SARS-CoV-2: An overview of virus structure, transmission and detection
SARS-CoV-2, the causative agent of Coronavirus Disease 2019 (COVID-19), is an enveloped, single-stranded RNA Betacoronavirus. Seven coronaviruses have been identified as agents of human infection, causing disease ranging from mild common cold to severe respiratory failure.1
SARS-CoV-2 is transmitted primarily from person-to-person through respiratory droplets and aerosols.2,3 The incubation period from infection to detectable viral load in the host commonly ranges from two to 14 days.4,5 Detection of viral load can be associated with the onset of clinical signs and symptoms, although a considerable proportion of individuals remain asymptomatic or mildly symptomatic.6-8 The interval during which an individual with COVID-19 is infectious has not yet been clearly established, however, transmission from symptomatic, asymptomatic, and pre-symptomatic individuals has been well described.9-11
Coronavirus genomes encode 4 main structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N). The S protein is a very large transmembrane protein that assembles into trimers to form the distinctive surface spikes of coronaviruses. Each S monomer consists of an N-terminal S1 subunit and a membrane-proximal S2 subunit. The virus gains entry to the host cell through binding of the S protein to the angiotensin-converting enzyme 2 (ACE2) receptor, which is present on the surface of numerous cell types including the alveolar type II cells of the lung and epithelial cells of the oral mucosa.12,13 Mechanistically, ACE2 is engaged by the receptor-binding domain (RBD) on the S1 subunit.14,15
Upon infection with SARS-CoV-2, the host usually mounts an immune response against the virus, typically including production of specific antibodies against viral antigens. IgM and IgG antibodies against SARS-CoV-2 appear to arise nearly simultaneously in blood.16 There is significant inter-individual difference in the levels and chronological appearance of antibodies in COVID-19 patients, but median seroconversion has been observed at approximately two weeks.17-20
After infection or vaccination, the binding strength of antibodies to antigens increases over time - a process called affinity maturation21. High‑affinity antibodies can elicit neutralization by recognizing and binding specific viral epitopes22,23. Antibodies against SARS‑CoV‑2 with strong neutralizing capacity, especially potent if directed against the RBD, have been identified.24-27 Numerous vaccines for COVID-19 are in development, many of which focus on eliciting an immune response to the RBD.28-30