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My piece in The Wire on the third national serosurvey. The headline (~21% prevalence) is probably not a major underestimate. Apparently the antibody test used was less vulnerable to missing old infections than the one used in the second serosurvey. 1/6 https://science.thewire.in/health/third-national-seroprevalence-survey-icmr-covid-19-rural-prevalence-test-positivity/
The increase in prevalence from 2nd to 3rd survey is roughly consistent with the increase in cases over this period.
The breakdown of prevalence suggests that disease was moving towards rural areas even as daily cases peaked and declined nationally (September). 2/6
The breakdown of prevalence suggests that disease was moving towards rural areas even as daily cases peaked and declined nationally (September). 2/6
Weaker rural surveillance of infections and deaths could explain a moderate drop in detection, and a more noticeable drop in the naive infection fatality rate (recorded deaths over estimated infections) between 2nd and 3rd national serosurveys. 3/6
Is another surge likely? With upto 80% of people still susceptible it is certainly possible.
Whether a major new surge occurs depends on what transmission looks like outside the major metros, and on *why* cases have declined so convincingly since September... 4/6
Whether a major new surge occurs depends on what transmission looks like outside the major metros, and on *why* cases have declined so convincingly since September... 4/6
So... why the sharp decline nationally when prevalence is still quite low? Three possibilities (not exclusive):
- spread has been underestimated in the survey
- the herd immunity threshold is lower than we thought
- mitigation is still putting the brakes on transmission
5/6
- spread has been underestimated in the survey
- the herd immunity threshold is lower than we thought
- mitigation is still putting the brakes on transmission
5/6
Any key message from the survey? The decline in infections is real, but could have been magnified in case and death data by an increasing % of infections from rural areas where surveillance is weaker. With high numbers still susceptible, it is no cause for complacency. 6/6
