key: cord-0962970-y1bzjtbv
authors: Rawat, Kajal; Kumari, Puja; Saha, Lekha
title: COVID-19 vaccine: A recent update in pipeline vaccines, their design and development strategies
date: 2020-11-25
journal: Eur J Pharmacol
DOI: 10.1016/j.ejphar.2020.173751
sha: d3c543aadc861bd65f64d3866a0f9a757d5410d8
doc_id: 962970
cord_uid: y1bzjtbv

Coronavirus Disease 2019 named as COVID-19 imposing a huge burden on public health as well as global economies, is caused by a new strain of betacoronavirus named as SARS-CoV-2. The high transmission rate of the virus has resulted in current havoc which highlights the need for a fast and effective approach either to prevent or treat the deadly infection. Development of vaccines can be the most prominent approach to prevent the virus to cause COVID-19 and hence will play a vital role in controlling the spread of the virus and reducing mortality. The virus uses its spike proteins for entering into the host by interacting with a specific receptor called angiotensin converting enzyme-2 (ACE2) present on the surface of alveolar cells in the lungs. Researchers all over the world are targeting the spike protein for the development of potential vaccines. Here, we discuss the immunopathological basis of vaccine designing that can be approached for vaccine development against SARS-CoV-2 infection and different platforms that are being used for vaccine development. We believe this review will increase our understanding of the vaccine designing against SARS-CoV-2 and subsequently contribute to the control of SARS-CoV-2 infections. Also, it gives an insight into the current status of vaccine development and associated outcomes reported at different phases of trial.

By the end of year 2019, Wuhan city of China had witnessed several cases of pneumonia like conditions, which later on found to be caused by 2019-novel coronavirus (2019-nCoV) (P. Zhou et al., 2020) . Later, on February 11, 2020, the novel coronavirus was termed as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by the international virus classification commission and on the same day the disease caused by SARS-CoV-2 was termed as Coronavirus Disease 2019 (COVID-19) by World Health Organization (WHO) (X. . WHO had declared COVID-19 as a global health emergency on 30 January 2020 and characterized it pandemic on 11 March 2020. The case fatality ratio (CFR, defined as the proportion of individuals diagnosed with a disease who die from that disease) was reported as 1-3% and it was shown to be higher in elderly patients especially men (Spychalski et al., 2020) . CFR is therefore a measure of severity of infection among detected cases, which is found to be varying among the WHO defined region (summarized in Table 1 ).

The probable reason could be the region specific immune response of individuals towards the infection. Since the southeast Asian countries are well immune to SARS and MERS infection, the fatality ratio is comparatively lesser than those in WHO defined American and European region. This potentially fatal virus is known to transmit quickly and have spread across 216 countries and territories affecting more than 45 million people worldwide (WHO, 2020) . This came within the 11 months after the first case have reported in Wuhan. Given the severity of the disease, there is an urgent need of therapeutics or vaccines to treat or prevent the spread of SARS-CoV-2.

Currently, no potential drugs are available to treat COVID-19. Only strategies followed worldwide to combat COVID-19 is to relieve the patient's symptoms. There are many therapeutic approaches being tried all over the world to tackle SARS-CoV-2 infections. One J o u r n a l P r e -p r o o f approach is to understand the structure, life cycle and pathogenesis of the virus, which ultimately facilitates a deeper understanding of various targets to be approached for devising a specific drug for SARS-CoV-2. Another approach like repurposing of existing drugs, such as those for influenza, hepatitis B, hepatitis C, and filoviruses, was considered to allow more rapid development (Li and De Clercq, 2020) . Various therapeutic treatments like antiviral drugs (nucleotide analogue) Holshue et al., 2020; Wang et al., 2020) , antimalarial drugs (those inhibits viral cell entry and its replication) (Gao et al. 2020; Gautret et al. 2020) , immunomodulators (Bian et al. 2020 ) and cell and plasma-based therapy (Leng et al., 2020) are currently being used worldwide. There are a number of clinical trials ongoing all over the world in search of a specific treatment for COVID-19. Despite the various approaches there are number of cases where patients once declared clinically recovered from COVID-19 have suffered reinfection Xing et al., 2020; Yuan et al., 2020) and these shortcomings of current approaches highlight the necessity of vaccines against SARS-CoV-2. For the development of a vaccine against SARS-CoV-2, numbers of approaches are being tried all over the world like targeting S protein of the virus and generating antibodies against this. Besides this various other vaccines are designed against the virus, including RNA and DNA based vaccines and these are described further in details.

