This review summarises evidence relating to a potential role for vitamin D supplementation in the prevention or treatment of coronavirus disease 2019 (COVID-19). Laboratory studies show that the active vitamin D metabolite 1,25-dihydroxyvitamin D induces innate antiviral responses and regulates immunopathological inflammation with potentially favourable implications for the host response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Meta-analyses of cross-sectional, case-control and longitudinal studies report consistent protective associations between higher circulating 25-hydroxyvitamin D [25(OH)D] concentrations or vitamin D supplement use and reduced risk and severity of COVID-19. However, Mendelian randomisation studies testing for associations between genetically predicted circulating 25(OH)D concentrations and COVID-19 outcomes have yielded consistently null results. Positive findings from observational epidemiological studies may therefore have arisen as a result of residual or unmeasured confounding or reverse causality. Randomised controlled trials of prophylactic or therapeutic vitamin D supplementation to reduce risk or severity of COVID-19 reporting to date have yielded inconsistent findings. Results of further intervention studies are pending, but current evidence is insufficient to support routine use of vitamin D supplements as a therapeutic or prophylactic agent for COVID-19, or as an adjunct to augment immunogenicity of SARS-CoV-2 vaccination. Accordingly, national and international bodies have not made any recommendations regarding a role for vitamin D in the prevention or treatment of COVID-19.
