Vitamin D and SARS-CoV-2 infection—evolution of evidence supporting clinical practice and policy development

Observational ecological and epidemiological studies have suggested increased risks of SARS-CoV-2 infection and severity in groups with high prevalence of vitamin D deficiency (older adults, and those with obesity, pre-existing medical conditions or darker skin). Emerging observational clinical studies have confirmed these associations, reporting higher risk of SARS-CoV-2 infection, severe Covid-19 disease and mortality in those who are vitamin D deplete. Further experimental studies have described the immunological and metabolomic mechanisms by which vitamin D deficiency increases these risks, while recent prospective RCT data have shown reduced Covid-19 disease severity and mortality with vitamin D supplementation, further supporting a causal association. Dietary vitamin D intakes are low (~3-6μg/d (120-240 IU/d)) and sunshine exposure inadequate to achieve optimal vitamin D status in Ireland. Consequently, vitamin D deficiency (serum 25(OH)D50nmol/l on a year round basis. Supplementation with 20-25μg/d (800-1000 IU/d) is therefore required to avoid deficiency and meet the minimum 25(OH)D threshold of 50nmol/l for enhanced immunity to viral respiratory infection. For older adults, and those with obesity, underlying conditions or darker skin, supplementation at higher doses under medical supervision is indicated. Practice and policy development are now required to ensure that Irish adults are supplemented with vitamin D3 at these safe and effective doses to reduce their risks of SARS-CoV-2 infection and severe disease.