Scientists and researchers all over the world are trying to understand this novel virus, SARS-CoV-2, to develop specific treatments for controlling and preventing this potentially fatal disease. The immune system plays pivotal role in the pathogenesis of SARS-CoV-2 infection, and to develop understanding of the immune response and the underlying mechanism is very important in developing an effective vaccine. In this review, we discuss different targets that can be approached for vaccine design and development against SARS-CoV-2 infections.

Also, we discuss different platforms that are currently being used all over the world for the development of vaccine candidates. J o u r n a l P r e -p r o o f 7 codes for structural and accessory proteins (Adnan et al., 2020) . The four types of structural proteins are spike protein (S), envelope protein (E), membrane glycoprotein (M) and nucleocapsid protein (N). The genetic sequence of SARS-CoV-2 was published on 11 January 2020 by Wu et al. (F. Wu et al., 2020) . Studies have shown approximately 80% similarity in the RNA genome of SARS-CoV-2 and SARS-CoV yet notable variations are reported in the genome of both these viruses as well as with MERS-CoV genome as shown in Fig. 1A . These differences include the absence of 8a protein and difference in the number of amino acids that codes for 8b and 3c protein in SARS-CoV-2 (A. Wu et al., 2020) (Fig. 1A) .

It is also reported that genome of SARS-CoV-2 has undergone genetic recombination with SARS-related CoV and it might be a combination of SARS-CoV of bat and unknown beta-CoV . This new strain of SARS-CoV-2 can easily recognize the evolving human ACE2 receptor, and therefore its S protein possesses efficient transduction efficiency resulting in the fast transmission of SARS-CoV-2 in humans (Walls et al. 2020 ). The S protein comprises of two functional subunits S1 and S2 subunits, S1 subunit being responsible for intraction with the host's ACE2 receptor through Receptor Binding Domain (RBD) and S2 subunit responsible for the fusion of viral and cellular membranes. The S protein also contains a functional polybasic (furin) cleavage site at the S1-S2 subunit boundary which led to the acquisition of three O-linked glycans around the site (Fig. 1A ).

Both the polybasic cleavage site and the three adjacent predicted O-linked glycans are unique to SARS-CoV-2 and were not previously seen in lineage betacoronaviruses (Andersen et al., 2020; Hu et al., 2020) .

The continuous evolution of the novel coronavirus poses its ability to quickly transmit among population . The similarity between the two viruses can be helpful in vaccine development against SARS-CoV-2 with the help of previously researched protective immune response against SARS-CoV. Studies have demonstrated that S protein is a prime J o u r n a l P r e -p r o o f 8 target for the designing and developing vaccines (Ahmed et al., 2020) . It is responsible for the attachment of virus to the host's cell surface receptor (most likely ACE2). Just as in SARS-CoV, antibodies targeting the S protein of SARS-CoV-2 can interfere with the virus resulting in neutralization of the infection caused by the virus (Liu et al., 2017) . Hence, many vaccines, developing platforms containing target antigen commonly target the S protein (Fig.   1B ).

