Background
Vitamin D deficiency (serum 25(OH)D<50nmol/l) is common in Ireland, particularly amongst older adults, hospital inpatients and nursing home residents. Vitamin D deficiency is associated with increased risk of acute viral respiratory infection and community acquired pneumonia, with several molecular mechanisms proposed to explain this association. Vitamin D supplementation has also been shown to reduce the risk of respiratory infection.

Vitamin D and Covid-19
Correction of vitamin D deficiency is thought to suppress CD26, a putative adhesion molecule for Covid-19 host cell invasion. Vitamin D may also attenuate interferon gamma (IFNγ) and interleukin-6 (IL-6) inflammatory responses, both potent predictors of poorer outcome in critically-ill ventilated patients including those with Covid-19.

Vitamin D Requirements
Irish adults require 25-30µg/d of vitamin D3, an intake not achievable by diet alone, to reliably maintain serum 25(OH)D levels >50nmol/l. Supplementation with doses up to 100µg/d has been shown to be safe for adults, and many agencies and expert groups now advocate supplementation in older adults, albeit at lower levels than this.

Conclusions and Recommendations
Vitamin D deficiency is common and may contribute to increased risk of respiratory infection including Covid-19. We recommend that all older adults, hospital inpatients, nursing home residents and other vulnerable groups (e.g. those with diabetes mellitus or compromised immune function, those with darker skin, vegetarians and vegans, those who are overweight or obese, smokers and healthcare workers) be urgently supplemented with 20-50µg/d of vitamin D to enhance their resistance to Covid-19, and that this advice be quickly extended to the general adult population.