1. Introduction
The outbreak of rapidly spreading severe respiratory disease caused by novel coronavirus (CoV) in December of 2019 in Wuhan, China has put whole world on high alert. The World Health Organization (WHO) named it COVID-19 and declared it a global pandemic on 30thJanuary 2020 (Kim et al., 2020). The disease has spread to more than 200 countries with over 5.12 million cases and more than 3.30 lakh deaths worldwide by 21st May 2020 (Worldometer, 2020). These numbers are still increasing rapidly. The normal life has been disrupted as a result of mandatory lockdowns, isolations, and quarantines in response to COVID-19 pandemic and has imposed a serious challenge to global health system and economy (Gorbalenya et al., 2020; Kupferschmidt et al., 2020). The coronaviruses represent an ongoing as well as future threat to human health because of their wide distribution, higher prevalence, higher recombination potential of their genomes, profound genetic diversity, and increasing human-animal interaction (Hui et al., 2020; Zhu et al., 2020). SARS-CoV-2 is relatively a large virus of betacoronavirus genus containing a single-stranded positive-sense RNA genome encapsulated within a membrane envelope (Morse et al., 2020). It is one of the seven discrete coronavirus species capable of causing disease in humans (Zhu et al., 2020). The Coronavirus Study Group (CSG) of International Committee on Taxonomy of Viruses (ICTV) changed its name from 2019-nCoV to SARS-CoV-2 because it was found to be a sister virus of severe acute respiratory syndrome coronavirus (SARS-CoV) after genomic analysis (Gorbalenya et al., 2020). The genomic analysis revealed that SARS-CoV-19 genome is more closely related to bat RaTG13 SARS-CoV (96%) in particular and SARS-CoV (80%) in general compared to middle east respiratory syndrome (MERS) CoV (54%) genome (Zhou et al., 2020).
The genomic analysis reveals similarities in receptor-binding motif of SARS-CoV-2 and SARS-CoV which directly binds human receptor protein called angiotensin-converting enzyme-2 (ACE-2) to enter target cells. This has direct bearing on its vaccine design, rapidity of vaccine development, and prediction of its pandemic potential (Kim et al., 2020). Similar to SARS-CoV and MERS-CoV, the predilection site of SARS-CoV-2 is lower respiratory system and viral pneumonia occurs. However, SARS-CoV-2 may also attack gastrointestinal tract, heart, kidney, liver, and central nervous system resulting in death due to multiple organ failure (Zhu et al., 2019). Though, the exact reservoir of SARS-CoV-2 could not be established yet with surety, its human-to-human transmission is now well established, and the verified cases are growing at an alarming rate. After crossing the species barrier, within a month human to human transmission became responsible for widespread and rapid dissemination of virus across the entire population of China with no pre-existing immunity (Chen, 2020; Wu et al., 2020). The current ferocious spread of COVID-19 indicates that SARS-CoV-2 is more transmissible or contagious than SARS-and MERS-CoVs. This grim situation of COVID-19 demands the generation of targeted vaccines and therapies for reducing the associated morbidity and mortality. However, the risk of contracting COVID-19 is highest in healthcare workers, elderly people over 60 years of age, and people suffering from diabetes and hypertension (Huang et al., 2020). Thus, these people need to be prioritized for vaccine trials and licensure (Chen et al., 2020).