Immunopathology and vaccine designing 4.1. COVID-19 immunopathology : The immunopathology of COVID-19 explains the reaction of the immune system against novel coronavirus infection. SARS-CoV-2 attacks hosts' immune system, leading to an uncontrolled inflammatory response, lymphocytopenia, high cytokine levels and increased antibodies. The most prominent immuno invasion processes observed in patients were: Depletion of lymphocytes, increased neutrophils, cytokine storm and antibody dependent enhancement (L. Yang et al., 2020) . The SARS-CoV-2 virion attacks ACE2 receptors on type 2 alveolar cells and release the viral genome (RNA) into the cytoplasm of host cell, the RNA further gets translated into structural proteins and then begins to replicate (Perlman and Netland, 2009) . The viral particles subsequently undergo transformation in the endoplasmic reticulum-Golgi intermediate compartment and released entrapped into vesicles, which on fusion with plasma membrane release the viral particles (De Wit et al., 2016) . Simultaneously, while virus enters the cells, a part of it as the antigen is presented on the surface of antigen presentation cells (APC), which is crucial for the body's immune response. Antigen presentation on B and T cells ultimately leads to the production of the typical pattern of IgM and IgG antibodies. Researchers have found that these IgM antibodies lasts until week 12, while the IgG antibodies remain long lasting and can provide prolonged protection (Li et al., 2003) . These type of virus-specific neutralizing J o u r n a l P r e -p r o o f IgG antibodies play major role by regulating the infection at later phase and prevents reinfection in the future (De Wit et al., 2016) . In general, T cells are the major immune cells those fight against viral infections in body either by production of virus specific antibodies (CD4+ helper type T cells mediated response) or by killing the virus-infected cells (CD8+ cytotoxic type T cells mediated response) (Mubarak et al., 2019) . However, the report of COVID-19 patients shows marked reduction in CD4+ and CD8+ T cells population, whereas the high proportions of HLA-DR (CD4 3.47%) and CD38 (CD8 39.4%) double positive fractions indicates the hyperactivated status of T cells imparting severe immune injury (Xu et al., 2020) . There could be the several possible mehanisms involved in the depletion and dysfunctioning of T lymphocytes induced by SARS-CoV-2 such as, (1) SARS-CoV-2 can directly infect T cells and macrophages by binding to the surface markers(CD 26 and CD 147) (García, 2020) or the ACE-2 receptors present on their surface (Dandekar and Perlman, 2005) as well as SARS-CoV-2 induce spleen atrophy and lymph node necrosis thereby destroying lymphatic organs (Cao, 2020; D. Li et al., 2020; Tan et al., 2020) ; (2) another mechanism is cytokine storm induced by SARS-CoV-2, the increased level of cytokines like TNFα, IL-6, and IL-10 is inversely correlated with the decreased T cell population (Moon, 2020). The lymphocytopenia occurring in the patients further explains the increased neutrophil count in patients, since the depletion of T cells make patients more suspectible to microbial infections and thereby promoting the recruitment and activation of neutrophils in patients' blood (Deshmukh et al., 2014) . The clinical outcomes and associated manifestations are briefly summarized in Table 3 .

Apart from lymphocytopenia, cytokine storm is another major clinical outcome observed in patients with severe COVID-19. SARS-CoV-2 after invading the host, activates host's immune system first inducing innate immune response and then subsequently generating the adaptive immunity. The innate response is marked by the production of cytokines specifically J o u r n a l P r e -p r o o f induce infiltration of more innate cells like monocytes, Natural Killer cells, dendritic cells, polymorphonuclear leukocytes, which produces more chemokines (García, 2020) . On the other hand direct activation of T cells by the SARS-CoV-2 infection leads to the activation of pathogenic T helper 1 (Th1) cells that secrete GM-CSF and produce inflammatory cytokine IL-17 which recruits macrophages and neutrophils to the site of infection and stimulates IL-1β and IL-6 cytokine cascades, where IL-6 plays the major role in development of cytokine storm in COVID-19 patients (Wu and Yang, 2020; Y. Zhou et al., 2020) . T cells ultimately led to the activation of B lymphocytes, which induce humoral response by producing neutralizing antibodies. COVID-19 patients were found to have poor outcome even after antibody upregulation, this can be explained by the phenomenon Antibody Depenedent Enhancement (ADE) (Ulrich et al., 2020) . Various infections like Ebola, Dengue, MERS have shown the antibody dependent enhancement of viral infection where it was observed that the viral entry and replication is being enhanced by sub-neutralizing antibodies (Cao, 2020; Katzelnick et al., 2017; Roncati et al., 2020) . However, the ADE mediated inflammatory response and its association with disease severity and progression requires further investigation (L. Yang et al., 2020) . The interconnection between all these processes collectively giving rise to lung injury and disease progression is briefly schematized in Fig. 2 .

Studies on SARS-CoV patients have shown the presence of memory T cells after the four years from recovery and those memory T cells remain able to respond against the S protein of SARS-Cov (Fan et al., 2009; Tang et al., 2011) . The similar results were reported in MERS coronavirus infection in mice (Zhao et al., 2014) . The aftermath, strongly indicates the importance of T cells in controlling the previously existing SARS and MERS coronavirus infections; hence there is high possibility that it can also provide protection against novel coronavirus, SARS-CoV-2 infection. The epitopes identified for SARS and MERS J o u r n a l P r e -p r o o f coronaviruses mostly concentrate on structural proteins especially S protein, hence mapping these epitopes against SARS-CoV-2 can be beneficial in designing the vaccine against all three human coronaviruses in the future (Mubarak et al., 2019) . J o u r n a l P r e -p r o o f

The rapid transmission of SAR-CoV-2 infection across the world ignited extensive efforts toward the development of COVID-19 vaccines that can be used globally for ending the pandemic. The full length spike glycoprotein or RBD of virion are able prevent host and virus interaction by inducing neutralizing antibodies and hence is considered as most important vaccine target antigen (Prompetchara and Chutitorn Ketloy, 2020) . Most of the COVID-19

vaccine development projects ongoing all over the world are using S protein as target antigen (Table 4 ).

