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

We investigated the negative relationship between mortality and COVID-19 vaccination at ecological level, which has been established through clinical trials and other investigations at the individual level. 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 of the pandemic as function of the vaccination rate in October 2021. In order to disentangle the protective relationship from confounding processes, 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. Controlling for confounding factors did not highlight a statistically significant global relationship between vaccination and cumulative mortality in the total country sample. As suggested by previous estimates at country level, a strong, significant, negative relationship between cumulative mortality 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 countries that use Western-only vaccines. This may partly reflect the higher effectiveness of Western vaccines in comparison with the average of all vaccines in use; it may also derive from the lower social heterogeneity of countries included in this segment. COVID-19 testing has a significant and positive relationship with cumulative mortality for all subsamples. This indicates that testing intensity should be controlled as a potential confounder in future ecological analyses of COVID-19 mortality.