Patterns of protection, infection, and detection: Country-level effectiveness of COVID-19 vaccination in reducing mortality worldwide

Background: The relationship between COVID-19 vaccination and mortality has been established through clinical trials and other investigations at the individual level. In this study, we aimed to investigate the negative relationship between mortality and COVID-19 vaccination at country level. Study design: We conducted an exploratory, correlational, country-level analysis of open data centralized by Our World in Data concerning the cumulative COVID-19 mortality for the winter wave (October 2021-March 2022) of the pandemic as function of the vaccination rate in October 2021. Methods: We controlled variables that capture country-level social development and level of testing. We also deployed three segmentation tactics, distinguishing among countries based on their level of COVID-19 testing, age structure, and types of vaccines used. Results: Controlling for confounding factors did not highlight a statistically significant relationship between vaccination and cumulative mortality in the total country sample. Still, a strong, significant, negative relationship between cumulative mortality (log scale) and vaccination was highlighted through segmentation analysis for countries positioned at the higher end of the social development spectrum. The strongest estimate for vaccine effectiveness at ecological level was obtained for the set of countries that used Western-only vaccines. Conclusions: COVID-19 testing (log scale) has a significant and positive relationship with cumulative mortality for all subsamples, consistent with patterns of under- and overreporting of COVID-19 deaths at country level, partly driven by testing. This indicates that testing intensity should be controlled as a potential confounder in future ecological analyses of COVID-19 mortality.