key: cord-0701633-63ynchqb authors: Salian, Vrishali S.; Wright, Jessica A.; Vedell, Peter T.; Nair, Sanjana; Li, Chenxu; Kandimalla, Mahathi; Tang, Xiaojia; Carmona Porquera, Eva M.; Kalari, Krishna R.; Kandimalla, Karunya K. title: COVID-19 Transmission, Current Treatment, and Future Therapeutic Strategies date: 2021-01-19 journal: Mol Pharm DOI: 10.1021/acs.molpharmaceut.0c00608 sha: 78171b6cbedb4e8be2689c89a62b152c3659c5c8 doc_id: 701633 cord_uid: 63ynchqb [Image: see text] At the stroke of the New Year 2020, COVID-19, a zoonotic disease that would turn into a global pandemic, was identified in the Chinese city of Wuhan. Although unique in its transmission and virulence, COVID-19 is similar to zoonotic diseases, including other SARS variants (e.g., SARS-CoV) and MERS, in exhibiting severe flu-like symptoms and acute respiratory distress. Even at the molecular level, many parallels have been identified between SARS and COVID-19 so much so that the COVID-19 virus has been named SARS-CoV-2. These similarities have provided several opportunities to treat COVID-19 patients using clinical approaches that were proven to be effective against SARS. Importantly, the identification of similarities in how SARS-CoV and SARS-CoV-2 access the host, replicate, and trigger life-threatening pathological conditions have revealed opportunities to repurpose drugs that were proven to be effective against SARS. In this article, we first provided an overview of COVID-19 etiology vis-à-vis other zoonotic diseases, particularly SARS and MERS. Then, we summarized the characteristics of droplets/aerosols emitted by COVID-19 patients and how they aid in the transmission of the virus among people. Moreover, we discussed the molecular mechanisms that enable SARS-CoV-2 to access the host and become more contagious than other betacoronaviruses such as SARS-CoV. Further, we outlined various approaches that are currently being employed to diagnose and symptomatically treat COVID-19 in the clinic. Finally, we reviewed various approaches and technologies employed to develop vaccines against COVID-19 and summarized the attempts to repurpose various classes of drugs and novel therapeutic approaches. In this article, we reviewed the current state of knowledge on the transmission of SARS-CoV-2 virus from patient to host, assessed mathematical models employed to evaluate the risk of viral aerosol/droplet traγnsmission, and discussed potential routes of SARS-CoV-2 viral entry into the human host and the underlying cellular mechanisms. In addition, we outlined the clinical manifestations of COVID-19 and commented on the capabilities of existing diagnostic methods to detect the virus in humans. Further, we discussed novel therapeutic strategies to curb the virus, specifically focusing on the current efforts employed for developing an effective vaccine and drug repurposing strategies to combat the virus. As published research on COVID-19 is extensive and continues to grow in volume, a complete review of the existing knowledge is not practically feasible. Hence, we direct the readers' attention to excellent articles that provided informative reviews on various topics that are not adequately covered in this review. Those include the following: i. Effective methods for prevention of person-to-person transmission of SARS-CoV-2. 1 ii. Clinical features of COVID-19 in symptomatic 2−4 and asymptomatic patients. 5 iii. Review on the estimates of incubation 6 and infectious 7 periods of COVID-19. iv. Human immune responses to SARS-CoV-2. 8 v. Association between COVID-19 mortality and preexisting comorbidities. 9−12 vi. Various types of vaccines in clinical development. 13 vii. Human immune response against COVID-19 vaccines. 8 viii. Monoclonal antibody therapy for SARS-CoV-2 versus SARS-CoV and MERS-CoV. 14 ix. Effect of experimental treatments, which are currently in clinical trials, on mortality, and length of hospital stay. 15 ■ COVID-19 OVERVIEW Pandemics: A Historical Perspective. Disease outbreaks appear as a sudden spike of illness in a particular area or community. When the outbreak is not contained, it spreads over a large population and affects an entire region or community of people, causing an epidemic. As infected people and/or objects contaminated with infectious material spread across the globe, an epidemic turns into a pandemic. 16 Through the 16th and 19th centuries, pandemics such as smallpox, plagues, and cholera destroyed many cities throughout Europe and Asia. The 20th century witnessed the spread of the Spanish Flu (1918−1919), a pandemic caused by the H1N1 strain of influenza that is most likely spread by soldiers returning home from World War I. 17, 18 Influenza recurred in the human population with a variety of mutations causing pandemics such as Asian flu (1957−1958) and swine flu in 2009 that together killed more than a million people. 19, 20 Some of the influenza strains persisted with humanity, causing seasonal influenza, leading to thousands of deaths every year. Concerted efforts of the scientific community to tackle such pandemics have eventually led to research advances, which ultimately helped to develop vaccines for combating the seasonal flu. Zoonotic Origins of Pandemics. Most pandemics encountered by humanity in recent times are zoonoses that are generally transmitted to humans via direct contact with animal body fluids or via vectors that carry zoonotic pathogens. 21 For example, the HIV/AIDS pandemic is believed to have originated in chimpanzees. 22 Another example is Ebola (2014−2016) that spread from bats to humans. 23 Influenza often originates in avian or swine hosts before being transmitted to humans. Severe acute respiratory syndrome (SARS), in 2002−2003, and Middle-East respiratory syndrome (MERS), which has continued to spread in the Arabian peninsula since 2012, were believed to have been transmitted to humans via palm civet cats 24 and dromedary camels, 25 respectively. Zoonoses amplify in the bodies of animals (animal reservoirs) without being fatal to the host. Bats, for example, serve as perfect reservoirs as they have adequate interferons to protect themselves from the actual disease while still amplifying the pathogen load. As zoonoses migrate across species, the pathogens carve out robust evolutionary routes and transform themselves into the most virulent and contagious strains. 21 Origins of Coronavirus Disease-19 . The current pandemic, COVID-19, is believed to have emerged from an animal host. The COVID-19 is caused by the SARS-CoV-2 virus, whose genome shares 96% similarity with betacoronavirus isolated from a bat in 2013 (RaTG13). The sequence of the receptor-binding motif (RBM) of SARS-CoV-2, which is critical for host infection, also shares a highsequence similarity with the betacoronavirus isolated from a Malayan pangolin. 24 In fact, the percentage of bases identical to the SARS-CoV-2 RBM sequence is higher for pangolin-CoV (75/76 = 98.7%) than for RaTG13 (59/76 = 77.6%). Wong et al. (2020) observed that 5 of the amino acids shared uniquely between SARS-CoV-2 and pangolin-CoV occur at the key sites engaged in host binding. Therefore, it has been speculated that SARS-CoV-2 originated in bats and went through multiple recombination events as it migrated through other mammals. 24 The Similarity and Differences among SARS-CoV-2 and Other Coronaviruses. Coronaviruses contain a positivesense single-stranded RNA (+ssRNA) enclosed in a capsid with spikes, which resemble solar corona. Relative to other positive RNA viruses, coronaviruses have a large genome and possess sophisticated machinery to overtake host cells. They are known to cross species barriers, infect humans, and hijack the host cells to replicate further and spread. As of now, there are no effective means of prevention or treatments against coronaviruses, which have become a significant source of respiratory disease outbreaks. Four of the six coronaviruses that were previously known to infect humans cause common colds, upper respiratory, and intestinal illnesses. Of these, betacoronaviruses like SARS-CoV and Middle-East respiratory syndrome coronavirus (MERS-CoV) cause severe and often fatal lower respiratory tract infections. 26 Viral RNA isolated Table 1 . Similarities and Differences among Betacoronaviruses a a from COVID-19 patients in Wuhan was sequenced and a betacoronavirus with unique genomic features, including a couple of novel putative short proteins that potentiate the replication and transmission of the viral proteins was identified. 27 Multiple independent genomic sequencing studies conducted on SARS-CoV-2 viral RNA isolated from several COVID-19 patients have demonstrated a phylogenetic relationship to a bat coronavirus (bat-CoV-RATG13) and a pangolin coronavirus (pangolin-CoV) at the whole-genome level and a very close association to SARS−Co-V at the molecular level (Table 1) . 27−30 In particular, these two coronaviruses exhibited similarities in the coding region of the spike protein (S-protein), which enables the virus to bind to the cell surface receptors and facilitate its entry into the human host. 30 Zhou et al. (2020) conducted a series of in vitro experiments to show that SARS-CoV-2 infected cells that express angiotensin-converting enzyme 2 (ACE2) receptors, thus providing strong evidence that the virus enters cells by binding to the ACE2 receptor, which was also shown to mediate SARS−Co-V internalization. 31 Comparative information on these and other essential features between SARS−Co-V and SARS-CoV-2 are provided in Table 1 . Similarities and Differences between SARS and COVID-19. Similarities between SARS and COVID-19. Like the more advanced cases of COVID-19, SARS manifested as a rapidly progressing viral pneumonia. The primary mode of transmission of SARS and COVID-19 appears to be via infectious respiratory droplets dispersed from the mucous membranes (Table 2) . SARS-CoV and SARS-CoV-2 are reported to have similar stability and decay rate in aerosols and on several surfaces. 32, 33 It has been demonstrated that both can survive for up to 3 days on plastic and up to 2 days on stainless steel, with similar decay profiles of the virus titer on each surface. 32 The median incubation period, which is the time from the initial exposure until the onset of symptoms, appears to be around 4−7 days, 33 and the maximum incubation period could be up to 14 days for both SARS and COVID-19. 