In the last decade, there is the marked evolution of vaccine platforms including the development of nucleic acid based vaccine candidates, vectored vaccines and recombinant protein vaccines. SARS-CoV-2 vaccine candidates currently being developed worldwide and under evaluation are based on different vaccine platforms briefly detailed in Fig. 1B . Some of the platforms are already in use for different vaccines and are licensed for use by different regulatory bodies while some of them are still in trials. However, the understanding of these in the field of oncology has encouraged researchers to use next-generation approaches in order to increase the speed of development and manufacture.

Based on their capability to replicate in the host cell, these can be replicating and non replicating recombinant viral vectors. These are evaluated as delivery of viral genome encoding the gene of interest. The longevity of the immune response generated by vaccine depends on the type of viral vector used. Most commonly used viral vector is Adenoviral (Ad) vector. The major advantage of this platform is the capability to induce both humoral and cellular immunity. Despite the complex production of viral vector based vaccines, these are known to induce strong immunological response (Geiben-Lynn et al., 2008; Rollier et al., 2011) . However, sometimes these are not J o u r n a l P r e -p r o o f capable of inducing immunogenicity due to the presence of preexistent immunity (Sekaly, 2008) . A recombinant replicating vaccine, Ervebo is licensed in the European Union as ebolavirus vaccine (Marzi et al., 2011) . The adenoviral vector Ad5 being used for COVID-19 vaccine development is cost effective approach and already been used for Ebola virus. One of the limitations of Ad5 vectors is their association with high prevalence in the human population and therefore an additional trial using chimpanzee derived adenoviruses (ChAd) is being conducted to combat preexisting immunity. Currently, there are 9 COVID-19 vaccine candidates developed using non replicating vector platform in clinical evaluation and 19 candidates in preclinical evaluation stage. Four COVID-19 vaccine candidates developed using replicating vector platform is in clinical evaluation stage and 18 candidates are in preclinical evaluation stage (World Health Organization, 2020).

These are protein multimers mimicking the structure of real virus but lacking genetic material and hence are non-infectious in nature. These can self assemble with more than one type of protein forming protein chimeras known as cVLPs (Roldão et al., 2010) . VLPs act by stimulating antigen presenting cells mediated activation of B-and T-cell immune responses. These are also involved in CD8+ cytotoxic T cell mediated killing of pathogenic cells. The immune system recognizes VLPs in the same way as it recognize original virus and thereby induce immune responses (Hemann et al. 2013) . VLP formulations because of their poor immunogenicity require adjuvants in most of the cases.

VLP-based vaccines are well established platform for prophylactic use. These are less time taking and production cost depends upon the expression system used which is comparatively low for bacterial system than the mammalian expression system. The licensed vaccines based on this platform are currently in use for human papillomavirus (HPV) (Zhao et al., 2013) .

Currently, there are two COVID-19 vaccine candidates developed as VLPs in clinical J o u r n a l P r e -p r o o f evaluation and 15 COVID-19 vaccine candidates in preclinical evaluation stage developed.

(World Health Organization, 2020) 5.3. DNA Platform: DNA-based vaccines were introduced two decades ago and these are non-infectious and non-replicating. These are easy to produce within a short duration and are stable and cost-effective at the same time. These confer long term immunogenicity to the host, however these remain hopeless when used in humans due to their poor immunogenic property. Also these are easily degraded by host enzymes and there is always the risk of its integration into host DNA. While it has shown efficacy in animal models, but yet to establish the potency in human trials (Kim J.H., 2009) 

The most common traditional method which involves manually weakened live pathogen which is no longer able to induce infection but able to induce immune response and hence mimic features of natural infection. It is capable of inducing both humoral and cellular immune response. Live attenuated vaccines on intranasal administration induces secretion of IgA and hence provide local mucosal immunity (Barría et al., 2013) . These vaccines are popular to induce strong lifelong immune responses within two doses. These are easy to produce for some viruses but challenging for complex pathogens.

