Coronaviruses (CoVs) are a large group of enveloped viruses with a positive-sense RNA that have characteristic spikes projecting from their surface. CoVs are well known for their large RNA genome (26-32 kb). They primarily affect mammals and birds, causing infections of the respiratory and gastrointestinal tracts. The emergence of human CoVs (HCoVs) has been reported once every ten years for the last three decades. The most recent emergence occurred in December 2019, when a new strain of CoVs named SARS-CoV-2 caused the coronavirus disease 2019 (COVID-19) pandemic, leaving a devastating impact on the global healthcare. The early cases were associated with the Huanan seafood market in Wuhan, although the exact origin of the virus is still being debated. Phylogenetic analysis reveals bats to be the reservoir hosts, but the intermediate host responsible for spill-over into the human population remains debatable. Accumulating evidence cites pangolins based on the similarity of receptor binding domain in spike protein; however, the search for a conclusive intermediate host that aided in the inter-species crossover is still underway. Advances have been made in our understanding of the functions of each structural protein, but certain non-structural proteins and accessory proteins are yet to be characterised. Owing to the large genetic diversity of CoVs that arise through recombination, genetic variation, or gene gains/losses, future re-emergence of CoVs are most likely.  In this review, we provide an introduction to CoVs and discuss the origin, virology, genetics, phylogeny, and pathogenesis of SARS-CoV-2 based on relevant literature.

Background: The emergence and the spread of the novel coronavirus or the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a devastating impact on the economy and has become a pressing issue globally. Due to the significant increase in the number of confirmed cases and death tolls worldwide, and certain countries reporting second waves, there is an immediate need for an effective vaccine or other therapeutic intervention to control the spread of the disease. Improving our understanding on the host’s anti-viral immune response on SARS-CoV-2 infection, the potential immune evasion mechanisms adopted by the virus, and the speculated role of antibody dependent enhancement (ADE) in coronavirus disease 2019 (COVID-19) pathogenesis will aid in identifying and designing effective therapeutics. Aim: This review aims to provide an in-depth view of the current knowledge available on the range of host defense mechanisms activated by SARS-CoV-2 infection and various immune evasion mechanisms utilized by the virus. In addition, it also highlights the postulated role of ADE in viral pathogenesis and covers the different preventive and therapeutic options available for the treatment of COVID-19 based on current literature. Discussion: The ongoing COVID-19 pandemic serves as a timely reminder on the constant evolutionary process the virus undergoes to emerge as a novel strain and to spread undetected within the population. Similar to other infectious diseases, the host defence mechanism is triggered, and it plays a central role in dampening viral replication by recruiting immune cells and activating anti-viral mechanisms to control the spread of infection by SARS-CoV-2. However, the virus has adopted different immune evasion mechanisms to circumvent host surveillance to successfully establish infection. Hence, understanding the host’s immune responses triggered by SARS-CoV-2 infection is critical for identifying and designing novel and effective therapeutics. Currently, over 70% of the population are either asymptomatic or they showcase mild to moderate symptoms and reasons for why some people can mount immune responses more quickly than others are unknown. However, a growing body of research speculates that the ADE mechanism may facilitate the SARS-CoV-2 entry and can contribute to severe clinical manifestations. With the constant rise in the number of confirmed cases, there is an immediate need for an effective vaccine to mitigate the spread of the virus. Presently, there is no treatment for COVID-19 although several vaccine candidates are in clinical trials. Therefore, preventive measures like social distancing, isolation, and travel restrictions, may be the key to controlling the rapid spread of COVID-19.