Background: SARS‐CoV‐2 coronavirus infection ranges from asymptomatic through to fatal COVID‐19 characterized by a ‘cytokine storm’ and lung failure. Vitamin D deficiency has been postulated as a determinant of severity.Objectives: To review the evidence relevant to vitamin D and COVID‐19.Methods: Narrative review.Results: Regression modelling shows that more northerly countries in the Northern Hemisphere are currently (May 2020) showing relatively high COVID‐19 mortality, with an estimated 4.4% increase in mortality for each 1 degree latitude north of 28 degrees North (P = 0.031) after adjustment for age of population. This supports a role for ultraviolet B acting via vitamin D synthesis. Factors associated with worse COVID‐19 prognosis include old age, ethnicity, male sex, obesity, diabetes and hypertension and these also associate with deficiency of vitamin D or its response. Vitamin D deficiency is also linked to severity of childhood respiratory illness. Experimentally, vitamin D increases the ratio of angiotensin‐converting enzyme 2 (ACE2) to ACE, thus increasing angiotensin II hydrolysis and reducing subsequent inflammatory cytokine response to pathogens and lung injury.Conclusions: Substantial evidence supports a link between vitamin D deficiency and COVID‐19 severity but it is all indirect. Community‐based placebo‐controlled trials of vitamin D supplementation may be difficult. Further evidence could come from study of COVID‐19 outcomes in large cohorts with information on prescribing data for vitamin D supplementation or assay of serum unbound 25(OH) vitamin D levels. Meanwhile, vitamin D supplementation should be strongly advised for people likely to be deficient.

LINKED CONTENTThis article is linked to Al‐Ani et al and Garg et al papers. To view these articles, visit https://doi.org/10.1111/apt.15779 and https://doi.org/10.1111/apt.15796.

Mucosa‐associated E Coli are reported to be increased in Crohn's disease