IL-1 levels were reported in the lungs of the C3 deficient mice (235) . The study also reported lower weight loss in mice deficient in Factor B or C4. In view of the critical role of the complement system in SARS-CoV infection since the first day of infection, it raised possibility for complement involvement in SARS-CoV-2. Levels of the terminal component of the complement system (MAC) and C5a are increased in patients with ARDS (236, 237) . MAC is known to damage endothelial cells, and thus, regulation or inhibition of MAC by its known regulators such as CD59 or clusterin could be a potential treatment for endothelial dysfunction/damage in ARDS or COVID-19 (238) (239) (240) .

Considering the lectin pathway of the complement system, MBL was shown to bind SARS-CoV in vitro and inhibit its infectivity (241) . The N-protein of SARS-CoV and SARS-CoV-2 has been shown to interact with MBL-associated serine proteases-2 (MASP2), which is known to initiate the lectin pathway (242) , leading to over-activation of the complement system. This same study also highlighted excess complement proteins found in post-mortem COVID-19 patient lungs (242) . Furthermore, deletion of Masp2 or perturbance of the MASP-2-N protein interaction was found to reduce lung injury. These studies, along with human proteomic studies, demonstrate the activation of multiple complement pathways during a coronavirus infection. In case of COVID-19, the alternative and lectin pathways of the complement system seem to be preferentially activated (243) . Increased levels of plasma C5a and MAC were recorded in patients with moderate and severe COVID-19 (244) . A post-mortem study of lung and skin vasculature in 5 COVID-19 patients showed significant deposits of MAC and C4d that colocalized with the SARS-CoV-2 S-protein, and MASP2 in the micro-vasculature. This study did not find prominent classical features of ARDS such as hyaline membranes and inflammation in the histopathological examination (245) . A recent study reported an increase in levels of C5a, which correlated with increased COVID-19 disease severity, as well as high levels of expression of C5aR1 in blood and pulmonary myeloid cells of COVID-19 patients (246) . Furthermore, use of anti-C5aR1 monoclonal antibodies in human C5aR1 knock-in mice was found to successfully prevent C5a-mediated myeloid cell recruitment and activation, thereby inhibiting acute lung injury (246) . A recent genetic study in COVID-19 patients as reported that gene variants associated with complement regulatory protein, CD55 (decayaccelerating factor, which accelerates the decay of complement proteins, and thus inhibits complement activation) is associated with increased risk in clinical outcome (odds ratio 2.34-2.4); gene variants that map to C3 showed some protective effect (odds ratio 0.66-0.68) (247) .

Neutrophils along with the complement system are another important component in the defence of the host against invading pathogens. Neutrophil infiltration in pulmonary capillaries, acute capillaritis with fibrin deposition, extravasation of neutrophils into the alveolar space, and neutrophilic mucositis have been reported in the case of SARS-CoV-2 infection (158) . The neutrophilic extracellular traps (NETs) and the neutrophils activated by SARS-CoV-2 infection contain C3, factor B and properdin, triggering the alternative pathway of the complement system (243) . While this is usually beneficial, the sustained activation and NETs formation leads to a hyper-inflammatory immune response that damages and destroy surrounding tissue. This aberrant behaviour, in concert with the abnormal complement activation, leads to the well recorded clinical manifestations observed in the case of COVID-19 such as ARDS and pulmonary inflammation (248) . Additionally, NETs have been reported to induce the production of excessive thrombin and subsequently generate C5a (248) . Hence, it has been speculated that feedback loop that begins with complement activation leading to NETosis causing an increases in thrombin production, that further stimulates the complement activation causing enhanced NET formation (243) .

Microangiopathy refers to a disease of the small blood vessels. The term is used when small blood vessels pathologically thicken or weaken, which leads to impaired flow of blood as well as leaking of cells and proteins. Sustained inflammation in the vascular system due to the SARS-CoV-2 infection leads to thrombosis and microangiopathy (109, 249) . This is supported by reports of increased lactate dehydrogenase, bilirubin, activation of platelets, elevated D-dimer levels and hyper-fibrinolysis (250) . A post-mortem case series of 4 patients with COVID-19 found thrombotic microangiopathy, which was restricted to the lungs, along with diffuse alveolar damage could have contributed to causing death (251) . Another such study of 21 cases found similar diffuse alveolar damage with significant capillary congestion and microthrombi despite anti-coagulation therapy (252) .

Due to the presence of severe pulmonary vascular dysfunction in ARDS, it has been argued that ARDS is a type of vascular microthrombotic disease with lung phenotype involvement. This argument is supported by the association of mortality in ARDS with thrombocytopenia and MOF as a result of disseminated intravascular coagulation (253) (254) (255) . In recent times, a couple of theories on the pathogenesis of ARDS in sepsis have evolved: the 'two-path unifying theory' in which certain homeostasis mechanisms lead to microthrombogenesis, and the 'twoactivation theory of the endothelium' in which the complement MAC leads to inflammation via cytokines and microthrombogenesis via platelet activation (256, 257 ). The complement system plays a key role in the pathogenesis of thrombotic microangiopathy. This is a syndrome characterised by thrombocytopenia (low platelet count), microangiopathic haemolytic anaemia and systemic organ damage. Atypical haemolytic uremic syndrome (aHUS) is an example of such a disorder that typically leads to kidney damage. It is caused by excessive activation of the alternative pathway due to mutations in complement regulators factor H (common), factor I, membrane-cofactor protein, or C3. Analysis of renal tissue morphology from autopsies of COVID-19 patients revealed strong C5b-9 staining (via the alternative pathway) on the apical brush border of tubular epithelial cells with minimal deposition on glomeruli and capillaries of the kidney (258) . Treatment with eculizumab (C5 inhibitor) dramatically improved outcomes of survival in aHUS. Features similar to complement-mediated thrombotic angiopathy such as kidney and cardiac injury increased lactate dehydrogenase and d-dimer, and decreased platelets were observed in COVID-19 (137, 259) . Eculizumab was used successfully as part of management of four COVID-19 patients with severe pneumonia or ARDS in the intensive care unit, and this preliminary data is being used to conduct further full-fledged clinical trials with eculizumab (260) .

Considering the overlap with complement-mediated thrombotic angiopathy in COVID-19, few studies are underway to test the effectiveness of complement regulators. A recent case study demonstrated a favourable outcome for the compstatin-based C3 inhibitor AMY-101. The study, which involved a 71-year-old Caucasian male with severe pneumonia and systemic inflammation, found that AMY-101 was safe and had a favourable outcome in improving the clinical presentation of the patient significantly (261) . Furthermore, treatment with a recombinant C5a antibody on 2 male COVID-19 patients aged 54 and 67 years showed significant benefit in suppressing complement hyperactivation, which contributes to the excessive immune response causing aggravated inflammatory lung injury, a hallmark of SARS-CoV-2 pathogenesis and lethality (242) . One of the many challenges includes determining patients who have a dysregulated complement activation. C3 bound to erythrocytes has been detected in patients with COVID-19 (262) , which may prove to be a useful blood marker as well as in identifying patients who potentially merit intervention with complement regulators (263) .

In COVID-19, endothelial injury has been found to be a key pathophysiological feature. A case series found direct evidence of viral infection of endothelial cells and endothelial inflammation, leading to endothelial cell death (264) . In COVID-19 patients, endothelial cell abnormalities were recorded in the kidney, lung, heart, small bowel, and liver. 5 of 26 deceased COVID-19 patients were found to have suffered endothelial cell swelling with variable foamy degeneration in the glomeruli and an additional 3 patients were found to have severe injury to the endothelium due to segmental fibrin thrombi in glomerular capillary loops (264) (265) (266) . MAC deposition has been observed in the endothelium of COVID-19 patients (245) . Such studies have led to notion that in COVID-19, there are strong vascular and inflammatory components as well, which play a significant role in the pathophysiology of the illness (267) . Consistent with endothelial injury, the significantly elevated levels of von Willebrand factor found in the patient with severe COVID-19 has led to the idea that the infection of the ACE2 expressing endothelium by SARS-CoV-2 induces injury and activates the complement , which sets up a feedback loop that maintains a state of inflammation (243, (268) (269) (270) . It is worth noting that ARDS may occur in COVID-19 despite well-preserved lung gas volume, which could indicate a key role for inflammatory processes, leading to vascular constriction and subsequent low oxygen levels in the blood (271) . Furthermore, d-dimer (a fibrin degradation product) levels are also found to be elevated in COVID-19 and are associated with poorer prognosis (268, 272) . These factors add to the importance of understanding vascular changes in this disease, including microangiopathic processes and coagulopathies in patients with COVID-19.

Pregnancy is associated with several maternal adaptations in both immune function (immunosuppression) and cardiovascular physiology (increased cardiac output, physiological anaemia, cardiac hypertrophy) that would likely alter susceptibility to viral respiratory infections including SARS-CoV-2. Maternal death occurred in 15% of patients admitted to the ICU for COVID-19 and in 25% of those who required invasive mechanical ventilation (273) . To date, the literature consists of case reports, case series and retrospective studies.

The most common presenting symptoms of maternal COVID-19 are fever, cough, dyspnoea, and gastrointestinal symptoms (274) . Clinical findings of respiratory manifestations were similar to those seen in the non-pregnant populations, with similar CT findings together with elevations in C-Reactive protein with decreased white blood cell counts (275) . Although the portal of entry is inhalational, there are widespread systemic effects. The immobility, hypoxia and acute inflammation lead to a prothrombotic hypercoagulable state, and indeed, elevated Ddimers are correlated with disease severity (276) . COVID-19 is thus associated with venous or arterial thromboembolism (277) . The mechanism by which this occurs is currently thought to be as a result of inflammatory cytokines (203) inducing production of tissue factor with subsequent thrombin activation. The elevations of D-dimer (often seen in acute phases of infection) may be related to this increased thrombin generation.