33, 34 Differences between SARS and COVID-19. SARS had a mortality rate of about 9%, which is 4−10 times higher than that of COVID-19. Unlike SARS-CoV-2, there were no reports of SARS-CoV transmission before symptoms appeared, and mild SARS-CoV infections were believed to be not transmittable (Table 2) . 33 The basic reproduction number, R 0 , defined as the average number of secondary infections produced by an infected person, is used to describe the transmission potential of infectious diseases. Using the World Health Organization (WHO) estimates, Petrosillo predicted that the R 0 for SARS is in the range of 1.7−1.9, whereas, for COVID-19, it was predicted to range between 2.0 and 2.5. 35 However, the Centers for Disease Control and Prevention (CDC) estimated that the R 0 for COVID-19 is much higher (5.7; 95% CI: 3.8− 8.9). 36 A larger difference in the frequency of COVID-19 cases compared to SARS cases and its ability to spread rapidly across the globe indicates that the R 0 value for COVID-19 is most likely closer to the CDC estimate (Table 2) . Transmission of SARS-CoV-2 from the Patients to the Host. Frequent sneezing and dry coughing exhibited by the COVID-19 patient generate viral plumes of thousands of droplets per cubic centimeter. Since SARS-CoV-2 infection is believed to be transmitted by aerosols and/or droplets, it is imperative to assess their particle characteristics, aerodynamic behavior, and their propensity to bypass various physiological barriers to enter the host body. 37 Characteristics of Droplets/Aerosols Emitted by COVID-19 Patients. It was initially thought that the pathogens are carried from the patient via larger droplets, which settle on the surfaces and are then carried to the host by the dust rising from the dried droplets. It has recently been identified that sneezing and dry cough suffered by COVID-19 patients generate droplet sizes ranging between 0.6 and 100 μm, and the number of droplets increases proportionately with coughing rate. 38 More than 97% of these droplets tend to be lower than 50 μm, and a majority of them are smaller than 10 μm. 39−41 Pre-or asymptomatic patients can also generate and emit large quantities of droplets, smaller than 1 μm, through normal breathing and speech. 42 The particle size distribution may shift even lower when the airborne droplets are evaporated to form droplet nuclei. The droplet nuclei formation is dependent on the ambient temperature and humidity, as well as on the particle size of the droplet. The droplets less than 10 μm have a greater potential to turn into droplet nuclei before settling. These droplets remain suspended in the cloud of air emitted by the cough or due to the ambient airflow. The droplets with a diameter less than 50 μm survive longer in the plume without any significant evaporation 43, 44 and contaminate distant Table 2 . Similarities and Differences between SARS and COVID-19 a a Purple-shaded items indicate similarity; yellow-shaded represent relative levels, and blue-shaded items are unique to COVID-19. pubs.acs.org/molecularpharmaceutics Review surfaces as well as ventilation systems. 41 Like most viruses, the average size of SARS-CoV-2 is around 0.1 μm. 37 Therefore, even 1−10 μm aerosol particles are sufficiently large to carry a viable viral particle load. 45, 46 Transmission of Airborne Viral Particles. Expulsion of air due to exhalation, sneezing, and coughing results in the release of multiphase turbulent flow, which is generally composed of hot moist air. The locally moist and warm atmosphere within the turbulent air helps the droplets escape evaporation much longer; this considerably extends the lifetime of the droplet from a fraction of a second to minutes. 47 Additionally, coughing and sneezing also generate the aerosol plumes at a high enough velocity to infect someone who is standing in proximity to the patient. Under optimal conditions of humidity and temperature, the aerosol droplets of all sizes can travel up to 7−8 m. 47, 48 Risk of Infection. A recent report has shown that SARS-CoV-2 aerosols remain viable in the air for a duration of at least 3 h with a half-life of about 1 h and is contagious to infect the human host. 32 The risk of infection when in close proximity with a COVID-19 patient was assessed by several mathematical models, of which the Wells and Riley model is the most widely used. Wells and Riley have conducted seminal research in quantifying airborne infection rates in confined spaces. 49,50 Their work has culminated in the Wells−Riley eq (eq 1), which computes the number of new cases infected (N c ) over time (t) based on the infective (I) and susceptible (S) people in a space with ventilation rate, Q, typically expressed in m 3 /S, and quantity of infectious material in the air, q where the pulmonary ventilation rate of susceptible individuals is p m 3 The Wells−Riley equation has been successfully employed to predict the measles outbreak in schools and has also been employed to evaluate the impact of airflow and ventilation on infection rates. 51 One of the limitations of this model is that it assumes a well-mixed room with uniform distribution of the aerosol particles throughout the space, which is not always possible even with a well-designed ventilation system. Therefore, the model may fall short of predicting the risk of COVID-19 infection in workspaces with several ventilation zones and in public spaces with activities, such as loud speaking and signing, that generate large droplets. Another major limitation of the Wells−Riley model is the representation of the infectious dose as the "quantum" of infection (q), which is defined as the number of infectious droplet nuclei required to infect 1−1/e (about 63.2%) susceptible people. 52 While this is a simple approach that is analogous to the quantity and virulence of infectious material in the air, it cannot fully capture the complex interaction between various physicochemical and biological factors that drive the infection. This limitation is being addressed by the development of detailed dose−response models and stochastic modeling approaches. 53−56 Cellular Mechanisms Underlying SARS-CoV-2 Entry into the Human Host. Viruses have been known to enter the host cells via receptor-mediated endocytosis, 57 which is triggered when the receptor-binding domain of the virus binds to the corresponding receptor on the host cell. Role of Angiotensin-Converting Enzyme-2 Receptor. It has been previously reported that the S-protein of SARS-CoV demonstrated an affinity for the ACE-2 receptor, 58 which served as an entry point for SARS-CoV viral RNA into the host cells. 59 Due to the high structural homology between the S- pubs.acs.org/molecularpharmaceutics Review protein of SARS-CoV and SARS-CoV-2, the ACE-2 receptor also facilitates the cellular entry of SARS-CoV-2 60 ( Figure 1 ). Studies conducted on human HeLa cells and in mice with and without ACE-2 expression have provided experimental evidence that the ACE-2 receptors are most likely involved in the cellular entry of the SARS-CoV-2 virus (Wuhan strain). 31 Similarly, the SARS-CoV-2 infection of BHK21 cells that were transfected with human and bat ACE-2 receptors was higher compared to the BHK21 cells that do not express ACE-2 receptors. 61 Biophysical and structural evidence demonstrates that the ACE-2 binding affinity of the SARS-CoV-2 S-protein ectodomain is 10−20-fold higher than that of SARS-CoV Sprotein, 62 which is claimed to be responsible for differences in the contagious nature of SARS-CoV-2 and SARS-CoV. 63 Although the ACE-2 receptor shares a considerable homology with the ACE-1 receptor, due to the smaller size of the active site on ACE-2 receptors and amino acid differences in the binding pocket, ACE-2 cannot be inhibited by the conventional ACE inhibitors like lisinopril, enalapril, and ramipril. 64 Additionally, there is no evidence that angiotensin receptor blockers (ARBs) such as losartan block ACE-2. Further discussion regarding ARBs is provided in the later sections. The Link between the SARS-CoV-2 and TMPRSS2. Uptake of SARS-CoV-2 by the host cells is not only dependent on the binding of S-protein of the virus to the ACE-2 receptor but also requires the S-protein priming by transmembrane protease serine-2 (TMPRSS2), 30, 31 which is critical for the fusion of the virus with the host cell membrane and its subsequent entry into the host cell ( Figure 1 ). Thus, the synergistic activity of the ACE-2 receptor and TMPRSS2 is needed for SARS-CoV-2 entry into the host. It was further noted that TMPRSS2 is highly expressed and widely distributed compared to the ACE2 receptors, suggesting that the ACE2 receptor might be the rate-limiting factor for SARS-CoV-2 entry during the initial stage of infection. 30 Although TMPRSS2 is a key component for the viral infection, other proteases such as cathepsin B/L could act as a substitute for TMPRSS2. Hence, it may be important to inhibit both these proteases to prevent the cellular entry of SARS-CoV-2. However, when it comes to the transmission and pathogenesis of the virus, TMPRSS2 is believed to play a more prominent role compared to the cathepsin B/L. 61 The Link between the SARS-CoV-2 and Furin. Extensive bioinformatics analyses identified the presence of a unique amino acid (PRRA) sequence between the S1 and S2 subunits in the S-protein of SARS-CoV-2. This amino acid sequence could be cleaved by furin, a type-1 membrane-bound protease, 65 expressed in various organs such as the brain, lungs, gastrointestinal (GI) tract, liver, and pancreas that are vulnerable to viral entry. The action of furin has been documented in other coronaviruses as well as in HIV, where it acts on the viral envelope protein. 66 The action of furin on the S-protein of SARS-CoV-2 could enhance its cellular entry by exposing the binding and fusion domains and enhance the viral transmissibility and pathogenesis. 65 Furin has also been detected in T cells, which circulate throughout the body. The circulating cells could form a feed-forward loop, which could facilitate the furin-dependent viral replication and contribute to the cytokine storm in some patients. 66 Therefore, furin presents an additional pathway that could be pharmacologically targeted to curb the spread of SARS-CoV-2. 