  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.
