Low levels of vitamin D are associated with an increase in inflammatory cytokines and a considerably higher risk of pneumonia and viral upper respiratory tract infections. Vitamin D deficiency has been found to occur more frequently in patients with obesity and diabetes. These conditions are said to lead to an increased mortality in COVID-19. 

It has been hypothesized that vitamin D may reduce the severity of COVID-19 with respect to pneumonia/ARDS, inflammation, inflammatory cytokines and thrombosis. The efficacy of vitamin D supplements in the prevention of acute respiratory tract infections has been shown with the chronic intake of low doses, instead of the bolus administration of large doses. 

Resources: Weir EK, Thenappan T, Bhargava M, et al. Does vitamin D deficiency increase the severity of COVID 19? Clin Med. 2020; 20: e107-e108.