While serious maternal morbidity has been seen, the vast majority of pregnant women with SARS-CoV-2 infection remained asymptomatic for respiratory symptoms (278) (279) (280) . Pregnancy is coupled to physiological changes in cardiorespiratory status (281) which might be expected to alter susceptibility to a respiratory upset. Nevertheless, the evidence suggests that this is less prevalent than thought. However, the changes in immune function and coagulation in pregnancy appear to increase some complications. Similarly, the coagulopathies seen in COVID-19 in the non-pregnant population might be expected to have deleterious effects in pregnancy, which is already a prothrombotic state.

COVID-19 has been seen to be associated with preeclampsia (274, 282) with one non-peerreviewed report suggesting a causal link (283) . The placenta has also been reported as having vascular malperfusion and thrombosis (284, 285) , which may provide an underlying explanation for the preeclampsia, a disease, associated with poor placental perfusion and altered vascular function (286) . Evidence of increased clotting at the placental surface suggests that this mechanism may be responsible (in part) for the increased incidence of preeclampsia. SARS-CoV-2 virions have been seen in the Syncytiotrophoblast, the part of the placenta responsible for the angiogenic imbalance seen in preeclampsia and effects on the release of known factors associated with the disease (sFlt-1 -Soluble Fms Like Tyrosine kinase 1 and PlGF -Placental Growth Factor) are unknown. The disease is also linked to preterm premature rupture of membrane (PPROM) (104, 282) , and preterm labour (287) , both of which are linked to inflammation. The underlying mechanism by which PPROM occurs is not entirely elucidated. However, reports have suggested that activation of the coagulation system and thrombin causes fetal membrane weakening and subsequent rupture of membranes (288, 289) . The alterations in clotting and thrombin seen in COVID-19 may well provide a mechanism for this. Similarly, thrombin has been related to the induction of preterm labour and weakened fetal membranes by induction of decidual colony-stimulating factor (CSF)-2 (290).

At present, there are no drugs, therapeutics, or vaccines approved for curing, preventing, or treating SARS-CoV-2 specifically. As of June 2020, a total of 239 (147 in human trails, 92 in preclinical development) therapeutic drugs are under development against COVID-19. The current treatment for SARS-CoV-2 patients involves the management of clinical symptoms and providing supportive care. While research into developing new drugs and treatments against SARS-CoV-2 are ongoing, much of the effort currently focuses on the repurposing existing medicines used against viruses, multiple sclerosis, arthritis, blood plasma derivatives and malaria. Moreover, although immunosuppressive treatments, e.g. corticosteroids have shown promise for COVID-19, there is considerable concern about possible side effects. Other immunotherapeutic approaches given as adjunct therapy and based on neutralizing inflammatory cytokines and other immunomodulators, passive viral neutralization to reduce lung pathology and viral load, could be a promising approach (291) . A number of these approaches are discussed below.

The antiviral drug Remdesivir, developed by Gilead Sciences, is an adenosine analogue, which incorporates into nascent viral RNA chains and results in premature termination, effectively inhibiting viral RNA synthesis (292) . It was developed for the treatment of Ebola and Marburg virus infections (293) , and animal studies have shown that it is effective against the other coronavirus (294) . In vitro studies have established its efficacy against SARS-CoV-2 (295) . An open-label trial across the United States, Europe, Canada and Japan showed clinical improvement of 36 of the 53 COVID-19 patients who were treated with a 10-day course of Remdesivir on a compassionate basis (296) . However, a follow-up multi-centre, randomized, double-blinded, placebo-controlled trial of 237 patients showed that the drug was not associated with a difference in time to clinical improvement. Compared to the placebo, the drug was found to have a non-significant but, numerically faster time to clinical improvement in patients with a symptom duration of 10 days or lower (297) . Currently, Japan and the USA have allowed the use of the drug under emergency use authorization for the treatment of COVID-19. In a randomized, open-label, multi-centre Phase 3 clinical trials, a 5-day course Remdesivir brought about a significant clinical improvement compared to standard alone in patients with moderate COVID-19. This clinical study assessed the effect of 5-day (n=191) and 10-day (n=193) investigational Remdesivir courses plus standard of care, versus standard of care alone (n=200) on clinical improvement on Day 11 (298) . In case of patients with severe disease, both 5 day and 10 courses of the drug have been found to have similar clinical outcomes, but as the study lacked placebo control, the magnitude of benefit cannot be determined (299) .

Umifenovir, marketed as Arbidol, is a derivative of indole carboxylic acids used for the treatment of influenza A and B virus infection, and other arboviruses (300) . It functions by incorporating into cell membranes and interfering with the hydrogen bonding network of phospholipids, blocking both the fusion of the virus to the cell membrane and the virusendosome fusion (301) . In vitro studies have established anti-viral efficacy against Ebola virus, human herpesvirus 8, hepatitis C virus, Tacaribe arena virus, SARS-CoV and SARS-CoV-2 (302) (303) (304) . A retrospective study on 81 SARS-CoV-2 patients treated with umifenovir did not reveal any improvement in clinical prognosis or accelerated viral clearance (304) . Currently, two randomized and open-label trials to determine the safety and efficiency of the drug are ongoing in China.

Favipiravir, another anti-viral drug, developed by Fujifilm Toyama Chemical (as Avigan) and Zhejiang Hisun Pharmaceutical, is a pyrazinecarboxamide derivative. It is converted into an active phosphoribosylated form (favipiravir-RTP) in cells and is recognized as a substrate by viral RNA polymerase, thereby blocking the activity of RNA-dependent RNA polymerase. It was developed as a treatment against influenza. The drug is currently approved for the treatment of SARS-CoV-2 in China and Italy. A study with 80 SARS-CoV-2 patients treated using the drug has reported that better efficacy was observed in anti-viral activity and lower adverse reactions compared to the control group that was treated with lopinavir/ritonavir (303) . Another prospective, multi-centre, open-label, randomized superiority study with 240 SARS-CoV-2 infected patients was conducted at three hospitals. They showed faster recovery from clinical symptoms when compared to the controls that were treated with umifenovir, even though similar numbers required the use of ventilators and oxygen (305) . There are currently six trials ongoing in China evaluating the efficiency of this drug against other antivirals for the treatment of COVID-19 and a Phase 3 clinal trial to assess its effectiveness and safety is scheduled in Japan and USA.

Anti-malaria drugs, Chloroquine and Hydroxychloroquine, are lysosomotropic agents that function by increasing late endosomal and lysosomal pH, which results in impaired viral release from the endosome or lysosome (306) (307) (308) . In vitro studies have shown antiviral activity against SARS-CoV-2 with Hydroxychloroquine, a weak diprotic base, to have higher potency against the virus (295, 309) . In SARS-CoV-2, Chloroquine, along with its lysosomotropic activity, is believed to reduce glycosylation of ACE2 affecting the binding of the virus to the cells (310) . Furthermore, Chloroquine is also shown to block the production of proinflammatory cytokines such as IL-6 preventing ARDS (311); Hydroxychloroquine was found to possess an anti-inflammatory effect on Th17-related cytokines (IL-6, IL-17 and IL-22) (312) . Initial clinical studies in China involving 100 SARS-CoV-2 infected patients, who were treated with Chloroquine, showed amelioration of pneumonia, shortened disease progression, increased resolution of lung lesions on CT, and a better virus-negative conversion (313, 314) . Hydroxychloroquine and combination therapy with azithromycin was found to reduce viral load in a French open-label non-randomised clinical trial and in an observational pilot study (315, 316) . Nevertheless, these studies were plagued with several limitations, such as small sample size, very short observation period, no randomisation, lack of reports on clinical progression, poorly described inclusion and exclusion criteria, and low National Early Warning Score (87, 315, 316) . Another trial with 30 SARS-CoV-2 infected patients treated with Hydroxychloroquine for seven days in China and a study with effectively 10 SARS-CoV-2 patients, revealed no significant difference in the nasopharyngeal viral carriage when compared to the controls that were provided with the local standard care (317, 318) . A third randomized clinical trial conducted in China with 62 patients exhibiting mild SARS-CoV-2 when treated with Hydroxychloroquine were found to have recovered faster from cough and fever when compared to the placebo. However, the result of this study cannot be extrapolated to patients with severe SARS-CoV-2 (319). A retrospective cohort study of 1438 random sample of inpatients with laboratory-confirmed SARS-COV-2 admitted to hospitals in the New York City was conducted. It did not find any significant differences in in-hospital mortality associated with the treatment with Hydroxychloroquine, azithromycin, or both, compared to the controls where the patients were given neither of the drugs (320). The US FDA and European Medicines Agency (EMA) and many other countries like India and Poland have authorized emergency use of Hydroxychloroquine to treat SARS-COV-2 infected patients. However, the FDA and EMA have issued warnings against the reported side effects of the drugs. These include abnormal electrical activity that affects the heart rhythm (QT interval prolongation, ventricular tachycardia, and ventricular fibrillation), particularly when taken at high doses or in combination with the antibiotic azithromycin. Other side effects reported are liver and kidney problems, nerve cell damage that can lead to seizures and hypoglycaemia (321, 322) . Around 30 clinical trials have been registered to study the effects of Hydroxychloroquine and Chloroquine independently or in combination with each other on SARS-CoV-2 have been registered in the USA and China (323) . Another anti-parasitic drug, ivermectin, has been shown to be effective against SARS-CoV-2 in vitro (324) . A clinical trial to assess the efficiency of ivermectin against SARS-CoV-2 has been planned to take place in Japan soon.