Most common example is Oral Polio Vaccine (Kumar et al. 2018) . Currently, there are 3 J o u r n a l P r e -p r o o f COVID-19 vaccine candidates in preclinical evaluation stage developed using this platform (World Health Organization, 2020).

These are produced by completely inactivating or killing the pathogen, on injecting it to the host, they primarily induce protective antibodies against epitopes on hemagglutinin glycoprotein on surface of virus. The subunit formulations that primarily consist of a part of killed pathogen in spite of the whole pathogen are used more frequently than the other formulations because potential association of whole virus formulation with increased reactogenicity. These vaccines tend to produce a weaker immune response than live attenuated vaccines, thus adjuvants are required to provide an effective immune response (Kumar et al. 2018) . Inactivated Polio Vaccine is the classical example where the whole killed pathogen is provided and Diphtheria and Tetanus vaccination is the example of subunit formulation (Nascimento and Leite, 2012) . Currently, there are 7 COVID-19 vaccine candidates in clinical evaluation and 12 candidates in preclinical evaluation stage developed using this platform (World Health Organization, 2020). According to the enterprise, VLPs have a multi-modal mechanism of action that works differently to inactivated vaccines, by activating both antibody and cell-mediated responses.

The adjuvants that will be used alongside the CoVLP vaccine candidate are GlaxoSmithKline's pandemic adjuvant technology and Dynanvax's CpG 1018 (Balfour, 2020).

Natural products mainly comprising of Ayurvedic medicines, and Traditional Chinese medicines (TCM) are used for decades as the regulator of the immune system. These Table 4 . The time required by these vaccine candidates to become available in the market depends on the success of all the phases of clinical trials. As discussed further in this section, 10 of the vaccine candidates have successfully passed phase 1 and phase 2 trials and found to have generated anti-SARS-CoV-2 immune responses and are safe to use, however phase 3 trials for the same are ongoing.

ChAdOx1 nCoV-19 vaccine (AZD1222) consists of the non replicating simian adenovirus vector ChAdOx1 containing the full-length structural spike protein of SARS-CoV-2 developed by University of Oxford and AstraZeneca. The preliminary report released by the research group shows that the candidate ChAdOx1 nCoV-19 vaccine given at a dose of 5 × 10¹⁰ viral particles was safe and tolerated, however a higher reactogenicity profile was observed than the control vaccine, meningococcal conjugate vaccine MenACWY. The reactogenicity was reduced on administration of 1 g paracetamol for the first 24 hours after vaccination. 14 days after the first dose of vaccine the cellular responses were observed and on day 28 post vaccination the humoral responses to spike protein, i.e. the spike specific antibodies were found to be at peak. The second dose of vaccine administered after 28 days of first dose induced neutralizing antibodies in all the participants and the reactogenicity was also found to be reduced after the second dose. Overall, the two doses of vaccine was found to be sufficient to induce potent cellular and humoral immunogenicity in all the participants and no serious adverse reactions occurred. The study was conducted in the healthy individuals excluding all the potential participants with recent COVID-19-like symptoms or J o u r n a l P r e -p r o o f with a history of positive PCR test for SARS-CoV-2, however a small number of participants found to have high levels of neutralising antibody at baseline which probably indicates prior asymptomatic infection (Folegatti et al., 2020) .

In conclusion, ChAdOx1 nCoV-19 showed an acceptable safety profile, and sufficient immunogenicity and reduced reactogenecity when given with paracetamol. A single dose found to elicit both humoral and cellular responses against SARS-CoV-2 and booster dose required to enhance neutralizing antibody concentration. The preliminary results supported the large scale evaluation of vaccine in an ongoing phase 3 program.

The mRNA-1273 vaccine is a lipid nanoparticle encapsulated, nucleoside-modified A phase 2 trial of mRNA-1273 in 600 healthy adults, evaluating doses of 50 μg and 100 μg is ongoing (ClinicalTrials.gov number, NCT04405076) and the phase 3 trial to evaluate 100 μg dose is expected to begin soon (Jackson et al., 2020) .

