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The current COVID-19 pandemic has urged the scientific community internationally to find answers in terms of therapeutics and vaccines to control SARS-CoV-2. Published investigations mostly on SARS-CoV and to some extent on MERS has taught lessons on vaccination strategies to this novel coronavirus. This is attributed to the fact that SARS-CoV-2 uses the same receptor as SARS-CoV on the host cell i.e. human Angiotensin Converting Enzyme 2 (hACE2) and is approximately 79% similar genetically to SARS-CoV. Though the efforts on COVID-19 vaccines started very early, initially in China, as soon as the outbreak of novel coronavirus erupted and then world-over as the disease was declared a pandemic by WHO. But we will not be having an effective COVID-19 vaccine before September, 2020 as per very optimistic estimates. This is because a successful COVID-19 vaccine will require a cautious validation of efficacy and adverse reactivity as the target vaccinee population include high-risk individuals over the age of 60, particularly those with chronic co-morbid conditions, frontline healthcare workers and those involved in essentials industries. Various platforms for vaccine development are available namely: virus vectored vaccines, protein subunit vaccines, genetic vaccines, and monoclonal antibodies for passive immunization which are under evaluations for SARS-CoV-2, with each having discrete benefits and hindrances. The COVID-19 pandemic which probably is the most devastating one in the last 100 years after Spanish flu mandates the speedy evaluation of the multiple approaches for competence to elicit protective immunity and safety to curtail unwanted immune-potentiation which plays an important role in the pathogenesis of this virus. This review is aimed at providing an overview of the efforts dedicated to an effective vaccine for this novel coronavirus which has crippled the world in terms of economy, human health and life.
Keywords: Vaccine, SARS-CoV-2, COVID-19, Clinical Trials, Convalescent Plasma Therapy, Monoclonal Antibodies
Abbreviations: SARS, Severe Acute Respiratory Syndrome; CoV, Coronavirus; SARS-CoV, Severe Acute Respiratory Syndrome Coronavirus; SARS-CoV-2, Severe Acute Respiratory Syndrome Coronavirus 2; MERS-CoV, Middle East Respiratory Syndrome Coronavirus; COVID-19, Coronavirus Disease 2019; hACE2, Human Angiotensin- Converting Enzyme 2; DPP4, Dipeptidyl Peptidase; ARDS, Acute Respiratory Distress Syndrome; RBD, Receptor Binding Domains; RBM, Receptor Binding Motif; NSP, Non-structural Proteins; NTD, N Terminal Domain; TMPRSS2, Transmembrane Protease Serine 2; VLP, Virus-Like Particle; TLR, Toll-Like Receptor; LAV, Live Attenuated Vaccine; CP, Convalescent Plasma; LNP, Lipid Nanoparticle; DC, Dendritic Cells; ADE, Antibody-Dependent Enhancement; nAb, Neutralizing Antibody
The novel beta-coronavirus SARS-CoV-2 is believed to have emerged last year in 2019 in Wuhan from Bats. Crossing the species barrier it entered human beings with furtherance of infection through human to human transmission. The beta-coronaviruses have jumped between the species and have caused three zoonotic outbreaks namely, SARS CoV (2002-03), MERS-CoV (2012), and SARS-CoV-2 (2019- till date) in the last 2 decades. The existence of a myriad of coronaviruses in bats, including many SARS-related CoV (Severe Acute Respiratory Syndrome related Coronaviruses) and the sporadic crossing over of the species barriers of the coronaviruses to humans, suggest that the future occurrences of zoonotic transmission events may sustain (Ou et al., 2020).
Since its emergence in Nov 2019, it has spread to 188 countries and 25 territories around the globe, despite elaborate efforts by WHO and Governments to contain the infection, primarily owing to the highly infectious nature of this virus (Anon, 2020a; Anon, 2020b). As of 2 July 2020, 10,533,779 cases have been reported globally with 512,842 deaths ((WHO) World Health Organisation, 2020). There has been a monumental increase in the number of infected patients, with a 7-day moving average of 210,209 cases per day, as of 2 July 2020 (Anon, 2020a). SARS-CoV-2, a highly contagious virus, tends to spread by the inhalation of the respiratory aerosols, direct human contact, and via fomites. Social distancing, personal hygiene, frequent hand washing or sanitizing using the alcohol (61-70%) based hand-sanitizers, and disinfection of the surfaces are some steps which can protect the individuals from getting infected ((CDC), Centers for Disease Control and Prevention, 2020). R0 is an epidemiological scale; used to measure the contagiousness of an infectious agent. Its magnitude depends upon various biological, environmental, and socio-behavioral factors. It can be defined as “the average number of secondary cases one would produce in a completely susceptible population in the absence of any deliberate intervention in disease transmission (Delamater et al., 2019).” SARS-CoV-2 has an R0 value range of 2-3 (Park, 2020) which is significantly higher in comparison to Spanish flu for which the R0 was recorded at 0.9-2.1 (Pyrek, 2018). According to WHO, people living with non-communicable diseases (co-morbid conditions) are prone to severe illness due to COVID-19 infection. The incubation period of the virus ranges from 2-14 days with a median of 5.1 days (Lauer et al., 2020). The symptoms include fever, dry cough, fatigue, shortness of breath, chills, muscles pain, headache, gastric disturbances and weight loss (CDC, 2020). Some patients may have lymphopenia and bilateral ground-glass opacity changes in the chest CT scans. The histological examinations of the lungs’ biopsy samples have shown a bilaterally diffused alveolar damage with cellular fibromyxoid exudates. A few interstitial mononuclear inflammatory infiltrates were observed both in the liver and the heart specimens (Xu et al., 2020). However, a large population of the infected patients have no or mild symptoms and remain asymptomatic (Shang et al., 2020).