The corticosteroid, Dexamethasone, functions as an immunosuppressant. The drug is believed to modulate the lung injury caused by a dysregulated immune system, thereby reducing the progression to respiratory failure and death (325) . In a randomized, controlled, open-label, adaptive, platform trial, 2,104 patients treated with 6 mg of Dexamethasone (orally or intravenously) for 10 days were found to have a significantly reduced 28 day mortality rate among those receiving mechanical ventilation by 33.33%, and by 20% among those receiving oxygen without mechanical ventilation, compared to 4,321 patients treated with standard care (325) . Treatment with the drug did not provide any benefit to patients who did not require oxygen or mechanical ventilation, hinting at possible harm. The use of corticosteroid in the case of severe respiratory infections requires the use of "the right dose, at the right time, in the right patient" (325) . This is because a high or an early dose may help the virus proliferate by suppressing the immune system, instead of reducing inflammation. In case of COVID-19, the peak of viral shedding is higher early in the disease. The benefit of dexamethasone when patients require respiratory support or after the first week of the disease suggest that this stage is dominated by an irrepressible immune response versus active viral replication (325) . Dexamethasone is the first drug found to reduce mortality in COVID-19 (326).

Lopinavir/ritonavir is a drug combination. Lopinavir is a protease inhibitor, developed by Abbott Laboratories against HIV-1 that functions by blocking essential viral proteases (327) . Due to poor pharmacokinetics, it is administered exclusively in combination with ritonavir which increases Lopinavir's plasma half-life through inhibition of CYP3A-mediated metabolism of Lopinavir, thereby increasing its exposure and improving the anti-viral activity of the drug (327) . In vitro studies have revealed that Lopinavir inhibited the replication of the SARS-CoV-2 virus in Vero E6 cell (328) . In a randomized, controlled, open-label trial with 199 patients with laboratory-confirmed SARS-CoV-2 infection, no benefit was observed with Lopinavir-ritonavir treatment beyond standard care (329) . Another single-blind randomised controlled trial in China treated 44 patients with mild/moderate COVID-19 for 14 days, or Umifenovir or standard care with no antiviral (219) . In the study, no differences were found in the time taken for viral clearance, as assessed by PCR of nasopharyngeal swabs, fever, cough, or lung CT findings. Clinical status deterioration to severe/critical from mild or moderate clinical status and gastrointestinal side effects was seen highest in patients treated with Lopinavir/ritonavir when compared to Umifenovir treated or those treated with standard care and no antivirals (219) . Both these randomised clinical trials suffer from small sample sizes and lack of blinding. A multi-centre, prospective, open-label, randomised, phase 2 trial in Hong Kong with 127 SARS-CoV-2 infected patients involved treatment for 14 days with only Lopinavir-Ritonavir (Control), or with a combination of Lopinavir-Ritonavir, ribavirin, an oral hepatitis C virus drug, and IFN-. It found that the combination treatment was effective in reducing symptoms and viral shedding faster, as well as duration of hospital stay (330) . Currently, about a dozen trials are studying the effect of the drug against SARS-CoV-2. One such study is a Phase 4 randomized controlled trial in China in which the effectiveness of lopinavir-ritonavir against influenza drugs, Umifenovir and oseltamivir, is to be studied. Another South Korean trial is looking to compare the efficacy of Lopinavir-Ritonavir against Hydroxychloroquine. The WHO SOLIDARITY trial and UK-based RECOVERY trial is looking to study the effectiveness of Lopinavir-Ritonavir independently; the WHO SOLIDARITY trial also looks to the explore the drug in combination with interferon-.

Another second-generation protease inhibitor against HIV-1, Darunavir, has shown significant inhibition of SARS-CoV-2 replication (in vitro). According to a press release by Johnson & Johnson, an unpublished single-centre, open-label, randomized, and controlled trial in China in which 30 SARS-CoV-2 patients were treated with darunavir and cobicistat was not effective in treating SARS-COV-2 (331). However, a further three clinical studies in China are scheduled.

Other drugs currently being tested against SARS-CoV-2 include Tocilizumab, a monoclonal antibody against IL-6 developed by Roche, which is used for the treatment of moderate to severe rheumatoid arthritis by blocking IL-6 activity. The drug was found to have helped cure 19 of 20 COVID-19 patients in a trial conducted in China (332) . Another open multi-centre randomized controlled trial French study awaits publication, in which 129 patients were split into two groups, i.e. routine treatment with and without Tocilizumab: in the group treated with Tocilizumab, the combination of ventilation requirement (mechanical or non-invasive) or death was achieved in a significantly lower proportion of patients (333) . A phase 3 trial to test its efficacy in treating patients with severe COVID-19 has been authorised by the FDA. Moreover, an Italian multi-centre, retrospective study of 544 patients with severe COVID-19 pneumonia, revealed that the use of Tocilizumab given either intravenously or subcutaneously was associated with reduced risk of mechanical ventilation and death (334) .

Anakinra is a recombinant IL-1 receptor antagonist that has shown promise in treating severe COVID-19 disease. In a retrospective cohort study of patients with COVID-19 and ARDS that were managed with non-invasive ventilation (outside the ICU), their treatment with high-dose anakinra was observed to be safe and associated with clinical improvement in 72% of patients (335) . Another study has also described the early use of anakinra in COVID-19 patients with cytokine storm syndrome (CSS) and acute hypoxic respiratory failure (AHRF) which may be beneficial in preventing the need for mechanical ventilation (336) . These results have encouraged further clinical trials to validate its safety and efficacy (337) .

Approaches targeting inhibition of Bruton tyrosine kinase (BTK) has also shown promise. BTK plays a significant role in human innate immune responses. TLRs recognize ssRNA of viruses like SARS-CoV-2 and induce signalling via BTK-dependent activation of NF-κB, initiating a pro-inflammatory response (338) (339) (340) (341) . BTK also plays a key role in the activation of the NLRP3 inflammasome, resulting in maturation and secretion of IL-1β, a key pro-inflammatory cytokine (342) (343) (344) . Thus, BTK seems a favourable target against the cytokine storm in COVID-19. In one study, Acalabrutinib (a selective inhibitor of BTK) was given to 19 patients with severe COVID-19 and clinical improvements were observed over a 2-week treatment period, with reduced biomarkers of inflammation (C-reactive protein and IL-6) to normal levels (345) . Other dual inhibitors e.g. ibrutinib which target BTK/IL-2-inducible T-cell kinase (ITK) signalling have also shown promise (346) . In one study of patients given ibutinib for treatment of B-cell malignancies and chronic graft-versus-host disease (cGVHD), there was evidence that ibutinib may also protect against pulmonary injury in COVID-19, which these patients subsequently had, suggesting ibutinib as a possible prophylactic for vulnerable patient groups (347) . Similar findings demonstrating a possible protective role of BTK inhibitors in cancer with COVID-19 have also been subsequently described (348) (349) (350) . These promising findings now merit a controlled randomised trial to demonstrate efficacy and drug safety of these BTK inhibitors.

Intravenous immunoglobulin (IVIG) is a pooled preparation of normal IgG obtained from several thousand healthy donors. It is generally used in the immunotherapy of several autoimmune and inflammatory diseases, (351) , and thus has been investigated for treating COVID-19 to mitigate the CSS. IVIG therapy has shown promise through several studies, although careful selection of COVID-19 patients and timing of IVIG administration appear to be the key for good clinical outcome. Preliminary findings from one multi-centre study showed that early administration of high dose IVIG improved the prognosis of critical patients with COVID-19 (352) . Similarly, 3 patients with severe COVID-19 who received high-dose IVIG made a satisfactory recovery (353) . In another study, the use of IVIG as an adjuvant treatment for COVID-19 pneumonia within 48 hours of admission to the ICU reduced the use of mechanical ventilation, ICU and hospital time, and the 28-day mortality rate of patients with severe COVID-19 pneumonia (354) . In a case study of a COVID-19 patient with respiratory failure and shock, treatment with plasma exchange before IVIG treatment resulted in prompt recovery without the need for mechanical ventilation and may be an additional early treatment step to treat critically ill COVID-19 patients (355) . IVIG treatment of severely-ill COVID-19 patients on mechanical ventilation has also shown promise. In one study of 5 patients, treatment with IVIG improved clinical and respiratory outcome, particularly saturation O2 levels, resulting in earlier extubation of the patients (356) . Furthermore, CT graphs obtained after IVIG therapy also revealed improvements in pulmonary lesions of these patients (356) .

Convalescent plasma therapy (CPT) is another classical adaptive immunotherapy used for the treatment of infectious disease for over a century. It has currently been approved for COVID-19 by the FDA under compassionate use rules. The treatment involves the transfusion of high neutralizing antibody titre containing blood plasma from SARS-CoV-2 recovered patients. This provides immediate short-term immunity. This is accomplished by binding of the pathogen to the antibody, which results in the activation of the immune system causing cellular cytotoxicity, phagocytosis, or direct pathogen neutralisation. Five clinical studies, conducted involving 27 COVID-19 patients who were treated with CPT, revealed significantly lower viral titres, increased levels of neutralizing antibody, improved clinical symptoms such as apyrexia, resolved ARDS and unassisted breathing (357) (358) (359) (360) (361) . Among the 27 CPT-treated patients, no fatalities were recorded, and no severe adverse reactions or treatment complications associated with CPT were reported (357) (358) (359) (360) (361) . While providing with valuable initial data, these studies suffer from several limitations such as lack of proper control groups, non-randomized evaluations, concomitant drug treatments, poor participant selection, lack of proper CPT dosage, and duration of therapy (362) . Three clinical trials are currently being evaluated by the FDA to test the safety and efficiency of CPT in patients who have been exposed to the virus and are at high risk of developing severe COVID-19, patients who are admitted in hospital with acute respiratory symptoms, and for COVID 19 patients under mechanical ventilation (363) . Further trials are also planned or ongoing in China, Columbia, Iran, Mexico and the Netherlands (363) . Early safety indicators of COVID-19 CPT were evaluated in a study of 5,000 patients and showed that the mortality rate was not unduly high and concluded that transfusion of convalescent plasma appears safe in hospitalized patients with COVID-19 (364) .

