Abstract
The pleiotropic effects of Vitamin D (or rather its active metabolite calcitriol) on the immune system and the lung suggest a preventive and therapeutic role for the COVID-19 disease. Epidemiologic studies demonstrate an association between vitamin D deficiency and the incidence of COVID-19 and the severity of the illness. However, reverse causality and other underlying causes (age, obesity) might be responsible for the association. Controlled prospective studies using vitamin D for the prevention of COVID-19 and for the treatment of infected patients demonstrated favourable effects for vitamin D with clinical relevance. However, this could not be confirmed by all studies. Open questions remain: which population groups should be supplemented and what is the optimal dosing regimen for vitamin D?