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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?
Evidence to date suggests that, similar to other respiratory viruses, SARS-CoV-2 - the virus that causes coronavirus (COVID-19) - is mainly transmitted by respiratory droplets which are spread from an infected person to others during talking, shouting, singing, coughing or sneezing.
These droplets can also land on objects or surfaces, where the virus can be transmitted through contact with a contaminated surface or object. Experts agree there is a gradient from large droplets to aerosols, however those who have been in close or direct contact with a confirmed coronavirus (COVID-19) case are at highest risk.
Transmission can also occur via aerosols in specific circumstances. The extent of this mode of transmission is still being researched but is well recognised during aerosol generating procedures in a healthcare setting. It is likely important in the context of other aerosol generating behaviours such as singing or shouting.
Risk of aerosol transmission may be higher in certain conditions such as poorly ventilated, crowded indoor environments.
Given the potential for aerosol spread, a precautionary approach has been taken toward controlling this risk in Victoria. This includes the requirement of particular filter respirators (e.g. N95/P2 respirators) in all care interactions for patients with coronavirus (COVID-19).
The most effective individual measures to prevent the spread of SARS-CoV-2 are good hand and respiratory hygiene, physical distancing, wearing a face mask, staying home and getting tested if you are unwell. Together, these measures minimise the risk of transmission of SARS-CoV-2.
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.