Structurally coronaviruses are pleomorphic, enveloped viruses with a characteristic fringe of projections composed of S protein on their surface. These viruses are equipped with a positive sense ssRNA genome, which is complexed with the nucleocapsid (N) protein forming helical nucleocapsids. The genome is both capped and polyadenylated (Carter and Saunders, 2007). The genetic analysis of SARS-CoV-2 and SARS-CoV has revealed 79% similarity with a total of 380 amino acid substitutions condensed mainly within the NSP genes. Out of these substitutions, there are 27 amino acid replacements in the immune-dominant S protein while 102 and 61 amino acid substitutions are found in the NSP3 and NSP2. Whereas, NSP7, NSP13, E protein, and some accessory proteins are devoid of any amino acid substitutions (Wu et al., 2020). SARS-CoV and SARS-CoV-2 bind a common host receptor, hACE2, to gain entry into the cell but SARS-CoV-2 binds the receptor with a higher affinity than the SARS-CoV. MERS-CoV uses an entirely different receptor that is, Dipeptidyl Peptidase 4 (DPP4) (Wan et al., 2020) and the virus is distantly related to SARS-CoV-2 with around 50% similarity as per the sequence analysis of the two viruses (Prof Roujian et al., 2020).
The genome of SARS-CoV-2 is transcribed in at least 10 Open Reading Frames (ORFs). ORF1ab translates into a polyprotein which is processed into 16 non-structural proteins (NSPs) (Yoshimoto, 2020). The NSPs perform various functions like genome replication, inducing the cleavage of host mRNA, membrane rearrangement, generation of the autophagosome, cleavage of the NSP polyprotein, capping, tailing, methylation, unwinding of the RNA duplex, etc. which are essential for the viral life cycle (da Silva et al., 2020). Besides, the SARS-CoV-2 virus contains four structural proteins namely, spike (S), nucleocapsid (N), envelope (E), and membrane (M) proteins which are encoded by the 3’-end of the viral genome (Wrapp et al., 2020). Amongst the 4 structural proteins the S glycoprotein, being a large multi-functional trans-membrane protein, plays the vital role of viral attachment, fusion, and entry into the host cell (Wrapp et al., 2020). The S protein consists of S1 and S2 subunits, which are further split into different functional domains. The S1 subunit has two functional domains viz. N-terminal Domain (NTD) and Receptor Binding Domain (RBD) and the latter contains conserved receptor binding motif (RBM) (Jiang et al., 2020). The alignment studies have revealed that the region of RBD sequence lies between the residues 331 and 524 of the S protein (Tai et al., 2020). Whereas, the S2 subunit has three operational domains namely, fusion peptide (FP), heptad repeat (HR) 1, and 2. The S1 protein trimer aligns itself at the top of the trimeric S2 stalk to form the immune-dominant S protein (Jiang et al., 2020). Interestingly, a furin cleavage site is observed within the spike protein of SARS-CoV-2 while it is absent in the SARS-CoV which may be a possible explanation of the variation in the pathogenicity of the virus (Walls et al., 2020). A host trans-membrane protease serine 2, (TMPRSS2) is responsible for the initial priming of the spike protein. The virus can utilize both TMPRSS2 and endosomal cysteine proteases cathepsin B and L (CatB/L) to initiate entry into the cell. The TMPRSS2 is responsible for the cleavage of the S protein to expose the FP region of the S2 subunit which is responsible for the initiation of the endosome mediated entry into the host cell. This indicates that TMPRSS2 is a host factor that is essential for viral entry; therefore, the drugs approved for the inhibition of this protease (like camostatmesylate) could be used for therapeutic purposes (Hoffmann and Kleine-Weber, 2020). SARS-CoV-2 uses the human angiotensin-converting enzyme 2 (hACE2) receptor to seize the target cell through the spike glycoprotein (S-Protein), . It has been suggested that the coronaviruses exercise the use of conformational masking and glycan shielding of the spike protein to circumvent the host immune cells. The Cryo-EM structures have revealed the presence of two distinct: closed and open conformations of the S-Protein ectodomain trimer, as a consequence of the opening of the structure at the trimer apex. This conformational diversification is necessary for the receptor binding as the trimer opening exposes the RBM which is present at the interface between the protomers in the closed trimers (Walls, 2020).
