COVID infection before a rapid antigen test

Probability:
0.5%
Odds:
1 to 199
dBan
−23 dBan

Imagine 100,000 asymptomatic people with suspected exposure taking a COVID test. At the hypothetical prevalence of 0.5%, 500 of them are infected before testing.

Calculation

prior p = 0.005; prior odds = 0.005 / 0.995

Assumptions

The 0.5% prevalence is a hypothetical input, not a measured current prevalence or a personal risk estimate. The same infection hypothesis and population are used for both test-result updates.

Sources

  1. Cochrane: accuracy of rapid antigen tests for COVID-19 Dinnes et al., version 4, published 28 November 2025, with studies searched through 17 February 2022