67 Potential Routes of SARS-CoV-2 Entry in the Human Host. Given the significance of cell surface proteins ACE2, TMPRSS2, and furin in enabling the entry of SARS-CoV-2 into the human host, 68 we assessed the expression levels of the genes that code for these proteins in several human bulk tissue and cell types. Transcriptomic gene expression was assessed in 54 healthy tissue types from the Genotype-Tissue Expression (GTEx) project. 69 According to the GTEx data, ACE2 is highly expressed in the testis and ileum (>10 transcripts per million, or TPM) as well as in adipose tissue, kidney, heart, and thyroid (>5 TPM). Further, there are 19 GTEx tissue types, including lung, with reasonable ACE2 expression (1 TPM). On the other hand, TMPRSS2 expression is at least 1 TPM in about 18 GTEx tissue types, and the highest levels were found in the prostate, stomach, colon, pancreas, lung, small intestine, and salivary glands (>40 TPM). While furin is ubiquitously expressed in all 54 GTEx tissue types, the highest furin expression is observed in the liver, lung, thyroid, whole blood, and skin (>100 TPM) ( Figure S1 and Table S1 ). Zou et al. (2020) reported a single-cell (RNA-Seq) study measuring relative transcription levels of ACE2 in various tissues. 70 In the lung, ACE2 was most highly expressed in alveolar type 2 (AT2) epithelial cells and respiratory epithelial cells. In the heart, ACE2 was highly expressed in myocardial cells; in the digestive system, ACE2 transcription was the most prominent in ileal and esophageal epithelial cells. ACE2 transcription within the urinary system was the most prolific in proximal tubule cells of the kidney and urothelial cells of the bladder (Table S2 ). Since SARS-CoV-2 enters the host through ACE2 receptor-mediated uptake, higher expression of ACE2 in the cells that form physiological barriers protecting various organs from viral entry could make the human host susceptible to SARS-CoV-2 infection. Oral Mucosa. H. Xu et al. (2020) investigated ACE2 expression in the oral cavity using 13 normal-adjacent tissues from The Cancer Genome Atlas (TCGA) and 14 tissue types from Functional Annotation of The Mammalian Genome Cap Analysis of Gene Expression (FANTOM5 CAGE) data sets. 71 The results demonstrated that the ACE2 is expressed on the oral mucosa and is highly enriched in the tongue epithelial cells. These findings emphasize the susceptibility to the oral cavity for SARS-CoV-2 infection. Upper Respiratory Tract. The SARS-CoV-2 viral RNA was detected in the upper respiratory tract in both symptomatic and asymptomatic patients, highlighting the possible role of the nasal epithelium as a viral reservoir and in spreading the virus across the respiratory mucosa. 30 The presence of high viral titer in the nasal epithelium could be due to the high expression of ACE2 receptors in the respiratory mucosa. 30, 72 Lower Respiratory Tract. Since lungs are one of the first organs affected in COVID-19, 73, 74 it is crucial to recognize the mechanisms of viral entry into the lower respiratory tract and identify cellular targets in the lung susceptible to SARS-CoV-2 infection. The majority of SARS-CoV-2-laden droplets/ aerosols generated by the patient are in the ideal size range (1−10 μm) to gain access to the deeper quarters of the lung. Single-cell transcriptomic study of healthy lung tissue samples obtained from multiple donors have shown that the AT2 epithelial cells expressed the highest levels of ACE2 compared to other lung cell types (Table S3) . 75 Despite relatively modest expression levels of ACE2 in bulk lung tissue, its higher expression in the AT2 cells may provide SARS-CoV-2 access to the lung tissue. Moreover, AT2 cells provide platforms for Molecular Pharmaceutics pubs.acs.org/molecularpharmaceutics Review SARS-CoV-2 viral replication and also manifest increased levels of inflammatory cytokines. 76 Entry into the Central Nervous System and Brain. In a recent study, neurological symptoms were evident in 36% out of 214 COVID-19 patients tested, and 46% of those patients exhibited severe neurological deficits. 77 Several other studies have also reported the prevalence of neurological symptoms in COVID-19 patients. 78 In another large prospective study conducted among hospitalized COVID-19 patients in New York, neurologic disorders were detected in 13.5% of the patients. The occurrence of neurologic disorders was found to confer a higher risk of in-hospital mortality. 79 This clinical evidence may fuel speculation about the propensity of SARS-CoV-2 to permeate the brain barriers and trigger neurological damage. Once in the nasal cavity, the virus could infect the olfactory sensory neurons in the olfactory epithelium, the only part of the CNS exposed to the external environment, as well as the trigeminal nerve to gain access to the CNS. 80 The SARS-CoV-2 virus could also penetrate the olfactory mucosa lining the cribriform plate and traverse along the perineuronal space to access the cerebrospinal fluid (CSF) in the subarachnoid space. 81 Although the New York study reported no evidence of meningitis, encephalitis, and myelitis, which indicate SARS-CoV-2 invasion of the CNS, 79 researchers from Beijing Ditan hospital have demonstrated the presence of SARS-CoV-2 in the CSF of patients by genome sequencing. Moreover, the drainage of the virus-laden CSF to the cervical lymph nodes may lead to the activation of immune response 82 and most likely triggers SARS-CoV-2-associated encephalitis in COVID-19 patients. 83 The SARS-CoV-2 virus could also enter the brain from the systemic circulation via the blood−brain barrier. 84 Cytokine syndrome leads to the blood−brain barrier breakdown, which has been associated with the development of acute necrotizing encephalopathy in COVID-19 patients. 85 This could increase viral transmission via the paracellular Breaching GI Epithelium. The virus present in the nasal cavity is cleared into the GI tract by the mucociliary system. Moreover, large droplets/droplet nuclei that are unable to enter the deeper lungs tend to be cleared into the GI tract. Although viruses cannot survive the strongly acidic environment of the stomach, there is substantial evidence that the GI tract may be a potential transmission route and target organ of the SARS-CoV-2 virus. 86 A recent study by Lin et. al showed that 58 out of 95 patients exhibited GI symptoms. Gastroscopy examination of these patients showed SARS-CoV-2 infection in the esophagus, stomach, duodenum, and rectum. Another study conducted in 138 hospitalized COVID-19 patients has shown that a significant portion of patients (10.1%) initially presented diarrhea and nausea prior to the development of fever and dyspnea, thus suggesting the possibility of virus infection via the GI tract. 87 The expression of ACE2 and TMPRSS2 is high in absorptive enterocytes and in ileal epithelial cell subclusters, respectively. Moreover, ACE2 and TMPRSS2 were found to be highly coexpressed in colon enterocytes. 86 Since the coexpression of ACE2 and TMPRSS2 is essential for SARS-CoV-2 viral entry, the enteric symptoms observed in COVID-19 patients might be due to the invasion of SARS-CoV-2 across the gut epithelial barrier. In comparison to COVID-19 patients without diarrhea, COVID-19 patients with ceased diarrhea or with ongoing diarrhea displayed elevated fecal calprotectin concentrations, 88 which are significantly correlated with serum interleukin-6 (IL-6) concentrations. Clinical evidence of diarrhea as well as elevated levels of fecal calprotectin and serum interleukin-6 in COVID-19 patients suggest that SARS-CoV-2 most likely generates an acute intestinal inflammatory response. 89, 90 ■ COVID-19 CLINICAL MANIFESTATIONS, DIAGNOSIS, AND TREATMENT Clinical Manifestations. The clinical manifestations of COVID-19 are not specific but somewhat similar to many viral illnesses. A high-level description and visualization of the disease and its symptoms are presented in Figure 2 . After an incubation period of about 4−14 days, most individuals develop symptoms that can range from mild to very severe and even fulminant disease. 91−93 The most common manifestations are cough (46−82%), fever (77−98%), fatigue, anorexia, and myalgias (muscle pain), 77 although anosmia (loss of sense of smell) and dysgeusia (loss of sense of taste) are frequently seen and are believed to be characteristic, but not exclusive, to COVID-19. 94 Sore throat, headache, and rhinorrhea (runny nose) are also reported. Gastrointestinal symptoms such as nausea and diarrhea and accompanying abdominal pain may precede the respiratory symptoms in up to 10% of patients. 95 Asymptomatic individuals can test positive for COVID-19 (30%). However, the majority of individuals will present mild to moderate disease (55%). About 30% of patients may develop dyspnea (shortness of breath) around day 5 after the disease onset. Deterioration in the second week of illness is typical in patients with a more severe form of the disease. These patients commonly require hospitalization by day 7 or 8 28, 96 and manifest hypoxemia (low blood oxygen) as well as bilateral pneumonia (75%). 97 Elevation of the liver enzymes and creatinine are also common. Most hospitalized patients require a standard level of care, although about 20% may deteriorate quickly after the onset of dyspnea and develop severe respiratory failure. 95 Complications. Acute respiratory distress syndrome (ARDS) is one of the most severe complications of patients with COVID-19. It is associated with prolonged hospitalization and high mortality, especially if patients develop multiorgan system failure. 77 Respiratory support is crucial and ranges from high flow oxygen to providing noninvasive as well as invasive mechanical ventilation. Prone positioning has been noted to be beneficial in improving the oxygenation. 98 A subset of patients may develop an acute inflammatory state with fevers and increased expression of inflammatory markers as well as cytokines, similar to that observed in cytokine release syndrome. 