BCG vaccination vs Covid-19 global features: clearing up a controversial
issue involving trained immunity mechanisms.
Abstract
Background: The Covid-19 pandemic is characterized by extreme
variability in the outcome distribution and mortality rates across
different countries. Some recent studies suggested an inverse
correlation with BCG vaccination at population level, while others
denied this hypothesis. In order to address this controversial issue, we
performed a strict epidemiological study collecting data available on a
global scale, considering additional variables such as
cultural-political factors and adherence to other vaccination coverages.
Methods: Data on 121 countries, accounting for about 99% of Covid-19
cases and deaths globally, were from John’s Hopkins Coronavirus Resource
Center, World Bank, International Monetary Fund, United Nations, Human
Freedom Report, BCG Atlas. Statistical models were Ordinary Least
Squares, Tobit and Fractional Probit, implemented on Stata/MP16
software. Results: Countries where BCG vaccination is or has been
mandated in the last decades have seen a drastic reduction in Covid-19
diffusion (-80% on average) and mortality (-50% on average), even
controlling for relative wealth of countries and their governmental
health expenditure. A significant contribution to this reduction
(respectively -50% and -13% on average) was also associated to the
outbreak onset during summer, suggesting a possible influence of
seasonality. Other variables turned out to be associated, though to a
lesser extent. Conclusions: Relying on a very large dataset and a wide
array of control variables, our study confirms a strong and robust
association between Covid-19 diffusion and mortality with BCG
vaccination and a set socio-economic factors, opening new perspectives
for clinical speculations, experimental studies and public health
policies.