A low level of vitamin D is associated with an increased risk of respiratory viral infections[1]. Observational studies clearly show that people with insufficient vitamin D status have a higher incidence of respiratory viral infections[1]. Vitamin D affects the immune system by reducing the production of inflammatory cytokines and enhancing regulatory T-cells that can protect against the severity of infection[1][2]. In patients with COVID-19, the predicted infection rate in the vitamin D-deficient group was 21.6% compared to 12.2% in the vitamin D-sufficient group[2]. Although observational studies suggest an association between low vitamin D levels and respiratory infections, clinical trials have not yet demonstrated a consistent protective effect of vitamin D supplementation, except when the initial vitamin D status was deficient[1]. Researchers caution that it is not clear whether low vitamin D levels are a cause or a consequence of the infection[1]. Further randomized clinical trials are needed to determine whether vitamin D supplementation could reduce the incidence of respiratory infections in the population[2].