95 Higher incidence of cardiovascular complications such as arrhythmias, hypoxemic cardiomyopathy, and acute cardiac injury are frequently seen (22−44%) in the intensive care unit (ICU) patients compared to non-ICU patients (2%). These could precede or develop during the multiorgan system failure of ARDS. Healthcare providers caring for these patients should be aware of these complications; 77,99 however, this is not an indication for routine telemetry monitoring or the need to screen for biomarkers for myocardial injury (troponins or BMP) unless there is evidence of myocardial ischemia or worsening heart failure. Additionally, QT should be documented on every patient as some of the drugs used in these patients may cause QT-interval prolongation. A coagulopathic stage resulting in microvascular thrombosis or disseminated intravascular coagulation has been associated with COVID-19 infection. 100−102 Interestingly, while thrombotic complications are common, bleeding is a rare COVID-19 Diagnosis. Laboratory Findings. The complete blood cell count can be normal, but the most common abnormal laboratory findings are lymphopenia (63%), leukopenia (9−25%), leukocytosis (24−30%), and thrombocytopenia (36%). Liver enzymes are elevated in about 37% of the patients. Other inflammatory markers [erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP)], Ddimer, ferritin, and IL-6, are also commonly elevated. Procalcitonin is usually normal but can be high, especially if there is a superimposed bacterial infection. 77, 95, 104 Radiographic Findings and Other Imaging Studies. It is important to be aware that up to 50% of the patients may have a normal chest X-ray (CXR), especially in the early stages of the disease. However, for those that develop pneumonia, typical CXR findings reveal bilateral peripheral patchy opacities ( Figure 3A) . A high-resolution CT of the chest (HRCT) is more sensitive, especially in the early stages. Common findings include patchy areas of ground-glass opacities, mostly peripheral and with lower lobe predominance. Areas of consolidation may be present, especially as the disease progresses ( Figure 3B ). Some features resemble those of organizing pneumonia. These findings are not specific to COVID-19 and can be seen in other viral pneumonia. Therefore, HRCT of the chest should not be used as a screening test for patients with suspected COVID-19, but rather be employed to evaluate clinical deterioration. 97, 105 Point of Care Ultrasound. It is especially helpful for those patients in the ICU setting to assess the lung (presence of Blines, consolidations with air bronchogram, pleural effusions) and heart function without the risk of having to transport critically ill patients for radiologic procedures and to avoid unnecessary exposure. 106 Diagnosis and PCR Testing. Clinical presentation, laboratory, radiological features, and exposure history (travel, positive contact, etc.) should raise the suspicion for COVID-19 infection. However, a definitive diagnosis should be made with microbiologic testing by the confirmation of the presence of SARS-CoV-2 RNA in clinical specimens. Initially, a reversetranscription polymerase chain reaction (RT-PCR)-based test was only performed by the CDC. However, similar tests are now also available at several hospitals and commercial laboratories. The sensitivity of the test is about 70−75%. Common factors that can affect the positivity of the test are the type and quality of the specimen (nasopharyngeal has better sensitivity than oropharyngeal), stage and severity of the disease (in the early stages, viral concentrations are higher in the oropharynx, while sputum and bronchoalveolar lavage tend to have higher sensitivity as the disease progresses), and the characteristics of the specific test. 107 108 Current Treatment Strategies for COVID-19. Due to the unknown efficacy of the available antiviral drugs, the standard of care, especially for those patients with mild disease, should center on the prevention of transmission. Close monitoring is important for patients that are being managed at home, and prompt escalation of care is required if deterioration occurs. 107 Data regarding the risk of increasing viral replication versus anti-inflammatory benefits of corticosteroids is inconclusive. 109 However, they can be considered in the presence of other indications such as severe COPD. 110 The use of inhalers is preferred over nebulized therapies to avoid aerosol-generating procedures that could potentially increase airborne viral spread. 111 Nonsteroidal anti-inflammatory drugs (NSAIDs) have been deemed to affect the levels of ACE2 receptors in epithelial cells and potentially increase viral infection. However, this is debatable. It is uncertain if all NSAIDs have the same potential for adverse reactions in COVID-19. NSAIDs are proposed to lead to a theoretical increased risk of ARDS via leukotriene release and, subsequently, bronchoconstriction. 112, 113 Therefore, the use of NSAIDs for symptom relief should be individualized. The European Medicines Agency (EMA) and the World Health Organization (WHO) do not recommend that NSAIDs be avoided. The use of acetaminophen is generally preferred in hospital settings due to the increased risk of bleeding and renal injury associated with NSAIDs. The use of ACE inhibitors and angiotensin receptor blockers has also been controversial. However, the American Society of Cardiology and the European Society of Cardiology currently do not recommend initiation or discontinuation of these agents. 35, 114 The decision to administer antiviral and other anti-inflammatory therapies to COVID-19 patients should be made on a case-by-case basis, if possible, in consultation with infectious disease specialists, and preferably as part of a clinical trial or registry. Patients with moderate to severe illness frequently benefit from oxygen supplementation (nasal cannula and high flow oxygen), and for those with acute respiratory failure, noninvasive and invasive mechanical ventilation is frequently needed. Positive airway pressure (PAP) should be used with the recognition that it is an aerosol-generating procedure and requires a higher level of personal protection equipment (PPE) used by healthcare providers. 35, 107 Pharmacological prevention of venous thromboembolism should be offered to all hospitalized patients unless there are specific contraindications due to the increased risk of venous thromboembolism in these patients. (Table S4) . However, the promise of vaccines is alluring as they have the potential to prevent disease transmission in a larger population. Before deploying these vaccines in a larger population, their safety and efficacy should be thoroughly established; ineffective vaccines may not only fail to protect the individual from the virus but could also cause disease through antibody-dependent enhancement or other mechanisms. 115, 116 Technologies Employed in COVID-19 Vaccine Development. To create a safe and effective vaccine against SARS- 115 Protein Subunit Vaccines. A protein subunit vaccine formulation often incorporates components of the pathogen that activate the host immune system 117 in a novel delivery vehicle such as liposome, virosomes, or polymeric nanoparticles. 118 Of these, liposomes and virosomes are being widely employed in vaccine development against SARS-CoV-2 because they not only function as delivery systems for subunit antigens but also as highly versatile adjuvants. 119 Liu et al. have developed a cationic liposome protein subunit vaccine, which contains the S1 subunit of the SARS-CoV-2 virus and two types of adjuvants, monophosphoryl lipid A (MPLA), a tolllike receptor 4 (TLR4) agonist, and CpG ODN, a toll-like receptor 9 (TLR9) agonist. 120 In addition, the incorporation of cationic ingredients like 1,2-dioleoy-l-3-trimethylammoniumpropane (DOTAP) was shown to improve the liposome's interaction with antigen-presenting cells. 121 Compared to the traditional S1 subunit vaccine with alum adjuvant, the liposome vaccine produced a stronger T cell immunity in mice by enabling both CD4+ and CD8+ cells. The liposome can also induce the production of IgA, which will provide the host with possible mucosal defense. 120 Virosomes are lipid vesicles around 150 nm in size and contain viral proteins. Virosomes are biologically degradable, nontoxic, and do not form antiphospholipid antibodies. As an adjuvant, virosomes are superior to liposomes because they can protect pharmaceutically active substances within endosomes from proteolytic degradation until they enter the cytoplasm. 122 Virosomes were previously used to deliver vaccines against SARS-CoV and MERS-CoV. 123 Building on their previous efforts to develop the SARS-CoV vaccine, Texas Children's Hospital Center for Vaccine Development at the Baylor College of Medicine is developing a subunit vaccine against SARS-COV-2 that consists of the Sprotein receptor-binding domain (RBD). Like their previous vaccine against SARS-CoV, the current vaccine formulation is most likely composed of a recombinant RBD polypeptide formulated with alum or synthetic TLR4 agonist known as glucopyranosyl lipid A (GLA). 124 The University of Queensland and Novavax have been developing an immunogenic virus-like nanoparticle vaccine using a recombinant Sprotein. 125 The Novavax vaccine was currently in phase 3 trial at the time of writing, 126 NVX-CoV2373 demonstrated mild or no reactogenicity in a majority of patients; the adjuvanted regimen induced a T helper 1 response without causing serious adverse effects. 127 In addition, Clover Biopharmaceuticals has been developing a highly purified Strimer vaccine using their patented Trimer-Tag technology, 125 which was previously employed to develop subunit vaccines against HIV, RSV, and Influenza. 128 Clover Biopharmaceuticals completed enrollment of subjects in a phase 1 trial for dose escalation on their subunit vaccine with CpG 1018 adjuvants developed by GlaxoSmithKline (GSK) and Dynavax Technologies (Table S5) . 129 , 130 Dynavax's CpG 1018 adjuvant is a TLR9 agonist, which was shown to stimulate the CD4+ and CD8+ T cells, and has a good safety profile. 131 Inactivated Virus Vaccines. Inactivated vaccine consists of attenuated viral particles or bacterial pathogens that evoke an immune response but not the infection. Since these vaccines do not provide long-lasting immunity, booster doses are often required. Inactivated viral vaccines are formulated by propagating and concentrating large quantities of viral particles and inactivating them via chemical and/or physical methods. Often, ascorbic acid, 132 binary ethylenimine, 133 gamma irradiation, 134 or relatively high-temperature treatments 135 are employed to inactivate the viral particles. Analysis confirming virus inactivation needs to be carefully performed to achieve a high degree of safety. 