The candidate vaccine is a type of non replicating adenovirus type Ad5-vectored COVID-19 vaccine developed by CanSino Biological Inc. and Beijing Institute of Biotechnology. It is found to elicit both cellular and humoral response in 95% of the participants by the day 28 post vaccination in both doses i.e. 1 × 10¹¹ viral particles and 5 × 10¹⁰ viral particles. No serious adverse reactions were reported, those reported were all ranging from mild to moderate in severity and resolved within a short period of time maximum by 48 hours of reaction onset. Earlier three doses were given to the participants but higher proportions of grade 3 adverse reactions were reported in the higher dose of 1.5 × 10¹¹ viral particles compared with the low dose (5 × 10 10 viral particles) and middle dose (1 × 10 11 viral particles). Further investigation revealed that the vaccine at 5 × 10 10 viral particles had a better safety profile than the vaccine at 1 × 10 11 viral particles and also shows the better immunogenicity profile. The preliminary report suggest the acceptable safety profile of Ad5vectored COVID-19 vaccine. Apart from the results, the major obstacle that the researchers face with this vaccine candidate was the high pre-existing anti-Ad5 immunity in the participants with older age (55 years or older) due to which single dose of vaccine was not inducing high level of humoral immune responses. Therefore researchers proposed to use the additional dose of vaccine in the older age population to gain the required effects. They will J o u r n a l P r e -p r o o f 22 be evaluating it in phase 2b stage of the ongoing trial. Moreover rest of the population showed better results (in terms of safety profile and adequate immunogenicity) with the single immunization of Ad-5 vectored COVID-19 vaccine at dose 5 × 10 10 viral particles making it a potential candidate for emergency vaccination (Zhu et al., 2020) . In conclusion, the results of the trial have shown acceptable results supporting the testing of candidate vaccine at dose 5 × 10 10 viral particles further in phase 2b and phase 3 trial in healthy adults.

The candidate vaccine is an inactivated whole SARS-CoV-2 virus vaccine developed by the Wuhan Institute of Biological Products and Sinopharm. The whole virus pathogen was cultivated in vitro in cell lines and the infected cells further inactivated twice using βpropiolactone under specific conditions and further adsorbed to 0.5 mg alum. The phase 1 trial was conducted using three doses with 2.5 μg, 5 μg and 10 μg antigen protein content per dose and the results showed the inactivated vaccine candidate to have better safety profiles compared to the other vaccines under different platforms and the neutralizing antibody titers induced were also adequate in all the three doses. No serious adverse event was reported, only mild to moderate self limiting reactions were reported and the incidence rate of adverse reactions associated with the current vaccine candidate was very low nearly 15% as compared to the other vaccines where the incidence rate is reported around 60% to even 100% in some studies. Phase 2 trial was conducted using a middle dose of vaccine i.e. 5 μg antigen protein content and the results showed the vaccine candidate is able to effectively produce antibody titer with minimal adverse reactions reported. However researchers came to conclusion that booster dose is required to generate adequate immunogenic response, also the Phase 1 and Phase 2 trial indicated that with longer interval (21 days and 28 dya) between the first dose and subsequent booster dose generates higher antibody titers as compared to shorter interval group (14 day schedule) (Zhu et al., 2020) . In conclusion, the interim analysis of the J o u r n a l P r e -p r o o f inactivated vaccine showed the acceptable safety and better immunogenic profile of the candidate vaccine, supporting its longer term adverse event assessment in ongoing phase 3 trials.

The candidate vaccine is the lipid nanoparticle encapsulated mRNA based SARS-CoV-2 vaccine named as BNT162b1 developed by BioNTech, Fosun Pharma and Pfizer. It consists of the nucleoside modified messenger RNA that encodes the receptor binding domain of the SARS-CoV-2 spike protein. Researchers have administered the BNT162b1 vaccine candidate at three different doses 10 μg, 30 μg and 100 μg and found that the vaccine candidate exhibit acceptable safety profile and elicited adequate antibody titer. Based on the tolerability profile, the participants receiving two of the doses 10 μg and 30 μg was given second booster dose and found to show sharper dose vs immunogenic responses while no data for immunogenic response was reported for the dose 100 μg till now since no second vaccination was provided to the participants to avoid the adverse reactions and the study is ongoing. Results indicated higher reactogenicity after the booster dose, however symptoms were limited to mild to moderate in severity which were resolved within few days of onset (Zhu et al., 2020) . In conclusion, the RNA based vaccine candidate BNT162b1 showed acceptable safety profile and also found to produce adequate antibody titre after booster dose, hence it supports the long term assessment of the candidate vaccine for efficacy and tolerability in the ongoing phase 3 trials.

