Bacillus Calmette-Guérin vaccination and socioeconomic variables vs COVID-19 global features. Clearing up a controversial issue

Since the appearance of COVID-19 in Wuhan, China, in December 2019, more than two million cases have been confirmed worldwide, with a death toll about 140 000 by April 20, 2020. One of the puzzles associated to the outspread of this pandemic is the geographic variability of its outcomes in terms of incidence (IR), case fatality (CFR), and mortality rates (MRs). Especially, striking is the case of adjacent countries where low and high rates are recorded. Some have suggested that such variability can be explained by the protective role of the Bacillus Calmette-Guérin (BCG) vaccination. To robustly investigate the role of BCG vaccination on IR, CFR, and MR of COVID-19, we used linear and nonlinear statistical methods, accounting for demographic, socioeconomic, and health policy confounding factors which would otherwise, if not properly controlled for, impair the validity of results and the implications for the further immunological and clinical studies.