While the repertoire of antivirals and repurposed drugs tested against SARS-CoV-2 are expected to help manage the disease, the development of a safe and effective vaccine would help cut down the overall number of deaths and prevent the population from getting the disease in the first place. A recent study suggested that mandatory BCG vaccination can possibly be associated in flattening the curve in the spread of COVID-19. It analysed the rate of day-wise increase in positive cases in 135 countries and deaths in 134 countries for the first 30-day period (365) . While arguments for the potentially beneficial effects of pre-existing vaccines have been sporadically made, including giving MMR (Mumps, Measles and Rubella) vaccines to elderly population, generating a SARS-CoV-2 specific vaccine seems a logical and obligatory choice. As of 31 July 2020, the WHO landscape document reports 139 candidate vaccines developed on various platforms ( Figure 6 ) in preclinical stages of development: only 26 are under clinical evaluation.

mRNA-1273 vaccine is a sequence optimized mRNA/LNP expressing a perfusion stabilized form of SARS-CoV-2 S-2P a transmembrane anchored protein with the native furin cleavage site, developed by Moderna in collaboration with the National Institute of Allergy and Infectious Diseases Vaccine Research Center (366, 367) . The vaccine is undergoing an openlabel phase 1 clinical trial that started in March, 2020 with 45 healthy adult (18-55-year-old) volunteers for six weeks in three dose cohorts (25µg, 100µg and 250µg) as two doses approximately 28 days apart via intramuscular injection in the upper arm. Three cohorts of 56-70-year-old volunteers and three cohorts of healthy volunteers aged 71 and above are being enrolled in addition to the initial volunteers. The volunteers will be followed through 12 months after the second vaccination to assess safety data, common vaccination symptoms, review trial data and advise NIAID (367) . A Phase II trial with 600 healthy participants in two cohorts (18-55 years old adults and adults aged 55 years and above) treated with a placebo, a 50μg or a 250μg dose has begun from May, 29 th , 2020. The in vivo studies in murine models suggested the vaccine to be immunogenic and could elicit IgG2a and IgG1 subclass S-binding antibodies. mRNA-1273 immunized mice splenocytes showed higher secretion of IFN- than IL-4, IL-5 or IL-3 upon re-stimulation with peptide pools (S1 and S2). A dose of 1μg of mRNA-127 was found to induce robust CD8 + T cell response to the S1 peptide pool with balanced Th1/Th2 Ab isotype response in mice. Thus, a 100 μg dose of vaccine has been decided for human trial in Phase 3 study, which is equivalent to 1μg dose induced in mice (368) . The FDA has granted Fast Track designation for the vaccine.

The Pfizer licensed BioNTech's BNT162 vaccine development programme has developed four coronavirus vaccine candidates (366) . Two of the vaccines contain mRNA coding for the Spike protein of SARS-CoV-2, while the other two contain only the RBD of the spike protein (369) . Furthermore, the four vaccine candidates are made of three different mRNA formats. Two of the vaccine are based on nucleoside modified mRNA (modRNA), which incorporates modified nucleosides in the mRNA (370). This suppresses intrinsic immune activation and the production of anti-drug antibodies against the mRNA itself (370). The suppressed immune activity against the therapeutic mRNA helps produce the antigenic protein for longer periods (370). The next vaccine candidate is based on the Optimised unmodified mRNA (uRNA) format (370). uRNA uses uridine in the mRNA, making it more immunogenic (370). Finally, the last vaccine candidate uses self-amplifying mRNA (saRNA) (370). It is based on the principle of viral replication. The saRNA, in addition to encoding a protein of interest, also encodes, replicase (370). This enables the self-amplification of the mRNA inside the cell (370). The dsRNA intermediate created during the replication of the RNA triggers an immune response making saRNA a potent activator of the immune system (370). A Phase 1/2, randomized, placebo-controlled, observer-blind, dose-finding, and vaccine candidate-selection study to evaluate the safety, tolerability, immunogenicity, and potential efficacy of the candidate in 200 healthy adult volunteers is ongoing (100).

Another frontrunner among the candidates is CanSino Bio's Ad5-nCoV (366) . It is a genetically engineered vaccine candidate with the replication-defective adenovirus type 5 (live virus) as the vector to express SARS-CoV-2 spike protein. This would help the body to produce neutralizing antibodies against SARS-CoV-2. It has been shown to induce a strong anti-viral activity against SARS-CoV-2 in animal and in vitro studies. A single-centre, non-random, open, and dose-escalation phase I clinical trial for recombinant novel coronavirus vaccine (adenoviral vector) in 108 healthy adults aged between 18 and 60 years were conducted. The vaccine has been administered as a liquid formulation intramuscularly in the deltoid muscle (371) . Three different doses were chosen: (a) low dose of 5x10 10 viral particles/0.5ml; (b) intermediate dose of 1.5x10 11 viral particles/ml; and (c) high dose combines both low and intermediate dose (one in each arm). The volunteers are assessed for a period for 6 months to study any adverse reactions or other relevant outcomes (371) . Most common systematic adverse reaction observed were fever, fatigue, headache and muscular pain but with no serious adverse effect were noted within 28 days. Participants showed four-fold increase in anti-RBD antibodies in all the groups; neutralizing antibodies increased gradually being highest at 28 days post vaccination. Ad5 neutralizing antibody titres were boosted significantly postvaccination. IL-2 and TNF- were detected and polyfunctional memory CD4 + T cell phenotypes were higher than CD8 + T cells. This suggested Ad5 vectored COVID-19 vaccine to be immunogenic and capable of stimulating both B and T cell response. For phase 2 clinical trial, intermediate dose was chosen and is expected to be completed by 31 January 2021 (372) . The vaccine may have some negative effects in older age people thus in the 2 nd clinical trial participants above 60 years will be included. T cell response peaked earlier from 14 th day after the 1 st shot of vaccine whereas the antibodies production level peaked at 28 th day post vaccination. The study also highlighted the possibility of negative effect on vaccine elicited immune response due to pre-existing Ad5 immunity (372) .

ChAdOx1-nCov19 is being developed by Oxford University, UK (366) . It is a replication deficient simian adenovirus vector ChAdOx1, containing full length S-protein of SARS CoV-2 along with a tissue plasminogen activator leader sequence. The vaccine is reported to be effective in inducing an antiviral response in animal models (373) . ChAdOx1-nCov19 was found to be immunogenic in mice mounting robust anti-viral response. Single dose of this vaccine was capable of inducing humoral and cellular immune response in rhesus macaques (373) . A phase I/II single-blinded, randomised, multi-centre study to determine efficacy, safety and immunogenicity of the vaccine in about 1090 healthy adult volunteers aged 18-55 years was initiated on April 23 rd , 2020 (374). The volunteers have been subjected to either one dose of 5x10 10 vp of ChAdOx1 nCoV-19, an additional booster dose of 2.5x10 10 vp of ChAdOx1 nCoV-19, or a control of MenACWY vaccine delivered intramuscularly (374). The volunteers were assessed for a period for 6 months to study any adverse reactions or other relevant outcomes (375) . The results showed increase in S-specific antibodies with a single dose by 28 th day and increase in neutralizing antibodies with booster dose in all participants. ChAdOx1-nCov19 was also capable of inducing heightened effector T-cell response quite earlier than antibody response. T cell response peaked on day 14 th and sustained up to 56 days. The results showed ChAdOx1 nCoV-19 vaccine to be safe, tolerant and immunogenic, which further supported phase 3 trial which is now underway (375) .

PiCoVacc is a purified inactivated SARS-CoV-2 vaccine candidate which is capable of inducing neutralizing antibodies in mice, rats, and nonhuman primates specific to SARS-CoV-2. CN2 strain of SARS CoV-2 virus was chosen to develop PiCoVacc which was inactivated with β-propiolactone. This inactivated vaccine candidate was able to produce about 10-fold higher S-specific antibody titres in murine model when compared to COVID-19 recovered patients. Efficacy of PiCoVacc was also tested in rhesus macaques with an intramuscular low (1.5µg), medium (3 µg) and high (6 µg) dose administered three times (0, 7 th and 14 th day) and on day 22 nd SARS CoV-2 CN1 strain was inoculated through intratracheal (lungs) route. All vaccinated macaques showed protection towards SARS CoV-2 infection and their viral loads declined significantly. No notable haemato-and histopathological changes were observed; human clinical trials are awaited (376) .

A group of US scientists have come up with a series of prototype DNA vaccines expressing variants of the SARS-CoV-2 spike protein. The efficacy of the DNA vaccine candidates was evaluated in 35 rhesus macaques (6-12-year-old). Intramuscular dose (5mg) of DNA vaccine was administered, followed by booster dose on 3 rd week and antigenic challenge (1.2 x 10 8 viral particles) on 6 th week (both intranasal and intratracheal route). DNA vaccine was found to be protective with dramatic reduction of viral replication and enhanced production of Sspecific binding as well as neutralizing antibodies compared to controls. The study has not yet addressed the possibility of mutations that may emerge in escaping neutralizing antibodies, though it seems to be protective in primates against SARS-CoV-2 (377).