136 The first inactivated COVID-19 vaccine is being developed by the Wuhan Institute of Biological Products affiliated with the China National Pharmaceutical Group (Sinopharm). This vaccine is produced by propagating the virus in the Vero cell line, employing β-propiolactone as the inactivating agent, and incorporating alum as the adjuvant. In phase 1/2 clinical trials, the vaccine generated minimal adverse reactions and has been reported to produce antibodies in all participants. 137 The most common adverse events associated with the vaccine include injection site pain and fever, both of which were self-limited and mild. Its efficacy and long term adverse reactions are currently being assessed in phase 3 trials. 137, 138 Another inactivated vaccine, CoronaVac (formerly PiCoVacc), is being developed by China's Sinovac Biotech Ltd. The vaccine consists of the CN2 strain of the SARS-CoV-2 virus isolated from bronchoalveolar lavage fluid samples of hospitalized patients. A comparison of the whole genome from different passages of the viral stock suggested that the CN2 strain had optimum genetic stability. The safety and immunogenicity of CoronaVac were tested in a rhesus macaques model 139 and were further established in phase 2 clinical trials. Currently, phase 3 clinical trials are underway in 8870 participants. 140 The University of Wisconsin, Madison, in collaboration with vaccine companies FluGen and Bharat Biotech, has developed an inactivated vaccine against SARS-CoV-2 called CoroFlu for intranasal administration. 141 The vaccine is based on FluGen's influenza vaccine, M2SR, which induces the immune response against influenza. Now, FluGen has inserted SARS-CoV-2 Sprotein gene sequences into M2SR to induce an immune response against SARS-CoV-2. 141 Although vaccines are primarily administered via the invasive parenteral route, 142 a noninvasive nasal immunization has the potential to induce robust mucosal and systemic immune responses against respiratory viral infections. 143, 144 Adenovirus Vaccines. Adenoviruses have double-stranded linear DNA enveloped in an icosahedral capsid. 145 Adenoviruses activate the innate as well as adaptive immunity in mammalian hosts and trigger the release of pro-inflammatory cytokines. These inflammatory cytokines further elevate the immune response by stimulating immune cells such as cytotoxic T lymphocytes, which recognize and kill virusinfected cells. 146 Adenoviral vectors have been previously employed against diseases like influenza, Ebola, SARS, influenza, and HIV. Oxford University's, Jenner Institute, CanSino biologics, and Johnson and Johnson have been testing this vector to develop vaccines against COVID-19 (Table S5) . Cansino's vaccine, Ad5-nCoV is in phase 2 clinical trials. 147 In addition, Oxford University researchers employ a chimpanzee adenoviral vaccine vector, AZD1222, which is a nonreplicating virus. The genetic sequence encoding the S-protein of SARS-CoV-2 is encapsulated in the AZD1222 construct. 148 A single dose of this vaccine has been claimed to generate a strong immune response without causing infection in the vaccinated Molecular Pharmaceutics pubs.acs.org/molecularpharmaceutics Review patient. Hence, this vaccine is believed to be safer for children, the elderly, and individuals with pre-existing conditions such as diabetes. AstraZeneca and the University of Oxford have been collaborating in developing this vaccine; phase 1 and 2 trials demonstrated an acceptable safety profile and confirmed neutralizing antibody response against SARS-CoV-2. 148 Further, a recombinant novel coronavirus vaccine, which incorporates a replication-defective adenovirus type-5 as the vector to express the SARS-CoV-2 S-protein, has been developed in China by CanSino Biologics and Institute of Biotechnology of the Academy of Military Medical Sciences (Table S5) ; a randomized, double-blind, placebo-controlled, phase 2 trial is ongoing since April 2020. This vaccine has demonstrated a significant neutralizing antibody response against the SARS-CoV-2 spike protein in healthy adults of age 18 years or older. 149 Although adenoviral vectors are being widely considered to deliver COVID-19 vaccines, they are currently in the experimental phases, and no vaccine using this platform has been approved for human use against infectious diseases. In addition to the serious risk of inflammatory reactions posed by adenoviral vectors, like those observed in AstraZeneca trials, immunity against adenoviral vectors is also possible since humans are commonly exposed to adenoviruses. Gene Vaccines. Gene vaccines involve direct administration of a DNA plasmid, which codes for the particular target antigen. This type of vaccine has many potential advantages over conventional vaccines in terms of stimulating both B and T cell responses and has a better safety profile. Since they are devoid of any infectious agents, they could be administered to immunocompromised patients. 150 The DNA plasmid incorporated in the vaccine can be accurately designed using the knowledge of the viral genome. Synthetic DNA vaccines accelerate the developmental process of a vaccine as they allow for the quick design of multiple vaccine candidates for preclinical testing, facilitate scalable manufacturing of large quantities of vaccine products, and encounter less regulatory hurdles for clinical translation. Moreover, the synthetic DNA is temperature-stable and has a longer shelf life. 151 Gene-based vaccines against SARS-CoV-2 are mostly being developed against the S-protein. The vaccine is expected to trigger the expression of spike antigens in the host, which then induce antibodies capable of inhibiting S-protein recognition by the host receptors. Inovio Pharmaceuticals employs DNAplasmid pGX9501 in their vaccine candidate, which encodes for the S-protein of SARS-CoV-2. Their previous studies have shown that the immunization of animal models with DNA vaccines encoding MERS-CoV S-protein could protect the animal from the disease. Given the shared global protein fold architecture between SARS-CoV-2 and MERS-CoV S-proteins, a synthetic DNA vaccine, IN0−4800 (Inovio Pharmaceuticals), was formulated based on their prior vaccine constructs. 152 Currently, INO-4800 is in phase 2 clinical trials, where the vaccine is administered to healthy adults by an intradermal route followed by electroporation. 153 In addition to DNA, mRNA could also be used in gene vaccines. While DNA vaccines act on the nucleus, mRNA vaccines act in the cytosol; hence, they are not required to cross the nuclear membrane. Additionally, RNA vaccines are known to induce a more potent memory in the immune system and therefore require lower doses than the DNA vaccines. However, RNA vaccines are not as stable as DNA vaccines; they are heat-labile and are prone to hydrolysis by ribonucleases present in circulation. 154 To improve the stability and deliverability to the host, mRNA vaccines are formulated as lipid nanoparticles using cationic lipids and lipopolymers that can be electrostatically complexed with the negatively charged RNA. 155 Moderna (Cambridge, MA) uses this approach for its SARS-CoV-2 (Table S5) vaccine. Their lipid nanoparticle formulation consists of mRNA-1273, which encodes a stabilized prefusion spike trimer, S-2P, and is currently in phase 3 clinical trials. 156 In addition, Pfizer's nucleoside-modified mRNA (modRNA) candidate, BNT162b1 (encodes an optimized SARS-CoV-2 full-length S-protein), is also formulated as a lipid nanoparticle. The phase 1/2 studies 157 have shown that BNT162b1 induced a stronger CD8-T cell response, which could promote the generation of CD4 T cells and neutralizing antibodies, 156, 157 in comparison to Moderna's vaccine candidate. 156, 158 In phase 3 interim analysis, the mRNA-based vaccine candidate BNT162b2 demonstrated a 90% efficacy rate over the placebo, 7 days after the second dose. 159, 160 Even after the development of a safe and effective COVID-19 vaccine, the challenges encountered in manufacturing, distributing, and administering the vaccine to the vulnerable population throughout the world is fraught with many challenges. Specifically, distributing the vaccine in developing countries could be very challenging as the cold chain required for the stability and activity of the vaccine is not adequately established. 161 Repurposing Approved Drugs to Treat COVID-19 Patients. There are several investigational drugs currently in clinical trials (Table S4 ) to treat COVID-19 patients. These drugs were selected based on their putative mechanisms inhibiting viral entry into the host and subsequent viral replication and are currently being investigated or were already investigated in human clinical trials ( Figure 4 ). While some of these drugs have been used previously to treat SAR-CoV infections, a few of them are being used for the first time to target SAR-CoV-2 infection. Antiviral Drugs. The FDA approved remdesivir (Gilead Sciences, Inc.) for the treatment of COVID-19 requiring hospitalization in patients 12 years of age and older. 162 Remdesivir is an RNA-dependent RNA polymerase inhibitor, which is believed to incorporate itself into SARS-CoV-2 RNA and inhibit its further replication (Figure 4) . Remdesivir was also administered to one of the first COVID-19 patients in the United States via intravenous (IV) administration. No adverse effects were reported, and the patient's clinical condition improved even after the supplemental oxygen was discontinued. After the IV administration of 200 mg of remdesivir on the first day and 100 mg on each subsequent day for 9 days, the overall clinical improvement was observed in 36 of 53 COVID-19 patients. 163 Lopinavir and ritonavir in an antiretroviral therapy has also has also been studied in treating patients with COVID-19 but failed to show benefit over standard care. 164 Umifenovir inhibits the S-protein/ACE2 interaction and is approved in Russia and China for influenza prophylaxis. In a study conducted in Russia during the 2004 SARS outbreak, the activity of umifenovir against SARS-CoV has been established using in vitro models 165, 166 and is currently being investigated for COVID-19 treatment. 