It is the type of non replicating adenovirus (rAd26-S+rAd5-S) type vaccine developed by 

It is the India's first vaccine against COVID-19 and is an inactivated whole SARS-CoV-2 virus vaccine developed by Bharat BioTech, Indian Council of Medical Research (ICMR) and National Institute of Virology (NIV). It has successfully cleared Phase I/II human trial and is given permission for Phase III trial by DCGI (Drug Controller General of India). The vaccine trial Phase I/II has been conducted in 12 hospitals in different cities across country, in healthy voulnteers with no co-morbidity conditions. The vaccine was found to generate robust immune responses, thereby preventing infection and disease in the primates upon high amount of exposure to live SARS-CoV-2 virus. As per the vaccine makers the phase III study would cover around 28,500 subjects aged 18 years and above, however the complete safety J o u r n a l P r e -p r o o f and immunogenicity data of the phase II trial is still not revealed (Anulekha Ray, 2020; Sharad Sharma, 2020).

It is India's second vaccine, a type of DNA plasmid vaccine expressing SARS-CoV-2 S protein developed by Zydus Cadila Healthcare. Currently, the phase II trial of the candidate vaccine is underway. The Phase I trial of vaccine was conducted on healthy volunteers and was found safe in all the participants (Anulekha Ray, 2020; CTRI/2020/07/026352, 2020).

COVID-19 pandemic has imposed a huge financial and social burden to the world. With regard to its continuously evolving nature, it remains a challenging task to restrict the transmission of infection among the population. One of the prominent approaches for preventing SARS-CoV-2 infection is the development of vaccines against COVID-19. A great deal of work has done so far and till now 33 vaccine candidates have entered into a clinical trial and 143 are in the preclinical phase. Despite the extraordinary efforts of the firms and researchers across the globe for developing the COVID-19 vaccines, there is still a lot more to explore that could be helpful in developing more specific vaccine. Gathering of important information like immunization route, finding more target antigen(s) apart from targeting S protein as antigen only, can be helpful in eradicating the infection. The immunopathological basis of COVID-19 need to be explored more so that its immune evasion mechanism can be targeted and it will also provide deeper insights into vaccine designing strategies. The three main criteria which should be taken into consideration while developing a vaccine are: speed, scale up manufacturing and global access. The astonishing efforts by researchers across the globe in terms of scale and speed of vaccine development have fastened the vaccines development journey from bench to bedside within a few months J o u r n a l P r e -p r o o f 26 only. The work done so far in this regard is very commendable as 5 of the candidates are already in Phase 3 clinical trial and hopefully will be available by the end of 2020. This represents a huge change in the conventional pathway of vaccine development which usually takes 5 to 10 years timescale. Along with speeding up the development process, it is equally important to evaluate the effectiveness and safety of vaccine at each step, and this has been the major hurdle for researchers in establishing the vaccine's efficacy so far. However, the phase 3 trial will play a major role in establishing the vaccine that is safe and effective in a large and diverse population. Finally, the animal models specific for COVID-19 should be established in order to assess vaccine efficacy. Also, the regulatory authorities should need to ensure the equal distribution of vaccines to all the affected areas.

The authors declare no conflict of interest J o u r n a l P r e -p r o o f 

Variations in accessory proteins (yellow) in all the three strains of betacoronvirus organized within structural proteins, Spike (light blue, S), envelope (green, E), membrane (dark blue, M), nucleocapsid (purple, N) and non-structural proteins expressed in ORF 1a (brown) and ORF 1b (peach) are indicated. Untranslated regions at the N and C-terminals are represented respectively as 5'-UTR and 3'-UTR; kb indicates kilo base pairs; RBD is the Receptor binding domain present in the S1 subunit of S protein reported as the most variable part ranging from 460 to 500 amino acid sequence. 

