Editorial: low population mortality from COVID-19 in countries south of latitude 35 degrees North-supports vitamin D as a factor determining severity. Authors' reply.

We read with interest the comments by Rhodes et al, and Panarese and Shahini, regarding a potential association between vitamin D levels and risk of severe coronavirus‐19 disease (COVID‐19). , Their cogent arguments regarding low dose vitamin D supplementation during a period of lockdown, particularly in areas of low sunlight exposure and low baseline vitamin D levels, appear reasonable in the context of bone protection. However, whether this association carries forward to a protective effect against severe COVID‐19 remains tenuous, and best regarded with caution.

Vitamin D has been associated with multiple cellular processes implicated in innate and adaptive immunity, and multiple disease associations with vitamin D deficiency have been noted. However, despite more than a decade of interventional clinical studies, few have supported vitamin D supplementation for altering clinical outcomes for patients with inflammatory disease. Variable study methodology, including dose and method of vitamin D administration, or target 25‐hydroxy vitamin D levels, may have contributed to the many negative studies to date. However, it is more likely that laboratory data and clinical associations have failed to translate to causality or meaningful therapy.

The data regarding north‐south gradient and outcomes of COVID‐19 outlined by Panarese and Shahini, and Rhodes et al must be interpreted in the context of public health measures, population density, urban connectivity and spread of COVID‐19 across various countries. Strict physical distancing and shutdown measures were implemented much earlier in Australia (where a large proportion of the population lives in a latitude below 32 degrees south), New Zealand and Norway, accounting for improved outcomes. In comparison, countries with relatively high sunlight exposure such as Indonesia, Morocco and Egypt, are currently experiencing high case‐fatality rates (CFRs). Singapore, which acted swiftly, is currently experiencing a surge in cases.

Different practices in testing for the virus and in reporting medical outcomes will also skew comparison of mortality figures between nations. Furthermore, until cross‐sectional antibody testing has been performed, CFRs will exclude undiagnosed asymptomatic patients and are likely to be gross overestimates that should be interpreted with care.

Apart from physical distancing and shutdown measures after identification of the first few cases of COVID‐19, there are likely to be multiple confounders to any potential relationship between vitamin D and severe outcomes from vitamin D. Age of population is the strongest determinant of severe outcomes. , The median age of the population tends to be substantially higher in countries with higher than lower CFRs.

It may be premature to suggest widespread vitamin D supplementation with the aim to improve outcomes from COVID‐19. It would be reasonable, however, to consider vitamin D supplementation to protect musculoskeletal health in those at risk of deficiency due to being housebound, as recommended currently by the UK National Health Service (NHS online). Additionally, measured recommendations for a balanced nutritious diet, physically distanced exercise and sunlight exposure may be better for overall physical and mental health during this global crisis.