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What is the infective agent?
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the infective agent that causes coronavirus (COVID-19). It was first identified in humans in Wuhan, China, in December 2019. SARS-CoV-2 shares 79.6 per cent sequence identity to SARS-CoV-1.
Coronaviruses are a large family of viruses, some causing illness in people and others that circulate among animals, including camels, cats and bats. There are human coronaviruses that cause more mild illness in humans, such as those that cause the common cold.
Rarely, animal coronaviruses can evolve and infect people and then spread between people. This was observed during past outbreaks of severe acute respiratory disease due to MERS-CoV and SARS-CoV.
It is highly likely that the virus has come from an animal source. Genomic analysis suggests that bats appear to be the reservoir of SARS-CoV-2 virus, but the intermediate host has not yet been identified.
What is the mode of transmission?
Human-to-human transmission is via droplets and fomites from an infected person.
- There is some evidence that coronavirus (COVID-19) infection may lead to intestinal infection and virus can be present in the faeces of infected persons.
- Additionally, airborne transmission of coronavirus (COVID-19) may occur during aerosol-generating procedures.
Despite this, current evidence does not support faecal-oral or airborne spread as major drivers in transmission; however, aerosol-generating procedures should be undertaken with appropriate precautions.
What is the reproductive number?
The basic reproductive number (R0) measures the transmission potential of disease. It is the average number of secondary infections produced by a typical case of infection where the entire population is considered susceptible (no herd immunity).
- Estimates for the basic reproductive number (R0) of SARS-CoV-2 range from 2–4, with R0 for confined settings, e.g. cruise ships, at the higher end of this range.
In other outbreaks, not all people will become infected if they come in contact with a case due to previously acquired immunity (from past infection or vaccination). However, the recency of this virus means that acquired immunity is not prevalent in the population.
Thus, estimates of the effective reproductive number (Reff) vary between settings and at different time points. It is more dependent on a range of factors, including public health interventions such as isolation, quarantine and physical distancing to limit close contact between people.
What is the incubation period?
The incubation period is the duration between exposure to the virus and the onset of symptoms. The World Health Organization (WHO) currently estimates that the incubation period ranges from 1 to 14 days, with a median incubation period of 5 to 6 days. The advice the department issues uses an upper range of 14 days.
When is someone considered infectious?
The infectious period of coronavirus (COVID-19) is still being determined. Epidemiological data suggests that the majority of transmission occurs from symptomatic cases.
There are studies suggesting that pre-symptomatic, and possibly asymptomatic, transmission occurs. Taking a precautionary approach, cases are currently considered infectious from 48 hours prior to the onset of symptoms until they meet criteria for release from isolation.
Can reinfection occur?
After infection, the extent or duration of any acquired immunity is still unknown. Apparent re-infection has been reported in a small number of cases. However, most of these reports describe patients having tested positive within 7-14 days after apparent recovery.
The role of serology in determining immunity is currently unclear. Immunological studies indicate that people recovering from coronavirus (COVID-19) mount an antibody response. However, further studies are needed to indicate whether these antibodies are indicative of virus neutralisation, or protection from infection.