COVID-19 vaccination status and testing rates in Finland-A potential cause for bias in observational vaccine effectiveness analysis.

COVID-19 vaccination effectiveness has been monitored in observational studies (test-negativity design or traditional cohort design), but these studies have not addressed the potential behavioral bias between vaccinated and unvaccinated individuals. We aimed to address this by comparing COVID-19 testing rates between vaccination status and whether vaccination changes the testing rates. We found that three times vaccinated had least tests performed during the pandemic and unvaccinated had the highest testing rate. Each vaccination dose increased the testing rate. In conclusion the observational studies addressing vaccine effectiveness should also present testing rates between vaccinated and unvaccinated to address the potential behavioral bias.