Like other coronaviruses (order: Nidovirales, subfamily: Coronaviridae, family: Coronavirinae), SARS-CoV-2 is an envelope virus containing a single-stranded positive-sense RNA genome with a length of 30 kilobases (kb) – one of largest genomes among RNA viruses.[1] SARS-CoV-2 has been named because the RNA genome is about 82% identical to the original SARS-CoV (sometimes named SARS-CoV-1) and both viruses belong to the same Betacoronavirus genus.

The SARS-CoV-2 genome encodes four major structural proteins: (1) Nucleocapsid (N), (2) Membrane (M), (3) Envelope (E), and the (4) Spike (S) protein. Among the four coronavirus structural proteins, the spike (S) and the nucleocapsid (N) are the main immunogens (Figure 1). The spikes are composed of three identical spike proteins (i.e., homo-trimeric structure) and have a triangle shape when viewed from the top (Figure 2). The spike (S) protein is a class I virus fusion protein[2] that facilitates (a) attachment of the virus to the host cell surface, (b) cell entry, and (c) sometimes cell-cell fusion.