J o u r n a l P r e -p r o o f INO-4800, developed by Inovio, is a DNA vaccine candidate (366) . The optimized Spike protein of SARS-CoV-2 virus DNA plasmids are introduced into cells by the use of a proprietary platform, CELLECTRA®, via electroporation (378) . Once inserted, the plasmids are expected to strengthen the body's own natural response. A Phase I Open-label study to evaluate the safety, tolerability and immunogenicity of INO-4800 as a prophylactic vaccine against SARS-CoV-2 in 40 healthy volunteers aged 18-50 years is ongoing (378) . The volunteers will be treated with either one or two doses of 1 mg of vaccine administered intradermally followed by electroporation the following day (378) . The volunteers will be assessed for a period for 1 year to study any adverse reactions or other relevant outcomes (378) . Once the initial safety and immunogenicity of the vaccine are satisfied, Phase II efficacy studies are planned.

Qualitative and quantitative properties of CD4 + and CD8 + T cell responses in COVID-19 and prophylactic vaccine development necessitate identifying viral regions and potential epitopes. Thus, a total of 423 peptides (15-to 18-mer), which span the full proteome of the SARS-CoV-2 excluding ORF-1, were designed and used to assess the memory T cell responses upon challenge on 42 patients following recovery from COVID-19. 39 peptides were identified containing CD4 + and/or CD8 + epitopes. The memory of T cell responses from convalescent individuals with COVID-19 was found to be greater in severe COVID-19 cases compared to mild ones. Immunodominant epitope clusters and peptides were most markedly observed to belong to spike, M, and ORF3 proteins. In about 35% of study groups, strong CD8 + T cell responses specific to the NP protein were observed, suggesting the possibility of inclusion of non-spike proteins in future COVID-19 vaccine design (379) . In another study, a comprehensive immunogenicity map of the SARS-CoV-2 virus was carried out; 65 peptide sequences (33-mers) were generated based on computational algorithms. A single 33-mer peptide containing multiple epitopes that can possibly present on HLA class I and class II across majority of population and provide long-term immunity in most people acting as B and T cell epitopes had been identified. This in silico analysis needs further evaluation for safety and efficacy as a vaccine (380) .

In an unprecedented short span of time and speed since the beginning of the COVID-19 pandemic, significant progress has been made in our understanding of the pathogenesis of SARS-CoV-2 infection. However, there are endless unanswered questions; hopefully and most likely, they will be answered in near future. Why there are a huge population that are asymptomatic carriers? What are the genetic contributors to susceptibility and resistance to developing COVID-19? How pregnant women are so resilient to developing COVIDsymptoms; for that matter young children as well! What happens during the period of latency, i.e. between being infected and showing symptoms? How far the lung surfactant system gets affected during severe symptoms? What triggers thrombotic microangiopathy in addition to complement activation. On the adaptive immune aspects, what variations exist within population in terms of the proportion of neutralising antibodies? Persistence of neutralising antibodies and recall memory magnitude following second infection (on vaccination trials) will yield serious information about how to finetune the dose, duration and vaccination strategies. In this acute crisis, a number of existing drugs have been repurposed empirically; clinical trials have yielded variable results. It is becoming clear that combination therapies are more likely to be successful. Deciphering, at high resolution, the mechanisms and consequences of hostpathogen interactions in COVID-19 will lead to novel therapies and preventative vaccination strategies. Primary cellular host and co-receptor for SAR-CoV-2. 1) Attachment and entry of SAR2-CoV-2 requires priming by Transmembrane serine protease 2 (TMPRSS2) which cleaves the S protein into S1 and S2 portions, facilitating, 2), S1 targeting and binding of the receptor Angiotensin-converting enzyme 2 (ACE2), followed by receptor-mediated endocytosis of the virion into the host cell. J o u r n a l P r e -p r o o f TMPRSS2 is the key protease involved in priming SARS-CoV-2, which forms a receptorprotease complex with ACE2 on the host cell surface, thus facilitating viral targeting and entry to the host cell. Co-expression of AEC2 and TMPRSS2 has been found in proximal as well in distal airways. The nasal cavity has the highest expression of both the receptors in ciliated and secretory (goblet) cells compared to lung bronchi (ciliated and secretory cells) and lung parenchyma (alveolar type 2 progenitor cells, AT2). Structural conformation of receptor-binding domain (RBD) present in S1 region of SARS-CoV-2 spike protein is capable of influencing the ACE2-binding affinity. In case of SARS-CoV-2, the RBD contains a four-residue motif glycine-valine/glutamine-glutamate/threonineglycine which enables the binding loop to take a different conformation. It can undergo two possible conformational changes, a "lying down state" which has low affinity towards AEC2 and a "standing up state" with high binding affinity. SARS-CoV-2 RBD is found mostly in lying down state, and thus being less accessible to AEC2. This hidden conformation of RBD in the spike protein can possibly be a masking strategy for immune evasion by SARS-CoV-2. (1) The SARS-CoV-2 binds to the cell via the ACE2 receptor using the S1 subunit of the spike protein. Once bound, the S2 subunit facilitates virus-cell membrane fusion by two tandem domains, heptad repeats 1 (HR1) and heptad repeats 2 (HR2) to form a six-helix bundle (6-HB) fusion core, bringing viral and cellular membranes into close proximity for fusion and infection. Type II pneumocytes infected with SARS-CoV 2 trigger the release of cytokines, chemokines and interferons. The secreted inflammatory mediators recruit macrophages, neutrophils and activated T cells. The stimulated macrophages secrete IL-1, IL-6 and TNF-α. This increases capillary permeability, causing plasma to leak into the interstitial space and the alveolus. The stimulated neutrophils release reactive oxygen species and proteinases, which destroy infected cells. The cell debries and the plasma combine to form a protein-rich fluid. The increasing fluid leads to dyspnoea and pneumonia. It also dilutes the surfactant lining of the alveolus causing alveolar collapse, which leads to hypoxaemia and acute respiratory distress syndrome. The sustained inflammation leads to systemic inflammatory response syndrome, which develops into septic shock causing multi-organ failure and death. 

Coronavirus pathogenesis

The structure and functions of coronavirus genomic 3' and 5' ends

Novel Coronavirus: an Emerging Global Threat

Identification of a novel coronavirus in patients with severe acute respiratory syndrome

Isolation of a novel coronavirus from a man with pneumonia in Saudi Arabia

A novel coronavirus from patients with pneumonia in China

Epidemiologic clues to SARS origin in China

Discovery of a rich gene pool of bat SARS-related coronaviruses provides new insights into the origin of SARS coronavirus

Summary of probable SARS cases with onset of illness from 1

Host determinants of mers-CoV transmission and pathogenesis

Middle East respiratory syndrome, MERS situation upadate

The species Severe acute respiratory syndrome-related coronavirus: classifying 2019-nCoV and naming it SARS-CoV-2

A pneumonia outbreak associated with a new coronavirus of probable bat origin

Genome Composition and Divergence of the Novel Coronavirus (2019-nCoV) Originating in China

The Architecture of SARS-CoV-2 Transcriptome

Isolation and Characterization of 2019-nCoV-like Coronavirus from Malayan Pangolins

A novel coronavirus associated with severe acute respiratory syndrome

Bat-to-human: Spike features determining "host jump" of coronaviruses SARS-CoV, MERS-CoV, and beyond

Angiotensin-converting enzyme 2: A functional receptor for SARS coronavirus

Efficient Activation of the Severe Acute Respiratory Syndrome Coronavirus Spike Protein by the Transmembrane Protease TMPRSS2

Evidence that TMPRSS2 Activates the Severe Acute Respiratory Syndrome Coronavirus Spike Protein for Membrane Fusion and Reduces Viral Control by the Humoral Immune Response

A Transmembrane Serine Protease Is Linked to the Severe Acute Respiratory Syndrome Coronavirus Receptor and Activates Virus Entry

SARS-CoV-2 Cell Entry Depends on ACE2 and TMPRSS2 and Is Blocked by a Clinically Proven Protease Inhibitor

Structure, Function, and Antigenicity of the SARS-CoV-2 Spike Glycoprotein

The cell biology of receptor-mediated virus entry

A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirusinduced lung injury

Type II transmembrane serine proteases

Activation of influenza viruses by proteases from host cells and bacteria in the human airway epithelium

The spike glycoprotein of the new coronavirus 2019-nCoV contains a furin-like cleavage site absent in CoV of the same clade

Rare driver mutations in head and neck squamous cell carcinomas converge on NOTCH signaling

SARS-CoV-2 entry factors are highly expressed in nasal epithelial cells together with innate immune genes

Single cell RNA sequencing of 13 human tissues identify cell types and receptors of human coronaviruses

Air, Surface Environmental, and Personal Protective Equipment Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) from a Symptomatic Patient

Characteristics of pediatric SARS-CoV-2 infection and potential evidence for persistent fecal viral shedding

SARS-CoV-2 Receptor ACE2 Is an Interferon-Stimulated Gene in Human Airway Epithelial Cells and Is Detected in Specific Cell Subsets across Tissues

Structure of the Fusion Core and Inhibition of Fusion by a Heptad Repeat Peptide Derived from the S Protein of Middle East Respiratory Syndrome Coronavirus

Activation of the SARS coronavirus spike protein via sequential proteolytic cleavage at two distinct sites

Host cell entry of Middle East respiratory syndrome coronavirus after two-step, furin-mediated activation of the spike protein

Inhibitors of cathepsin L prevent severe acute respiratory syndrome coronavirus entry

Coronavirus membrane fusion mechanism offers a potential target for antiviral development

Protease-mediated enhancement of severe acute respiratory syndrome coronavirus infection

Cleavage of the SARS coronavirus spike glycoprotein by airway proteases enhances virus entry into human bronchial epithelial cells in vitro

Cleavage and Activation of the Severe Acute Respiratory Syndrome Coronavirus Spike Protein by Human Airway Trypsin-Like Protease