The E protein that forms E channels (called the viroporins), and is involved in a myriad of functions in the viral replication cycle involving assembly, release, pathogenesis, etc. (Gralinski and Menachery, 2020). These reprobate ion channels exist in the form of homo-pentamers with each subunit containing 50-120 amino acids. E channels contain at least one trans-membrane domain (TMD) which facilitates the linkage in host cell membranes. SARS CoVs generally contain three categories of ion channels namely: E, 8a, and 3a. The E and 8a ion channels contain the PDZ (Post Synaptic Density Protein; Disc Large Tumor Suppressor; Zonula Occludens-1 Protein) Domain Binding Motif (PBM) which is responsible for the over-expression of the inflammatory cytokines which may result in the cytokine storm (Pharmaceutical Targeting the Envelope Protein of SARS-CoV-2: the Screening for Inhibitors in Approved Drugs, 2020). From the sequence alignment study of the E protein, it was observed that a negatively charged glutamate residue (E69) in SARS-CoV corresponds to a positively charged arginine residue (R69) in SARS-CoV-2 (Yoshimoto, 2020). However, this mutation is remote from the inhibitor binding site; therefore, E protein can be used as a pharmaceutical target (Pharmaceutical Targeting the Envelope Protein of SARS-CoV-2: the Screening for Inhibitors in Approved Drugs, 2020).
M protein, the central organizer of CoV assembly, is most abundantly expressed in the virus particle. It functions crucially in the morphogenesis and assembly of the SARS-CoV-2 by interacting with the essential structural proteins (Conserved Protein Domain Family: SARS-like-CoV_M, 2020). The binding of the M and N protein stabilizes the N protein and RNA complex, and the internal core of the virus. In case of SARS-CoV, the M protein has also been shown to induce the process of apoptosis in the host cell (Yoshimoto, 2020).
In addition to stabilizing the ssRNA genome of the virus particle, the N protein is an antagonist of the antiviral RNAi. It is responsible for the inhibition of the cell cycle of the host cell as it can inhibit the entry of the cell into the S-phase (Yoshimoto, 2020).
Immunotherapy is considered as an effective method for the prophylaxis and treatment of various infectious diseases and cancers, which involves the artificial triggering of the immune system to elicit the immune response (Masihi, 2001). A vaccine that elicits the production of S protein neutralizing antibodies in the vaccinated subjects is the primary aim of all the programs for COVID-19 vaccines. Studies have revealed that there is a limited to no cross-neutralization between the sera of SARS-CoV and SARS-CoV-2, indicating that recovery from one infection may not shield against the other (Ou et al., 2020). Furthermore, a database of approximately 5500 full-length genomes of SARS-CoV-2 isolated from various countries is now available at NCBI which facilitates delineating the polymorphisms in S protein and other important proteins of the virus concerning vaccine development. The rationale for writing this review is to gather all the information about the COVID-19 vaccine development programs and give the readers and researchers insight into types of vaccines being worked upon and the current status of the clinical trials of these vaccines for ready reference.
Many efforts have been directed towards the development of the vaccines against COVID-19, to avert the pandemic and most of the developing vaccine candidates have been using the S-protein of SARS-CoV-2 (Dhama et al., 2020). As of July 2, 2020, the worldwide SARS-CoV-2 vaccine landscape includes 158 vaccine candidates, out of which 135 are in the preclinical or the exploratory stage of their development. Currently, mRNA-1273 (Moderna), Ad5-nCoV (CanSino Biologicals), INO-4800 (Inovio, Inc.), LV-SMENP-DC, Pathogen-specific aAPC (ShinzenGeno-Immune Medical Institute), and ChAdOx1 (University of Oxford) have entered the phase I/II clinical trials (WHO, 2020). The vaccines which are in the conduit are based upon inactivated or live attenuated viruses, protein sub-unit, virus-like particles (VLP), viral vector (replicating and non- replicating), DNA, RNA, nanoparticles, etc. with each exhibiting unique advantages and hindarances ( Table 1 ) (Ning et al., 2020). COVID-19 vaccine landscape with percentage share of different types of vaccine is represented in Fig. 1 . To enhance the immunogenicity, various adjuvant technologies like AS03 (GSK), MF-59 (Novartis), CpG 1018 (Dynavax), etc. are now accessible to the researchers for the vaccine development (Le et al., 2020). The immuno-informatics approach is also used for the epitope identification for the SARS-CoV-2 vaccine candidates. It can be used to identify the significant cytotoxic T cell and B-cell epitopes in the viral proteins (Gupta et al., 2006; Baruah and Bose, 2020).
Outline of the vaccine production platforms for SARS-CoV-2 and their advantages and limitations