Season of the year, vitamin D and COVID-19.

Vitamin D supplementation reduced the risk of acute respiratory tract infection in two meta-analyses. Mendelian randomization shows a causal effect of low vitamin D on bacterial pneumonias risk. These studies involved patients before COVID-19 pandemic. Several association studies found higher incidence of SARS-CoV-2 positivity, greater COVID-19 severity and higher risk of mortality in vitamin D deficient subjects compared to vitamin D non-deficient controls. We draw attention to the trend of inverse relative COVID-19 mortality in Europe versus the states of the Southern Hemisphere (Australia, Brazil, South Africa) in dependence on season, which may be associated with intensity of ultraviolet radiation and consequent seasonal fluctuation of serum vitamin D levels. Although we cannot yet confirm causal role of vitamin D in SARS-CoV-2 positivity or COVID-19, we recommend consumption of vitamin D rich food or vitamin D supplementation in the non-sunny season to prevent vitamin D deficiency.