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This series of articles provides solutions to some of the most frequently asked questions.
VIROLOGY AND TRANSMISSION
How is SARS-CoV-2 (the virus that causes COVID-19) transmitted?
- Direct person-to-person respiratory transmission is the primary means of transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
- It is thought to occur mainly through close-range contact (ie, within approximately six feet or two meters) via respiratory particles; virus released in the respiratory secretions when a person with infection coughs, sneezes, or talks can infect another person if it is inhaled or makes direct contact with the mucous membranes. Infection might also occur if a person’s hands are contaminated by these secretions or by touching contaminated surfaces and then they touch their eyes, nose, or mouth, although contaminated surfaces are not thought to be a major route of transmission.
- SARS-CoV-2 can also be transmitted longer distances through the airborne route (through inhalation of particles that remain in the air over time and distance), but the extent to which this mode of transmission has contributed to the pandemic is unclear. Scattered reports of SARS-CoV-2 outbreaks (eg, in a restaurant, on a bus) have highlighted the potential for longer-distance airborne transmission in enclosed, poorly ventilated spaces.
- While SARS-CoV-2 RNA has been detected in non-respiratory specimens (eg, stool, blood), neither faecal-oral nor bloodborne transmission appear to be significant sources of infection. SARS-CoV-2 infection has been described in animals, but there is no evidence to suggest that animals are a major source of transmission.
What is the incubation period for COVID-19?
The incubation period for COVID-19 is thought to be within 14 days following exposure, with most cases occurring approximately three to five days after exposure. The incubation period also varies by viral variant. For example, the incubation period for the Omicron variant (B.1.1.159) appears to be slightly shorter than other variants, with symptoms first appearing around three days after exposure.
What are some of the important SARS-CoV-2 variants?
Multiple SARS-CoV-2 variants are circulating globally. Some variants contain mutations in the surface spike protein, which mediates viral attachment to human cells and is a target for natural and vaccine-induced immunity. Thus, these variants have the potential to be more transmissible, cause more severe disease, and/or evade natural or vaccine-induced immune responses.
Some of the more important circulating variants are:
Alpha (B.1.1.7 lineage), also known as 20I/501Y.V1, was first identified in the United Kingdom in late 2020. This variant is estimated to be more transmissible than wild-type viruses. Some studies suggest this variant may cause more severe illness.
Delta (B.1.617.2 lineage), also known as 20A/S:478K, was identified in late 2020 in India. This variant is more transmissible than B.1.1.7 and is also associated with more severe disease.
Omicron (B.1.1.529 lineage) was first reported from southern Africa in November 2021, and it was promptly identified in multiple other countries. The variant contains >30 mutations in the spike protein, including mutations that have been found in other variants of concern and that have been associated with increased transmissibility and decreased susceptibility to neutralizing antibodies (including therapeutic monoclonal antibodies). Emerging data suggest that Omicron has a replication advantage over the Delta variant and evades infection- and vaccine-induced humoral immunity to a greater extent than prior variants. The risk of severe disease with the Omicron variant is more uncertain.