167 Favipiravir, which is approved in Japan for influenza, inhibits RNA polymerase and blocks viral replication (Figure 4 ). When compared with umifenovir, treatment with favipiravir resulted in the better clinical recovery of moderate COVID-19 infections in a prospective randomized study. 167 Antimalarial Drugs. Chloroquine and hydroxychloroquine have been approved by the Food and Drug Administration (FDA) for the treatment of malaria and inflammatory disorders such as rheumatoid arthritis. Being weak bases, chloroquine and hydroxychloroquine are believed to inhibit viral replication by increasing the endosomal vesicular pH and engendering the inactivation of proteases such as acid hydrolases, which are pH-dependent ( Figure 4) . Consequently, post-translational modifications of newly synthesized pro-teins, 168 including the formation of the S-protein envelope, which allows for SARS-CoV-2 binding to ACE2 receptors and subsequent endocytosis, 31 are inhibited. Clinical trials conducted in China demonstrated that chloroquine may reduce the progression of pneumonia and replication of the SARS-CoV-2 virus in a small group of 100 patients. 169 Despite these putative therapeutic benefits of chloroquine, hydroxychloroquine did not appear to have the same positive clinical effects. A randomized, double-blind, placebo-controlled clinical trial conducted in subjects that had high-risk or moderate-risk exposure to COVID-19 showed that hydroxychloroquine provided no significant benefits in preventing illness when used within 4 days of the exposure. 170 Hydroxychloroquine is a cautionary tale: despite promising in vitro data and its inhibition of viral entry mechanisms, patient outcome data did not necessarily show a benefit. Angiotensin Receptor Blockers and Statins. Therapeutic drug combinations such as statins and angiotensin receptor blockers (ARBs) are promising therapies for potentially averting ARDS in COVID-19 patients. Statins and ARBs could alleviate the host response to the infection through their immunomodulatory properties that reduce the release of inflammatory cytokines (Figure 4 ). 171 Pro-inflammatory cytokines, in due course, cause endothelial barrier leakage by disrupting the integrity of endothelial tight junctions and trigger pneumonia. 172 The leaky barrier allows for the accumulation of fluid from the blood in the interstitial lung tissue, presumably due to an increase in the expression of angiopoietin-2. 172 Statins and ARBs decrease the production of angiopoietin-2 and help restore the endothelial barrier integrity. 172 Although ARBs and ACE inhibitors appear to upregulate ACE2 expression and increase the potential of SARS-CoV-2 binding, 173 some argue that ACE inhibitors and ARBs may have a beneficial effect by decreasing overall inflammation. 174 Several cardiology associations strongly recommend continuing treatment with ACE inhibitors/ARB in patients who were previously taking these drugs as the benefit outweighed the risk. Khera and colleagues found that patients with both hypertension and COVID-19 treated with ACE inhibitors were 40% less likely to be hospitalized than those not treated with ACE inhibitors. Interestingly, this effect was not seen in patients treated with ARBs. 175 More information is needed to understand the clinical impact of ACE or ARBs on COVID-19 treatment. Impact of Formulation and Route of Administration on the Efficacy of Antivirals. As described earlier, pneumonia is one of the severe symptoms observed in COVID-19 patients. Administering therapeutic concentrations of remdesivir to the lungs is pivotal to avoid further spread of the virus into the lungs. Sun et al. (2020) have demonstrated that remdesivir IV administration alone cannot achieve clinical efficacy due to low lung distribution and poor cellular permeability of remdesivir as well as its active nucleoside triphosphate metabolite. 176 However, a combination drug delivery approach involving IV administration along with pulmonary nebulization of remdesivir was deemed effective in achieving therapeutic drug concentrations in the lungs to reduce SARS-CoV-2 viral replication. 176 The IV administration of remdesivir is only possible in a healthcare setting, whereas patients could selfadminister remdesivir via a nebulizer. However, further studies are needed to determine the role of nebulized remdesivir in early COVID-19. Novel Therapies. Convalescent Plasma Treatment. Convalescent plasma treatment is being developed at a remarkable speed. In August 2020, the FDA authorized the use of high titer and low titer convalescent plasma for the treatment of patients hospitalized with COVID-19. 177 The convalescent patient plasma may contain antibodies that not only block viral infection but also improve the clearance of cells infected with the virus. 178, 179 Thus, convalescent plasma from patients recovered from COVID-19 might be useful to alleviate symptoms in critically ill patients. 180 When five COVID-19 patients suffering from ARDS were infused with 400 mL of convalescent plasma (with SARS-CoV-2-specific antibody binding titer greater than 1:1000 and neutralization titer greater than 40), viral load decreased within 3−4 days after the infusion; moreover, the majority of the patients did not need mechanical ventilation following 12 days of plasma infusion. 180 In this small sample study, patients have also received steroids and other antiviral agents. In a larger study of 5000 patients, convalescent plasma appeared to be safe in hospitalized patients with COVID-19. 181 Recent reports from the follow-up study conducted on 35,322 transfused COVID-19 patients have shown that the patients receiving high IgG plasma (>18.45 signal-to-cutoff ratio (S/Co)) had lower mortality (8.9%) than those receiving medium (4.62−18.45 S/Co) or low IgG plasma (<4.62 S/Co). 182 Although data support the safety and potential efficacy of convalescent plasma, rigorous randomized clinical trials are needed to determine which subset of patients and to what extent they are most likely to benefit. Immunoglobulins. Monoclonal antibody therapy could potentially be an effective clinical treatment against COVID-19. 183 It targets a single epitope, which allows for a higher specificity on a predetermined target. Monoclonal antibodies that are produced on a large scale have reduced batch variations in comparison to polyclonal antibodies. 184 Lilly has developed a neutralizing monoclonal antibody drug candidate, bamlanivimab (LY-CoV555), from the convalescent plasma of COVID-19 patients. Bamlanivimab has an activity against the SARS-CoV2 receptor-binding domain. 185 It blocks the attachment of the virus to the host cell and prevents its entry into human cells. In the phase 2 clinical trial for bamlanivimab in outpatients, a reduction in viral load was observed at day 11 when compared to the placebo group. The symptom severity and COVID-19 hospitalization rate were also reduced in patients who received bamlanivimab. 185 It currently has received emergency use authorization by the FDA for the treatment of COVID-19 in recently diagnosed patients. 186 Regeneron has developed a novel two monoclonal antibody cocktail, REGN-COV2, from humanized VI mice and blood from recovered COVID-19 patients. It is known to reduce SARS-CoV-2 viral infectivity by binding to the RBD of the spike protein at two distinct, nonoverlapping locations. This interaction hinders the binding of the virus to the host cell and is able to neutralize the virus and prevent infection. 187 Moreover, the two-antibody cocktail combination prevents mutant viral escape, which is usually seen with single antibody therapies. REGN-COV2 prevents the escape of viral mutants by simultaneously binding to two distinct regions of the virus. 188 REGN-COV2 is being evaluated in four late-stage clinical trials. Two phase 2/3 clinical trials conducted on hospitalized and nonhospitalized patients have shown a reduction of viral symptoms in nonhospitalized patients. 189, 190 Two phase 3 recovery and prevention trials of hospitalized COVID-19 patients are currently underway. 191 Antibody immunotherapy is therapeutically promising against SARS-COV2, but still, there are certain challenges. Patients who have previously died of SARS-CoV infection have exhibited a strong neutralizing antibody response in addition to pulmonary inflammation. Due to a pathological link between the neutralizing antibody response and pulmonary inflammation, it is necessary to consider the patients' adaptive immune responses when administering an antiviral immunotherapy. 192 Additionally, patients with severe COVID-19 may not be responsive to antibody therapy because they would have developed several underlying conditions such as acute inflammation and coagulopathy during the course of infection. In such cases, the patient's condition deteriorates so drastically that decreasing viral load in these patients may not be helpful. 193 Despite being promising therapeutic agents to treat COVID-19, immunoglobulins pose several challenges for large scale manufacturing and quality control. Moreover, immunoglobulins have to be given intravenously, at doses as high as 8 g for REGN-COV2, which requires hospitalization of the patient. These constraints may increase the expense and limit the widespread use of immunoglobulins for COVID-19 treatment. 194 Inflammatory Modulators. A multicenter randomized controlled trial investigating the efficacy and safety of tocilizumab in COVID-19 treatment has been conducted in Wuhan, China. Tocilizumab is a monoclonal antibody which blocks IL-6 receptors. 195 Initial studies concluded that tocilizumab was associated with reduced mortality and clinical improvement in patients with severe COVID- 19. 196 Further studies are needed to confirm the efficacy and safety of tocilizumab prior to routine use in clinical practice. Interferons are a family of proteins produced by the host cells in response to a viral infection. Interferon-β is known to increase the production of anti-inflammatory cytokines and downregulate the production of pro-inflammatory cytokines. 197 Interferon-α is known to extend the activated T cell response, increase humoral immunity, and antigen-presenting cell response. 