Conclusions
Our study, by integrating a wide set of controls, attenuates if not eliminates altogether the effects of possible confounding factors affecting previous studies on BCG and Covid-19. The nonlinear, probabilistic methods used in the analyses confer additional statistical robustness to the results. This way, we are able to confirm a robust and large effect of BCG vaccination on Covid-19 diffusion and mortality, and to uncover other noteworthy and potentially relevant statistical relationships. In this wider context, the association between BCG vaccination and Covid-19 diffusion and mortality is fully addressed by a comprehensive epidemiological perspective, that can support novel hypothesis as well as clinical or experimental studies in the field.