COVID-19 infection : Origin , transmission , and characteristics of human coronaviruses

Preliminary Identification of Potential Vaccine Targets for the COVID-19 Coronavirus (SARS-CoV-2) Based on SARS-CoV Immunological Studies

Clinical characteristics of the recovered COVID-19 patients with re-detectable positive RNA test

Localized mucosal response to intranasal live attenuated influenza vaccine in adults

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

A Trial of Lopinavir-Ritonavir in Adults Hospitalized with Severe Covid-19

COVID-19: immunopathology and its implications for therapy

Origin and evolution of pathogenic coronaviruses

Immunopathogenesis of coronavirus infections: implications for SARS

The microbiota regulates neutrophil homeostasis and host resistance to Escherichia coli K1 sepsis in neonatal mice

Characterization of SARS-CoV-specific memory T cells from recovered individuals 4 years after infection

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

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

Immune response, inflammation, and the clinical spectrum of COVID-19

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

Kinetics of recombinant adenovirus type 5, vaccinia virus, modified vaccinia ankara virus, and DNA antigen expression in vivo and the induction of memory T-lymphocyte responses

Rapidly produced SAM ® vaccine against H7N9 influenza is immunogenic in mice

Protective CD8 T cell mediated immunity against influenza A virus infection following influenza virus-like particle vaccination1

First case of 2019 novel coronavirus in the United States

Double-stranded RNA sensors and modulators in innate immunity

An mRNA vaccine against SARS-CoV-2-preliminary report

Antibody-dependent enhancement of severe dengue disease in humans

DNA Vaccines Against Influenza Viruses

Novel Platforms for the Development of a Universal influenza vaccine

Transplantation of ACE2-Mesenchymal stem cells improves the outcome of patients with covid-19 pneumonia

Functional assessment of cell entry and receptor usage for SARS-CoV-2 and other lineage B betacoronaviruses

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

Profile of specific antibodies to the SARS-associated coronavirus

Therapeutic options for the 2019 novel coronavirus

Molecular immune pathogenesis and diagnosis of COVID-19

T-cell immunity of SARS-CoV: Implications for vaccine development against MERS-CoV

Genomic characterisation and epidemiology of 2019 novel coronavirus: implications for virus origins and receptor binding

Vesicular stomatitis virus-based Ebola vaccines with improved cross-protective efficacy

Moon C (2020) Fighting COVID-19 exhausts T cells

Middle East Respiratory Syndrome Coronavirus ( MERS-CoV ): Infection , Immunological Response , and Vaccine Development

Recombinant vaccines and the development of new vaccine strategies

Coronaviruses post-SARS: Update on replication and pathogenesis

Immune responses in COVID-19 and potential vaccines: Lessons learned from SARS and MERS epidemic

Development of nucleic acid vaccines: Use of self-amplifying RNA in lipid nanoparticles

Virus-like particles in vaccine development

Viral vectors as vaccine platforms: Deployment in sight

Signals of Th2 immune response from COVID-19 patients requiring intensive care

The failed HIV Merck vaccine study: A step back or a launching point for future vaccine development?

Estimating case fatality rates of COVID-19

Lymphopenia predicts disease severity of COVID-19: a descriptive and predictive study

Lack of Peripheral Memory B Cell Responses in Recovered Patients with Severe Acute Respiratory Syndrome: A Six-Year Follow-Up Study

Dengue Fever, COVID⁰19 (SARS⁰CoV⁰2), and Antibody⁰Dependent Enhancement (ADE): A Perspective

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

Receptor Recognition by the Novel Coronavirus from Wuhan: an Analysis Based on Decade-Long Structural Studies of SARS Coronavirus

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

SARS and MERS: Recent insights into emerging coronaviruses

World Health Organization (2020) Draft landscape of COVID 19 candidate vaccines

Genome Composition and Divergence of the Novel

2019-nCoV ) Originating in China

TH17 responses in cytokine storm of COVID-19: An emerging target of JAK2 inhibitor Fedratinib

A new coronavirus associated with human respiratory disease in China

Post-discharge surveillance and positive virus detection in two medical staff recovered from coronavirus disease 2019 (COVID-19)

Pathological findings of COVID-19 associated with acute respiratory distress syndrome

COVID-19: immunopathogenesis and Immunotherapeutics

Clinical Characteristics on 25 Discharged Patients with COVID-19 Virus Returning

The Continuous Evolution and Dissemination of 2019 Novel Human Coronavirus

Probable Pangolin Origin of SARS-CoV-2 Associated with the COVID-19 Outbreak

Pangolin homology associated with 2019-nCoV

Rapid generation of a mouse model for Middle East respiratory syndrome

Elevated IL-6, TNFα, IL-1β (Huang et al., 2020; Rodrigues et al., 2020; Zhou et al., 2020) Lymophocyte