Modulation of TNF-α-converting enzyme by the spike protein of SARS-CoV and ACE2 induces TNF-α production and facilitates viral entry

TMPRSS2 and ADAM17 Cleave ACE2 Differentially and Only Proteolysis by TMPRSS2 Augments Entry Driven by the Severe Acute Respiratory Syndrome Coronavirus Spike Protein

Entry from the Cell Surface of Severe Acute Respiratory Syndrome Coronavirus with Cleaved S Protein as Revealed by Pseudotype Virus Bearing Cleaved S Protein

Characterization of a Highly Conserved Domain within the Severe Acute Respiratory Syndrome Coronavirus Spike Protein S2 Domain with Characteristics of a Viral Fusion Peptide

Ultrastructure and Origin of Membrane Vesicles Associated with the Severe Acute Respiratory Syndrome Coronavirus Replication Complex

A Contemporary View of Coronavirus Transcription

Coronaviruses post-SARS: Update on replication and pathogenesis

Minus-strand copies of replicating coronavirus mRNAs contain antileaders

Recombination between nonsegmented RNA genomes of murine coronaviruses

RNA recombination of coronavirus

Replication of coronavirus MHV-A59 in sac-cells: determination of the first site of budding of progeny virions

Characterization of the budding compartment of mouse hepatitis virus: Evidence that transport from the RER to the Golgi complex requires only one vesicular transport step

Molecular Interactions in the Assembly of Coronaviruses

Modular organization of SARS coronavirus nucleocapsid protein

Identification of In Vivo-Interacting Domains of the Murine Coronavirus Nucleocapsid Protein

Coronaviruses: An overview of their replication and pathogenesis

A structural analysis of M protein in coronavirus assembly and morphology

Differential maturation and subcellular localization of severe acute respiratory syndrome coronavirus surface proteins S, M and E

The production of recombinant infectious DI-particles of a murine coronavirus in the absence of helper virus

Analysis of Constructed E Gene Mutants of Mouse Hepatitis Virus Confirms a Pivotal Role for E Protein in Coronavirus Assembly

Infectious Bronchitis Virus E Protein Is Targeted to the Golgi Complex and Directs Release of Virus-Like Particles

Envelope Protein Palmitoylations Are Crucial for Murine Coronavirus Assembly

Characterization of the Coronavirus Mouse Hepatitis Virus Strain A59 Small Membrane Protein E

The spike protein of SARS-CoV -A target for vaccine and therapeutic development

Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study

Dynamics of anti-SARS-Cov-2 IgM and IgG antibodies among COVID-19 patients

Antibody responses to SARS-CoV-2 in patients with COVID-19

Detection of antibodies against SARS-CoV-2 in patients with COVID-19

Clinical characteristics of coronavirus disease 2019 in China

Epidemiologic Features and Clinical Course of Patients Infected with SARS-CoV-2 in Singapore

Public health might be endangered by possible prolonged discharge of SARS-CoV-2 in stool

A rapid advice guideline for the diagnosis and treatment of 2019 novel coronavirus (2019-nCoV) infected pneumonia (standard version)

First known person-to-person transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the USA

Early transmission dynamics in Wuhan, China, of novel coronavirusinfected pneumonia

A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster

SARS-CoV-2 epidemic in India: epidemiological features and in silico analysis of the effect of interventions

Asymptomatic cases in a family cluster with SARS-CoV-2 infection

Asymptomatic SARS-CoV-2 infected patients with persistent negative CT findings

Transmission of 2019-NCOV infection from an asymptomatic contact in Germany

Systematic Urgent Review Group Effort (SURGE) study authors. Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis

Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1

TMPRSS2 and TMPRSS4 promote SARS-CoV-2 infection of human small intestinal enterocytes

First case of 2019 novel coronavirus in the United States

Royal College of Midwives and Royal College of Paediatrics and Child Health, with input from the Royal College of Anaesthetists PHE and HPS. COVID-19 Virus Infection and Pregnancy

Infants Born to Mothers With a New Coronavirus (COVID-19)

Clinical analysis of 10 neonates born to mothers with 2019-nCoV pneumonia

World Health Organization. FREQUENTLY ASKED QUESTIONS: Breastfeeding and COVID-19 For health care workers

If You Are Pregnant, Breastfeeding, or Caring for Young Children

Gender Differences in Patients With COVID-19: Focus on Severity and Mortality

Clinical features of patients infected with 2019 novel coronavirus in Wuhan

The trinity of COVID-19: immunity, inflammation and intervention

Sex hormones determine immune response

OpenSAFELY: factors associated with COVID-19 death in 17 million patients

The clinical dynamics of 18 cases of COVID-19 outside of Wuhan, China

Incubation period of 2019 novel coronavirus (2019-nCoV) infections among travellers from Wuhan, China

Diarrhea during COVID-19 infection: pathogenesis, epidemiology, prevention and management

Clinical Characteristics of 138 Hospitalized Patients with 2019 Novel Coronavirus-Infected Pneumonia in Wuhan, China

Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study

Autoimmune haemolytic anaemia associated with COVID-19 infection

The Ocular Manifestations and Transmission of COVID-19: Recommendations for Prevention

Taste Changes (Dysgeusia) in COVID-19: A systematic review and metaanalysis

Acute anosmia from COVID-19 infection

Early recovery following new onset anosmia during the COVID-19 pandemic -An observational cohort study

Loss of smell and taste as symptoms of COVID-19: what does the evidence say? -CEBM

Anosmia, hyposmia, and dysgeusia as indicators for positive SARS-CoV-2 infection

COVID-19 pandemic and the skin: what should dermatologists know?

Neurologic Manifestations of Hospitalized Patients with Coronavirus Disease

Neurological and neuropsychiatric complications of COVID-19 in 153 patients: a UK-wide surveillance study

Autoimmune and inflammatory diseases following COVID-19

Paediatric multisystem inflammatory syndrome temporally associated with SARS-CoV-2 mimicking Kawasaki disease (Kawa-COVID-19): A multicentre cohort

Kawasaki-like multisystem inflammatory syndrome in children during the covid-19 pandemic

An outbreak of severe Kawasaki-like disease at the Italian epicentre of the SARS-CoV-2 epidemic: an observational cohort study

Clinical Characteristics of 58 Children with a Pediatric Inflammatory Multisystem Syndrome Temporally Associated with SARS-CoV-2

Acute heart failure in multisystem inflammatory syndrome in children (MIS-C) in the context of global SARS-CoV-2 pandemic

Multisystem Inflammatory Syndrome in U.S. Children and Adolescents

Multisystem Inflammatory Syndrome in Children in New York State

Classification of the cutaneous manifestations of COVID-19: a rapid prospective nationwide consensus study in Spain with 375 cases

COVID-19 and dermatology: a comprehensive guide for dermatologists

Immune thrombocytopenic purpura in a patient with covid-19

Cutaneous manifestations in COVID-19: a first perspective

Appearance and management of COVID-19 laryngo-tracheitis: two case reports

Ribose 2'-O-methylation provides a molecular signature for the distinction of self and non-self mRNA dependent on the RNA sensor Mda5

Coronavirus Nonstructural Protein 16 Is a Cap-0 Binding Enzyme Possessing (Nucleoside-2′O)-Methyltransferase Activity

SARS-CoV-2 and COVID-19 in older adults: what we may expect regarding pathogenesis, immune responses, and outcomes

Papain-like protease regulates SARS-CoV-2 viral spread and innate immunity

Structural basis for translational shutdown and immune evasion by the Nsp1 protein of SARS-CoV-2

Comparative replication and immune activation profiles of SARS-CoV-2 and SARS-CoV in human lungs: an ex vivo study with implications for the pathogenesis of COVID-19

Comparative pathogenesis of COVID-19, MERS, and SARS in a nonhuman primate model

Cytokine release syndrome in severe COVID-19

CT imaging of the 2019 novel coronavirus (2019-NCoV) pneumonia. Radiology

COVID-19 associated with acute respiratory distress syndrome

Abnormal coagulation parameters are associated with poor prognosis in patients with novel coronavirus pneumonia

Pathogenesis of COVID-19 from a cell biology perspective

Targeting potential drivers of COVID-19: Neutrophil extracellular traps

Mechanisms of hypoxemia

Frequency and Distribution of Chest Radiographic Findings in COVID-19 Positive Patients

Covid-19 med stille hypoksemi

Effects of acute hypoxia on heart rate variability, sample entropy and cardiorespiratory phase synchronization

Cytokines and cytokine inducers stimulate prostaglandin E2 entry into the brain

Neutrophil extracellular traps in COVID-19

Comprehensive mapping of immune perturbations associated with severe COVID-19

COVID-19: immunopathology and its implications for therapy

Coronaviruses and the cardiovascular system: acute and long-term implications

COVID-19 and the cardiovascular system

Cold agglutinin autoimmune haemolytic anaemia associated with novel coronavirus (COVID-19)

COVID-19 infection associated with autoimmune hemolytic anemia

Incidence of thrombotic complications in critically ill ICU patients with COVID-19

Risk factors for severity and mortality in adult COVID-19 inpatients in Wuhan

The emerging spectrum of COVID-19 neurology: clinical, radiological and laboratory findings

SARS-CoV-2 can induce brain and spine demyelinating lesions

Generalized myoclonus in COVID-19

Neurologic features in severe SARS-COV-2 infection

Psychiatric and neuropsychiatric presentations associated with severe coronavirus infections: a systematic review and meta-analysis with comparison to the COVID-19 pandemic