198 Interferon-α has been used in combination with ribavirin to treat MERS-CoV. 199 Inhalation formulation of interferon-β-1a, which is expected to decrease symptoms of respiratory illness and pneumonia in COVID-19 patients, is currently in phase 2 clinical trials (Synairgen, England). Eculizumab, a monoclonal antibody that binds to complement component 5, is currently being evaluated to treat COVID-19. Preliminary evidence demonstrated that 4 patients with severe ARDS or pneumonia in the intensive care unit recovered after treatment with eculizumab. 200 Stem Cell Therapies. Expanded umbilical cord mesenchymal stem cells (UC-MSCs) have been regarded as a possible treatment for SARS-CoV-2. The MSCs have powerful immunomodulatory properties and can secrete anti-inflammatory factors. Theoretically, the accumulation of MSCs in the lung could protect alveolar epithelial cells and improve lung function. Promising preclinical and preliminary clinical data have demonstrated the feasibility of stem cell therapy to enhance the recovery of COVID-19 patients. 201 Among the types of stem cells that are available for clinical use, UC-MSCs appear to be the best candidates to treat coronavirus. 201 The UC-MSCs derived from umbilical cords have a rapid doubling time, which makes it easy to scale up in the lab. Moreover, they can be harvested noninvasively, unlike bone marrow stem cells. Chinese investigators have reported IV infusion to be the ideal route of administration for stem cells in COVID-19 patients as the stem cells are mainly confined to the lungs following IV administration. This approach may be particularly beneficial since the lungs are the most affected organs in COVID-19 patients. 201 Overall, using UC-MSCs is relatively inexpensive and presents a potential treatment option for COVID-19. 201 Despite similarities in the clinical manifestations and molecular mechanisms with other diseases caused by betacoronaviruses, COVID-19 turned out to be so contagious and deadly that it triggered commitment and collaboration among scientists across the globe to protect humanity against this juggernaut. This coordinated effort has not only been accelerating our understanding of COVID-19 pathophysiology and its clinical manifestations but also contributing to the better prognosis of hospitalized patients. Vaccine development against COVID-19 is in full swing, and several vaccine candidates are in phase 3 trials. On the other hand, high-throughput drug discovery platforms are being harnessed to repurpose existing drugs and develop formulation strategies for COVID-19 treatment. As of now, there are more than 300 clinical trials underway to test the safety and efficacy of various drug candidates in COVID-19 patients. A recent flurry of articles in various scientific journals on COVID-19 etiology, pathophysiology, clinical treatments, and drug discovery, as well as on repurposing efforts, are serving as beacons of optimism that a vaccine and/or effective treatment may soon become available for this devastating disease. The current review article is intended to summarize COVID-19 pathophysiology from the perspective of pharmacological interventions that are being investigated and provide a snapshot of this rapidly moving frontier in the fight against COVID-19. ■ ASSOCIATED CONTENT The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.molpharmaceut.0c00608. Transcriptomic gene expression of ACE2, TMPRSS2, and furin in various organs from the genotype-tissue expression (GTEx project); (PDF) Relative transcription levels of ACE2 in various tissues from a single-cell (RNA-Seq) study; ACE2, TMPRSS2, and furin expression levels in various cell types from a single-cell transcriptomic study; repurposed drugs in clinical trials for COVID-19; vaccines in clinical trials for COVID-19 (XLSX) Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and metaanalysis The prevalence of symptoms in 24,410 adults infected by the novel coronavirus (SARS-CoV-2; COVID-19): A systematic review and meta-analysis of 148 studies from 9 countries Risk Factors of Severe Disease and Efficacy of Treatment in Patients Infected with COVID-19: A Systematic Review, Meta-Analysis and Meta-Regression Analysis Correlations of Clinical and Laboratory Characteristics of COVID-19: A Systematic Review and Meta-Analysis Asymptomatic patients as a source of COVID-19 infections: A systematic review and meta-analysis Incubation period of COVID-19: a rapid systematic review and meta-analysis of observational research Inferred duration of infectious period of SARS-CoV-2: rapid scoping review and analysis of available evidence for asymptomatic and symptomatic COVID-19 cases COVID-19 and cardiovascular disease: from basic mechanisms to clinical perspectives Caring for patients with cancer in the COVID-19 era Diabetes and COVID-19: evidence, current status and unanswered research questions Association of cardiovascular disease and 10 other pre-existing comorbidities with COVID-19 mortality: A systematic review and meta-analysis Perspectives on monoclonal antibody therapy as potential therapeutic intervention for Coronavirus disease-19 (COVID-19) Drug treatments for covid-19: living systematic review and network meta-analysis Origins of the Spanish Influenza pandemic (1918−1920) and its relation to the First World War Phylogenetic analysis of HA and NA genes of influenza H1N1 viruses from 1918 to 2017 Pandemic (avian) influenza. Seminars in respiratory and critical care medicine Treatment and Prevention of Pandemic H1N1 Influenza Wildlife as source of zoonotic infections The Pathogenesis of Ebola Virus Disease Evolution of the novel coronavirus from the ongoing Wuhan outbreak and modeling of its spike protein for risk of human transmission Host Factors in Coronavirus Replication Genomic characterization of the 2019 novel human-pathogenic coronavirus isolated from a patient with atypical pneumonia after visiting Wuhan A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster Genome Composition and Divergence of the Novel Coronavirus (2019-nCoV) Originating in China SARS-CoV-2 Entry Genes Are Most Highly Expressed in Nasal Goblet and Ciliated Cells within Human Airways A pneumonia outbreak associated with a new coronavirus of probable bat origin The severe acute respiratory syndrome Estimated effectiveness of symptom and risk screening to prevent the spread of COVID-19 High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2 Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy The size and concentration of droplets generated by coughing in human subjects The numbers and the sites of origin of the droplets expelled during expiratory activities Cough aerosol in healthy participants: fundamental knowledge to optimize droplet-spread infectious respiratory disease management Violent expiratory events: on coughing and sneezing The coronavirus pandemic and aerosols: Does COVID-19 transmit via expiratory particles? Characterizations of particle size distribution of the droplets exhaled by sneeze Modality of human expired aerosol size distributions Deposition of particles in the human respiratory tract in the size range 0.005−15 μm The coronavirus pandemic and aerosols: Does COVID-19 transmit via expiratory particles? Turbulent Gas Clouds and Respiratory Pathogen Emissions: Potential Implications for Reducing Transmission of COVID-19 Airborne Contagion and Air Hygiene Airborne spread of measles in a suburban elementary school Mathematical models for assessing the role of airflow on the risk of airborne infection in hospital wards On the mechanics of droplet nuclei infection; quantitative experimental air-borne tuberculosis in rabbits Viral kinetics and exhaled droplet size affect indoor transmission dynamics of influenza infection Time-dose-response models for microbial risk assessment The effect of ongoing exposure dynamics in dose response relationships Review and comparison between the Wells-Riley and dose-response approaches to risk assessment of infectious respiratory diseases The cell biology of receptor-mediated virus entry Interaction of severe acute respiratory syndrome-coronavirus and NL63 coronavirus spike proteins with angiotensin converting enzyme-2 Angiotensinconverting enzyme 2 is a functional receptor for the SARS coronavirus Structure, Function, and Antigenicity of the SARS-CoV-2 Spike Glycoprotein SARS-CoV-2 Cell Entry Depends on ACE2 and TMPRSS2 and Is Blocked by a Clinically Proven Protease Inhibitor COVID-2019: The role of the nsp2 and nsp3 in its pathogenesis Angiotensin-Converting Enzyme-2 (ACE2): Comparative Modeling of the Active Site, Specificity Requirements, and Chloride Dependence The spike glycoprotein of the new coronavirus 2019-nCoV contains a furin-like cleavage site absent in CoV of the same clade Covid-19 infection and mortality: a physiologist's perspective enlightening clinical features and plausible interventional strategies Furin Inhibitors Block SARS-CoV-2 Spike Protein Cleavage to Suppress Virus Production and Cytopathic Effects Cell entry mechanisms of SARS-CoV-2 The Genotype-Tissue Expression (GTEx) project Single-cell RNA-seq data analysis on the receptor ACE2 expression reveals the potential risk of different human organs vulnerable to 2019-nCoV infection SARS-CoV-2 entry factors are highly expressed in nasal epithelial cells together with innate immune genes Pulmonary fibrosis secondary to COVID-19: a call to arms? Reverse Genetics Reveals a Variable Infection Gradient in the Respiratory Tract scRNA-seq assessment of the human lung, spleen, and esophagus tissue stability after cold preservation SARS-CoV-2 induces transcriptional signatures in human lung epithelial cells that promote lung fibrosis Neurotropism of SARS-CoV 2: Mechanisms and manifestations N Neurological manifestations of patients with COVID-19: potential routes of SARS-CoV-2 neuroinvasion from the periphery to the brain Tailoring Formulations for Intranasal Nose-to-Brain Delivery: A Review on Architecture, Physico-Chemical Characteristics and Mucociliary Clearance of the Nasal Olfactory Mucosa The immunogenicity of intracerebral virus infection depends on anatomical site Encephalitis as a clinical manifestation of COVID-19 Evidence of the COVID-19 Virus Targeting the CNS: Tissue Distribution, Host-Virus Interaction, and Proposed Neurotropic Mechanisms 19-associated Acute Hemorrhagic Necrotizing Encephalopathy: CT and MRI Features H Digestive system is a potential route of COVID-19: an analysis of single-cell coexpression