Rapid-test use may bias COVID-19 vaccine effectiveness estimates

Test-negative design studies estimate vaccine effectiveness by comparing the odds of infection in vaccinated and unvaccinated people who seek testing. Researchers used individual-level simulations based on national US COVID-19 surveillance data to examine how rapid diagnostic test (RDT) use and healthcare visits might affect these estimates. They simulated 21 scenarios varying the likelihood of visiting a healthcare facility after a positive or negative RDT by vaccination status. The abstract does not report the number of individuals represented in the simulations.

In scenarios where vaccinated people were more likely than unvaccinated people to seek care after a positive RDT, estimated effectiveness was 5.8 percentage points lower than in simulations without RDT testing (95% uncertainty interval [UI], −9.1 to −2.4). A similar downward bias of 6.1 points (95% UI, −9.9 to −2.5) occurred when unvaccinated people were more likely to seek care after a negative RDT. Other differences in care-seeking produced upward biases of 5.2 points (95% UI, 2.4 to 7.7) and 8.4 points (95% UI, 5.6 to 11.3).

The findings indicate that RDT use and subsequent healthcare-seeking patterns may affect estimates from test-negative studies in either direction. These are simulation results, and the abstract does not establish the size of any bias in a specific real-world vaccine effectiveness study.