COVID-19: consider cytokine storm syndromes and immunosuppression

Guillain-Barré Syndrome Associated with SARS-CoV-2

Brain MRI Findings in Severe COVID-19: A Retrospective Observational Study

Early evidence of pronounced brain involvement in fatal COVID-19 outcomes

Neuropathology of COVID-19: a spectrum of vascular and acute disseminated encephalomyelitis (ADEM)-like pathology

A rapid fatal evolution of Coronavirus Disease-19 (COVID-19) in an advanced lung cancer patient with a long time response to nivolumab

Clinical characteristics of 113 deceased patients with coronavirus disease 2019: Retrospective study

Concomitant acute aortic thrombosis and pulmonary embolism complicating COVID-19 pneumonia

COVID-19 and the liver: little cause for concern

Anosmia and Ageusia: Common Findings in COVID-19 Patients

Unilateral conjunctivitis as first presentation of Coronavirus Disease 2019 (COVID-19): A telemedicine diagnosis

Characteristics of Ocular Findings of Patients with Coronavirus Disease

Ophthalmologic evidence against the interpersonal transmission of 2019 novel coronavirus through conjunctiva

The infection evidence of SARS-COV-2 in ocular surface： a single-center cross-sectional study

Ocular manifestations of a hospitalised patient with confirmed 2019 novel coronavirus disease

SARS-CoV-2 Isolation From Ocular Secretions of a Patient With COVID-19 in Italy With Prolonged Viral RNA Detection

Kawasaki disease: Insights into pathogenesis and approaches to treatment

Kawasaki disease: epidemiology and the lessons from it

Epidemiologic features of Kawasaki disease in Japan: Results of the 2009-2010 nationwide survey

Association between a novel human coronavirus and kawasaki disease

Possible involvement of infection with human coronavirus 229E, but not NL63, in Kawasaki disease

SARS-CoV-2-related paediatric inflammatory multisystem syndrome, an epidemiological study

Why does COVID-19 disproportionately affect older people?

Aging immunity may exacerbate COVID-19

Inflammaging and 'Garbaging

Blocking elevated p38 MAPK restores efferocytosis and inflammatory resolution in the elderly

From discoveries in ageing research to therapeutics for healthy ageing

Impaired Antibody Response to Influenza Vaccine in HIV-Infected and Uninfected Aging Women Is Associated with Immune Activation and Inflammation

TORC1 inhibition enhances immune function and reduces infections in the elderly

Enhancement of cutaneous immunity during aging by blocking p38 mitogen-activated protein (MAP) kinaseinduced inflammation

A new coronavirus associated with human respiratory disease in China

Dysregulation of immune response in patients with COVID-19 in Wuhan, China

Immune response in COVID-19: addressing a pharmacological challenge by targeting pathways triggered by SARS-CoV-2

Immunopathogenesis of coronavirus infections: Implications for SARS

Distribution of ACE2, CD147, CD26 and other SARS-CoV-2 associated molecules in tissues and immune cells in health and in asthma, COPD, obesity, hypertension, and COVID-19 risk factors

Meplazumab treats COVID-19 pneumonia: an open-labelled, concurrent controlled add-on clinical trial

No evidence for basigin/CD147 as a direct SARS-CoV-2 spike binding receptor

T-cell exhaustion in the tumor microenvironment

Fighting COVID-19 exhausts T cells

Cause analysis and treatment strategies of "recurrence" with novel coronavirus pneumonia (covid-19) patients after discharge from hospital

Neutrophil-to-lymphocyte ratio predicts critical illness patients with 2019 coronavirus disease in the early stage

Immune dysfunction leads to mortality and organ injury in patients with COVID-19 in China: insights from ERS-COVID-19 study

SARS-CoV-2-specific T cell immunity in cases of COVID-19 and SARS, and uninfected controls

SARS-CoV-2-reactive T cells in patients and healthy donors

SARS-CoV-2-reactive T cells in healthy donors and patients with COVID-19

Linkage Camden COVID-19 Sub-study

Immune cell profiling of COVID-19 patients in the recovery stage by single-cell sequencing

An exploratory randomized, controlled study on the efficacy and safety of lopinavir/ritonavir or arbidol treating adult patients hospitalized with mild/moderate

SARS-CoV-2 infection protects against rechallenge in rhesus macaques

Primary exposure to SARS-CoV-2 protects against reinfection in rhesus macaques

Convergent antibody responses to SARS-CoV-2 in convalescent individuals

Transmission of MERS-coronavirus in household contacts

Anti-spike IgG causes severe acute lung injury by skewing macrophage responses during acute SARS-CoV infection

Feasibility of using convalescent plasma immunotherapy for MERS-CoV infection, Saudi Arabia

Transcriptional landscape of SARS-CoV-2 infection dismantles pathogenic pathways activated by the virus, proposes unique sex-specific differences and predicts tailored therapeutic strategies

Systematic comparison of two animal-to-human transmitted human coronaviruses: SARS-CoV-2 and SARS-CoV

Transcriptomic characteristics of bronchoalveolar lavage fluid and peripheral blood mononuclear cells in COVID-19 patients

Immune and Metabolic Signatures of COVID-19 Revealed by Transcriptomics Data Reuse

Heightened Innate Immune Responses in the Respiratory Tract of COVID-19 Patients

COVID-19 Hyperinflammation: What about Neutrophils? mSphere

Clinical characteristics and prognosis in cancer patients with COVID-19: A single center's retrospective study

Clinical characteristics of COVID-19-infected cancer patients: a retrospective case study in three hospitals within Wuhan, China

Complement system part I -molecular mechanisms of activation and regulation

Complement activation contributes to severe acute respiratory syndrome coronavirus pathogenesis

Accentuated formation of the terminal C5b-9 complement complex in patient plasma precedes development of the adult respiratory distress syndrome

Association of complement activation and elevated plasma-C5a with adult respiratory distress syndrome

Acute Respiratory Distress Syndrome as an Organ Phenotype of Vascular Microthrombotic Disease: Based on Hemostatic Theory and Endothelial Molecular Pathogenesis

Complement-mediated injury and protection of endothelium: Lessons from atypical haemolytic uraemic syndrome

Complement-mediated endothelial cell damage in immune complex vasculitis of the skin: Ultrastructurallocalization of the membrane attack complex

Mannose-Binding Lectin in Severe Acute Respiratory Syndrome Coronavirus Infection

Highly pathogenic coronavirus N protein aggravates lung injury by MASP-2-mediated complement over-activation

The complement system in COVID-19: friend and foe?

Complement activation in patients with COVID-19: A novel therapeutic target

Complement associated microvascular injury and thrombosis in the pathogenesis of severe COVID-19 infection: a report of five cases

Association of COVID-19 inflammation with activation of the C5a-C5aR1 axis

Immune complement and coagulation dysfunction in adverse outcomes of SARS-CoV-2 infection

NETosis, complement, and coagulation: a triangular relationship

The Science Underlying COVID-19: Implications for the Cardiovascular System

Clinical characteristics of 82 death cases with

Pulmonary and Cardiac Pathology in Covid-19: The First Autopsy Series from New Orleans

Post-mortem examination of COVID19 patients reveals diffuse alveolar damage with severe capillary congestion and variegated findings of lungs and other organs suggesting vascular dysfunction

The significance of disseminated intravascular coagulation on multiple organ dysfunction during the early stage of acute respiratory distress syndrome

Role of multiple organ dysfunction in ARDS mortality

Thrombocytopenia is associated with acute respiratory distress syndrome mortality: An international study

Hemostasis based on a novel "two-path unifying theory" and classification of hemostatic disorders

Sepsis and septic shock: Endothelial molecular pathogenesis associated with vascular microthrombotic disease

Human Kidney is a Target for Novel Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Infection. medRxiv

Incidence, clinical characteristics and prognostic factor of patients with COVID-19: a systematic review and meta-analysis

Eculizumab treatment in patients with COVID-19: Preliminary results from real life ASL Napoli 2 Nord experience

The first case of COVID-19 treated with the complement C3 inhibitor AMY-101

Erythrocytes Reveal Complement Activation in Patients with COVID-19

COVID-19: Complement, Coagulation, and Collateral Damage

Endothelial cell infection and endotheliitis in COVID-19

Pathophysiology of SARS-CoV-2: targeting of endothelial cells renders a complex disease with thrombotic microangiopathy and aberrant immune response. The Mount Sinai COVID-19 autopsy experience

Renal histopathological analysis of 26 postmortem findings of patients with COVID-19 in China

COVID-19 and its implications for thrombosis and anticoagulation

Severe COVID-19 infection associated with endothelial activation

Severe COVID-19 infection and thrombotic microangiopathy: success doesn't come easily

Hypercoagulability of COVID-19 patients in Intensive Care Unit. A Report of Thromboelastography Findings and other Parameters of Hemostasis

Covid-19 Does Not Lead to a "Typical" Acute Respiratory Distress Syndrome

Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study

Maternal Mortality Among Women with COVID-19 Admitted to the Intensive Care Unit

SARS-CoV-2 in Pregnancy: Where and What Is the Evidence?