pattern of key proteins in viral entry process Hospitalized Patients With 2019 Novel Coronavirus-Infected Pneumonia in Wuhan, China Comparison of different histological indexes in the assessment of UC activity and their accuracy regarding endoscopic outcomes and faecal calprotectin levels Faecal calprotectin indicates intestinal inflammation in COVID-19 Gastrointestinal symptoms of 95 cases with SARS-CoV-2 infection Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus-Infected Pneumonia Clinical Characteristics of Coronavirus Disease 2019 in China Q The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application Self-reported Olfactory and Taste Disorders in Patients With Severe Acute Respiratory Coronavirus 2 Infection: A Cross-sectional Study Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China Characteristics of and Important Lessons From the Coronavirus Disease 2019 (COVID-19) Outbreak in China: Summary of a Report of 72 314 Cases From the Chinese Center for Disease Control and Prevention Radiological findings from 81 patients with COVID-19 pneumonia in Wuhan, China: a descriptive study Prone positioning in nonintubated patients with COVID-19: raising the bar Risk Factors Associated With Acute Respiratory Distress Syndrome and Death in Patients With Coronavirus Disease Ntanasis-Stathopoulos, I; Elalamy, I Hematological findings and complications of COVID-19 venous thromboembolism in patients with severe novel coronavirus pneumonia R Prominent changes in blood coagulation of patients with SARS-CoV-2 infection Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study Relation Between Chest CT Findings and Clinical Conditions of Coronavirus Disease (COVID-19) Pneumonia: A Multicenter Study Findings of lung ultrasonography of novel corona virus pneumonia during the 2019− 2020 epidemic Interim Guidelines for Collecting, Handling, and Testing Clinical Specimens for COVID-19 Chest CT for Typical Coronavirus Disease 2019 (COVID-19) Pneumonia: Relationship to Negative RT-PCR Testing Role of corticosteroid in the management of COVID-19: A systemic review and a Clinician's perspective U Management of patients with COPD during the COVID-19 pandemic Airborne transmission of severe acute respiratory syndrome coronavirus-2 to healthcare workers: a narrative review NSAIDs may increase the risk of thrombosis and acute renal failure in patients with COVID-19 infection Corynebacterium equi infection complicating neoplastic disease SARS-CoV2: should inhibitors of the renin-angiotensin system be withdrawn in patients with COVID-19? Design of a peptide-based subunit vaccine against novel coronavirus SARS-CoV Rapid COVID-19 vaccine development Recent Advances in Subunit Vaccine Carriers. Vaccines (Basel, Switz Peptide Vaccine: Progress and Challenges. Vaccines (Basel, Switz Based Adjuvants for Subunit Vaccines: Formulation Strategies for Subunit Antigens and Immunostimulators Nanovaccine for COVID-19 Cationic liposomes as potential carriers for ocular administration of peptides with antiherpetic activity Nanoparticles in influenza subunit vaccine development: Immunogenicity enhancement. Influenza Other Respir Potential adjuvants for the development of a SARS-CoV-2 vaccine based on experimental results from similar coronaviruses Roadmap to developing a recombinant coronavirus S protein receptor-binding domain vaccine for severe acute respiratory syndrome The SARS-CoV-2 Vaccine Pipeline: an Overview. Curr Novavax initiates phase 3 efficacy trial of covid-19 vaccine in the united kingdom XXX, XXX−XXX releases/news-release-details/novavax-initiates-phase-3-efficacy-trialcovid-19-vaccine-united Phase 1−2 Trial of a SARS-CoV-2 Recombinant Spike Protein Nanoparticle Vaccine Clover Initiates Development of Recombinant Subunit-Trimer Vaccine for Wuhan Coronavirus (2019-nCoV) La Merie Business Intelligence. Pipelinereviews.com Clover Biopharmaceuticals starts Phase I Covid-19 vaccine trial. Verdict Media Limited Clover biopharmaceuticals announces positive preclinical data and updates on phase 1 study for its adjuvanted strimer covid-19 vaccine candidate CpG 1018 Dynavax's proprietary toll-like receptor 9 (TLR9) agonist adjuvant. Dynavax Technologies In vitro inactivation of the rabies virus by ascorbic acid Rabies virus inactivation by binary ethylenimine: new method for inactivated vaccine production The gamma-irradiated influenza vaccine and the prospect of producing safe vaccines in general Polyomavirus inactivation -a review Inactivated Viral Vaccines Y Effect of an Inactivated Vaccine Against SARS-CoV-2 on Safety and Immunogenicity Outcomes: Interim Analysis of 2 Randomized Clinical Trials Draft Landscape of COVID-19 candidate vaccines Development of an inactivated vaccine candidate for SARS-CoV-2 Clinical Trial of Efficacy and Safety of Sinovac's Adsorbed COVID-19 (Inactivated) Vaccine in Healthcare Professionals (PROFIS-COV) Bharat Biotech to develop CoroFlu, a coronavirus vaccine Route of Vaccine Administration Alters Antigen Trafficking but Not Innate or Adaptive Immunity Strategies for intranasal delivery of vaccines The establishment of resident memory B cells in the lung requires local antigen encounter Adenovirus vectors for gene therapy, vaccination and cancer gene therapy. Curr Adenoviral vector-based strategies against infectious disease and cancer. Hum. Vaccines Immunother Overview of the current promising approaches for the development of an effective severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine 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 Immunogenicity and safety of a recombinant adenovirus type-5-vectored COVID-19 vaccine in healthy adults aged 18 years or older: a randomised, double-blind, placebo-controlled, phase 2 trial DNA vaccines: roles against diseases Intradermal SynCon® Ebola GP DNA Vaccine Is Temperature Stable and Safely Demonstrates Cellular and Humoral Immunogenicity Advantages in Healthy Volunteers Tolerability and Immunogenicity of INO-4800 Followed by Electroporation in Healthy Volunteers for COVID19 Human extracellular ribonucleases: multiplicity, molecular diversity and catalytic properties of the major RNase types Jr Lipid-based nanoparticles for nucleic acid delivery An mRNA Vaccine against SARS-CoV-2 -Preliminary Report Phase 1/2 study of COVID-19 RNA vaccine BNT162b1 in adults Concurrent human antibody and T H 1 type T-cell responses elicited by a COVID-19 RNA vaccine Pfizer and biontech announce vaccine candidate against covid-19 achieved success in first interim analysis from phase 3 study A phase 1/2/3, placebo-controlled, randomized, observer-blind, dose-finding study to evaluate the safety, tolerability, immunogenicity, and efficacy of sars-cov-2 rna vaccine candidates against covid-19 in healthy individuals 3rd The COVID-19 Vaccine Race: Challenges and Opportunities in Vaccine Formulation Food & Drug Administration. FDA approves first treatment for covid-19 Use of Remdesivir for Patients with Severe Covid-19 A Trial of Lopinavir-Ritonavir in Adults Hospitalized with Severe Covid-19 Arbidol as a broadspectrum antiviral: an update Discovering drugs to treat coronavirus disease 2019 (COVID-19). Drug Discoveries Ther Pharmacologic Treatments for Coronavirus Disease 2019 (COVID-19): A Review Effects of chloroquine on viral infections: an old drug against today's diseases Breakthrough: Chloroquine phosphate has shown apparent efficacy in treatment of COVID-19 associated pneumonia in clinical studies Do outpatient statins and ACEIs/ARBs have synergistic effects in reducing the risk of pneumonia? A population-based case-control study Treating the host response to emerging virus diseases: lessons learned from sepsis, pneumonia, influenza and Ebola Interactions of coronaviruses with ACE2, angiotensin II, and RAS inhibitorslessons from available evidence and insights into COVID-19 Outcomes in Patients with COVID-19 Infection Taking ACEI/ARB Good or bad: Application of RAAS inhibitors in COVID-19 patients with cardiovascular comorbidities Remdesivir with IV Administration Alone is Unlikely to Achieve Adequate Efficacy and Pulmonary Delivery should be Investigated in COVID-19 Patients Plasma therapy against infectious pathogens, as of yesterday, today and tomorrow Convalescent plasma as a potential therapy for COVID-19 Treatment of 5 Critically Ill Patients With COVID-19 With Convalescent Plasma Early safety indicators of COVID-19 convalescent plasma in 5000 patients Effect of Convalescent Plasma on Mortality among Hospitalized Patients with COVID-19: Initial Three-Month Experience Monoclonal antibody as a potential anti-COVID-19 Monoclonal Antibodies for Emerging Infectious Diseases -Borrowing from History SARS-CoV-2 Neutralizing Antibody LY-CoV555 in Outpatients with Covid-19 Lilly's neutralizing antibody bamlanivimab (LY-CoV555) receives FDA emergency use authorization for the treatment of recently diagnosed COVID-19 Studies in humanized mice and convalescent humans yield a SARS-CoV-2 antibody cocktail 1014−1018. (189) REGENERON. Regeneron's regn-cov2 antibody cocktail reduced viral levels and improved symptoms in non-hospitalized covid-19 patients Regeneron's covid-19 outpatient trial prospectively demonstrates that regn-cov2 antibody cocktail significantly reduced virus levels and need for further medical attention Regeneron announces start of regn-cov2 phase 3 covid-19 prevention trial in collaboration with national institute of allergy and infectious diseases (niaid) Perspectives on therapeutic neutralizing antibodies against the Novel Coronavirus SARS-CoV-2 Antibodies for Prevention and Treatment of COVID-19 The race to make COVID antibody therapies cheaper and more potent A Review in Rheumatoid Arthritis Tocilizumab for treatment patients with COVID-19: recommended medication for novel disease The innovative development in interferon beta treatments of relapsing-remitting multiple sclerosis Interferons in Autoimmune and Inflammatory Diseases: Regulation and Roles Ribavirin and interferon alfa-2a for severe Middle East respiratory syndrome coronavirus infection: a retrospective cohort study Eculizumab treatment in patients with COVID-19: preliminary results from real life ASL Napoli 2 Nord experience Expanded Umbilical Cord Mesenchymal Stem Cells (UC-MSCs) as a Therapeutic Strategy in Managing Critically Ill COVID-19 Patients: The Case for Compassionate Use