Written by Clay Smith
If a susceptible person is exposed to someone with COVID-19, there is, on average, about a 3-4% chance of they will become infected. Several factors make this more or less likely.
Why does this matter?
A key question about COVID-19 is the secondary attack rate (sAR). That is, when a susceptible person is exposed to an infected individual, how likely are they to catch it? With measles, the sAR is >90% in a susceptible person, for example. Measles is a true airborne pathogen, spread via aerosol transmission. COVID-19 doesn’t behave this way…exactly. It seems to be primarily transmitted via respiratory droplets (≥5 microns). Although, it may rarely spread via even smaller droplets, possibly aerosols, over further distances. So, will you catch it?
Spreading the SARS-CoV-2
This was a prospective cohort study of patients in China based on contact tracing of 3410 persons exposed to 391 COVID-19 infected individuals. Out of the 3410, 127 developed infection. There are several important points from this study.
Overall sAR: 3.7%
Household sAR: 10.3%
sAR in a healthcare setting: 1%
sAR on public transportation (via mandatory cell phone tracking): 0.1% (n=1) Mask wearing in public was mandatory.
The chance of infection increased with severity of illness in the index patient: 0.3% (n=1) when exposed to asymptomatic index case vs 6.2% for severely or critically ill index case. It also increased when the index case had a productive cough with expectoration (aOR 4.39, 95%CI 2.92–6.61).
Contact Settings and Risk for Transmission in 3410 Close Contacts of Patients With COVID-19 in Guangzhou, China: A Prospective Cohort Study. Ann Intern Med. 2020 Aug 13. doi: 10.7326/M20-2671. Online ahead of print.
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