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Watch Burnet Virologist, Professor Gilda Tachedjian’s explanation.
Overdispersion of the Virus (Superspreading)
COVID-19, like other diseases caused by coronaviruses, such as SARS and MERS, tend to cluster rather than spread linearly like influenza. COVID-19 is also subject to large superspreading events where one person may infect dozens or hundreds of others. The metric R0 does not reflect the reality of SARS-CoV-2 transmission.
The over-dispersion factor k is a better metric to characterise transmission. The lower the value of k the fewer people are causing most spread of the virus. This value has been estimated as 0.12 for SARS and 0.25 for MERS while estimates for COVID-19 are as low as 0.1 meaning that just 10 percent of infected cases are infecting more than 80 percent of people.
Superspreading events have been associated with large indoor gatherings in poorly ventilated spaces and where people may be singing (choir practices and church services) or speaking loudly (nightclubs and bars). It is likely that in this (indoor) environment an individual with a high viral load in their pharynx has introduced the virus into an environment where it spreads rapidly.
The concept of over-dispersion has implications for contact tracing. Rather than focusing only on ‘forward’ tracing, namely finding people that have been in contact since the person was infected, there should also be efforts to see who first infected the subject. This may reveal new previously unidentified infected individuals within the cluster.
TECHNICAL BRIEFS and REPORTS
Professor Brendan Crabb AC
Director and CEO; Co-Head Malaria Research Laboratory; Chair, Victorian Chapter of the Association of Australian Medical Research Institutes (AAMRI)
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