Vitamin D is a key regulator for the development and maturation of all immune system lineages. In cases of deficiency, supplementation has shown positive effects in acute respiratory infections, even though it does not reduce the incidence of serious events.
Many reports, based on observations made during the first pandemic wave in Italy, suggest an association between vitamin D deficiency, risk of infection with SARS-CoV-2, incidence and severity of COVID-19, and mortality. Speculative observations have proposed a correlation between the fact that Italy has the highest prevalence of hypovitaminosis D among European countries and that the country experienced a very high incidence of infections with SARS-CoV-2 and COVID-19, above all in the northern regions. These studies, however, associate the two events without verifying the causal nexus and without excluding random factors. Vitamin D status, infection risk and development of serious pathological forms are complex phenomena which depend on countless variables, whose multifactorial relations of interdependence cannot be described by their mere summation. For this reason, only large cohort studies, which do not ignore fundamental variables, can take on epidemiological relevance.