Vaccination status, previous infection history, and underlying medical conditions were self-reported and might be subject to recall bias. This could lead to inaccurate VE estimates.
Limited duration of study
The study period was relatively short (September-November 2022) and might not capture the long-term effectiveness of the bivalent boosters, especially against newer variants.
Low bivalent booster uptake
Low acceptance of bivalent booster doses during the study period could bias the results if people getting vaccinated early differ systematically from those vaccinated later.
Information about SARS-CoV-2 exposure risk and mask use was not collected, leading to potential residual confounding.
Focus on specific variants
The study focused on BA.4/BA.5 predominance. Results might not generalize to future variants.
Tests were primarily conducted in areas with higher social vulnerability, potentially limiting the generalizability of the findings to the broader U.S. population.
Potential testing behavior bias
Differences in testing behaviors between vaccinated and unvaccinated individuals could have introduced bias.