Maternal and perinatal outcomes with COVID-19: A systematic review of 108 pregnancies

D-dimer levels on admission to predict in-hospital mortality in patients with Covid-19

Confirmation of the high cumulative incidence of thrombotic complications in critically ill ICU patients with COVID-19: An updated analysis

Coronavirus disease 2019 infection among asymptomatic and symptomatic pregnant women: two weeks of confirmed presentations to an affiliated pair of New York City hospitals

Maternal and neonatal outcomes of pregnant women with COVID-19 pneumonia: a case-control study

Asymptomatic COVID-19 infection in late pregnancy indicated no vertical transmission

Cardio-respiratory physiological adaptation of pregnancy

Clinical characteristics and intrauterine vertical transmission potential of COVID-19 infection in nine pregnant women: a retrospective review of medical records

SARS-CoV-2 Infection of the Placenta. medRxiv (2020)2020.04.30

Placental Pathology in Covid-19 Positive Mothers: Preliminary Findings

Placental Pathology in COVID-19

The role of angiogenic factors in the management of preeclampsia

Coronavirus disease 2019 (COVID-19) in pregnant women: A report based on 116 cases

Activation of coagulation system in preterm labor and preterm premature rupture of membranes

Elevated protease activities in human amnion and chorion correlate with preterm premature rupture of membranes

Thrombin-Induced Decidual Colony-Stimulating Factor-2 Promotes Abruption-Related Preterm Birth by Weakening Fetal Membranes

Adjunct Immunotherapies for the Management of Severely Ill COVID-19 Patients

Therapeutic efficacy of the small molecule GS-5734 against Ebola virus in rhesus monkeys

A randomized, controlled trial of Ebola virus disease therapeutics

Broad-spectrum antiviral GS-5734 inhibits both epidemic and zoonotic coronaviruses

Remdesivir and chloroquine effectively inhibit the recently emerged novel coronavirus (2019-nCoV) in vitro

Compassionate Use of Remdesivir for Patients with Severe Covid-19

Remdesivir in adults with severe COVID-19: a randomised, double-blind, placebocontrolled, multicentre trial

Gilead announces results from phase 3 trial of investigational antiviral Remdesivir in patients with Severe COVID-19

Remdesivir for 5 or 10 Days in Patients with Severe Covid-19

Arbidol: A Broad-Spectrum Antiviral Compound that Blocks Viral Fusion

Membranotropic effects of arbidol, a broad anti-viral molecule, on phospholipid model membranes

The Synthetic Antiviral Drug Arbidol Inhibits Globally Prevalent Pathogenic Viruses

Discovering drugs to treat coronavirus disease 2019 (COVID-19)

Umifenovir treatment is not associated outcomes in patients with coronavirus disease 2019: a retrospective study

Favipiravir versus Arbidol for COVID-19: A Randomized Clinical

Fluorescence probe measurement of the intralysosomal pH in living cells and the perturbation of pH by various agents

Targeting endosomal acidification by chloroquine analogs as a promising strategy for the treatment of emerging viral diseases

Inhibition of Endosomal/Lysosomal Degradation Increases the Infectivity of Human Immunodeficiency Virus

Hydroxychloroquine, a less toxic derivative of chloroquine, is effective in inhibiting SARS-CoV-2 infection in vitro

New insights on the antiviral effects of chloroquine against coronavirus: what to expect for COVID-19?

Effects of chloroquine on viral infections: An old drug against today's diseases?

Hydroxychloroquine decreases Th17-related cytokines in systemic lupus erythematosus and rheumatoid arthritis patients

A systematic review on the efficacy and safety of chloroquine for the treatment of COVID-19

Breakthrough: Chloroquine phosphate has shown apparent efficacy in treatment of COVID-19 associated pneumonia in clinical studies

Clinical and microbiological effect of a combination of hydroxychloroquine and azithromycin in 80 COVID-19 patients with at least a six-day follow up: A pilot observational study

Hydroxychloroquine and azithromycin as a treatment of COVID-19: results of an open-label non-randomized clinical trial

No evidence of rapid antiviral clearance or clinical benefit with the combination of hydroxychloroquine and azithromycin in patients with severe COVID-19 infection

A pilot study of hydroxychloroquine in treatment of patients with common coronavirus disease-19 (COVID-19)

Efficacy of hydroxychloroquine in patients with COVID-19: results of a randomized clinical

Association of Treatment with Hydroxychloroquine or Azithromycin with In-Hospital Mortality in Patients with COVID-19 in New York State

FDA Drug Safety Communication: FDA cautions against use of hydroxychloroquine or chloroquine for COVID-19 outside of the hospital setting or a clinical trial due to risk of heart rhythm problems

Chloroquine diphosphate in two different dosages as adjunctive therapy of hospitalized patients with severe respiratory syndrome in the context of coronavirus (SARS-CoV-2) infection: Preliminary safety results of a randomized, double-blinded, phase IIb cl

Hydroxychloroquine and covid-19

The FDA-approved drug ivermectin inhibits the replication of SARS-CoV-2 in vitro

Effect of Dexamethasone in Hospitalized Patients with

First drug to reduce mortality in hospitalised patients with respiratory complications of COVID-19 found

Epigenetic gene silencing underlies C-Fiber dysfunctions in neuropathic pain

Remdesivir, lopinavir, emetine, and homoharringtonine inhibit SARS-CoV-2 replication in vitro

A trial of lopinavir-ritonavir in adults hospitalized with severe covid-19

Triple combination of interferon beta-1b, lopinavir-ritonavir, and ribavirin in the treatment of patients admitted to hospital with COVID-19: an open-label, randomised, phase 2 trial

Lack of evidence to support use of darunavir-based treatments for SARS-CoV-2

Clinical trial analysis of 2019-nCoV therapy registered in China

Le tocilizumab améliore significativement le pronostic des patients avec pneumonie COVID moyenne ou sévère | APHP

Tocilizumab in patients with severe COVID-19: a retrospective cohort study

Interleukin-1 blockade with high-dose anakinra in patients with COVID-19, acute respiratory distress syndrome, and hyperinflammation: a retrospective cohort study

Use of Anakinra to Prevent Mechanical Ventilation in Severe COVID-19: A Case Series

Emerging pharmacotherapies for COVID-19

Macrophages eat cancer cells using their own calreticulin as a guide: Roles of TLR and Btk

Bruton's Tyrosine Kinase Is Required for Apoptotic Cell Uptake via Regulating the Phosphorylation and Localization of Calreticulin

Bruton's tyrosine kinase regulates TLR7/8-induced TNF transcription via nuclear factor-κB recruitment

BTK operates a phospho-tyrosine switch to regulate NLRP3 inflammasome activity

Bruton's tyrosine kinase is essential for NLRP3 inflammasome activation and contributes to ischaemic brain injury

Human NACHT, LRR, and PYD domain-containing protein 3 (NLRP3) inflammasome activity is regulated by and potentially targetable through Bruton tyrosine kinase

Inhibition of Bruton tyrosine kinase in patients with severe COVID-19

BTK/ITK dual inhibitors: Modulating immunopathology and lymphopenia for COVID-19 therapy

The BTK inhibitor ibrutinib may protect against pulmonary injury in COVID-19-infected patients

BTK Inhibitors in Cancer Patients with COVID-19: "The Winner Will be the One Who Controls That Chaos

Treating Leukemia in the Time of COVID-19

Protective role of Bruton tyrosine kinase inhibitors in patients with chronic lymphocytic leukaemia and COVID-19

IVIG-mediated effector functions in autoimmune and inflammatory diseases

Clinical Efficacy of Intravenous Immunoglobulin Therapy in Critical Patients with COVID-19: A Multicenter Retrospective Cohort Study

High-dose intravenous immunoglobulin as a therapeutic option for deteriorating patients with coronavirus disease 2019

Effect of regular intravenous immunoglobulin therapy on prognosis of severe pneumonia in patients with COVID-19

Successful treatment with plasma exchange followed by intravenous immunoglobulin in a critically ill patient with COVID-19

Recovery of severely ill COVID-19 patients by intravenous immunoglobulin (IVIG) treatment: A case series

Treatment of 5 Critically Ill Patients with COVID-19 with Convalescent Plasma

Effectiveness of convalescent plasma therapy in severe COVID-19 patients

Treatment with convalescent plasma for COVID-19 patients in Wuhan

Use of convalescent plasma therapy in two covid-19 patients with acute respiratory distress syndrome in Korea

Treatment With Convalescent Plasma for Critically Ill Patients With Severe Acute Respiratory Syndrome Coronavirus 2 Infection

Convalescent plasma transfusion for the treatment of COVID-19: Systematic review

The resurgence of convalescent plasma therapy

Early safety indicators of COVID-19 convalescent plasma in 5,000 patients

Mandated Bacillus Calmette-Guérin (BCG) vaccination predicts flattened curves for the spread of COVID-19

World Health Organization. Draft landscape of COVID-19 candidate vaccines -15

Phase I, Open-Label, Dose-Ranging Study of the Safety and Immunogenicity of 2019-nCoV Vaccine (mRNA-1273) in Healthy Adults

SARS-CoV-2 mRNA Vaccine Development Enabled by Prototype Pathogen Preparedness

A Close Look at the Frontrunning Coronavirus Vaccines As of May 1 (updated)

Chinese Clinical Trial Registry. A randomized, double-blinded, placebo-controlled phase II clinical trial for Recombinant Novel Coronavirus (2019-nCOV) Vaccine (Adenovirus Vector)

Safety, tolerability, and immunogenicity of a recombinant adenovirus type-5 vectored COVID-19 vaccine: a dose-escalation, open-label, non-randomised, first-in-human trial

ChAdOx1 nCoV-19 vaccination prevents SARS-CoV-2 pneumonia in rhesus macaques

A Study of a Candidate COVID-19 Vaccine (COV001) -Full Text View -ClinicalTrials.gov

Safety and immunogenicity of the ChAdOx1 nCoV-19 vaccine against SARS-CoV-2: a preliminary report of a phase 1/2, single-blind, randomised controlled trial

Development of an inactivated vaccine candidate for SARS-CoV-2

DNA vaccine protection against SARS-CoV-2 in rhesus macaques

Tolerability and Immunogenicity of INO-4800 for COVID-19 in Healthy Volunteers

Broad and strong memory CD4 + and CD8 + T cells induced by SARS-CoV-2 in UK convalescent COVID-19 patients

Identification of SARS-CoV-2 Vaccine Epitopes Predicted to Induce Long-Term Population-Scale Immunity