Vitamin D and COVID-19 in older age: evidence versus expectations

The current global pandemic of SARS-CoV-2 coronavirus infection originated in Wuhan, China, during December 2019; over 50 million cases have been diagnosed to date. Older age and comorbidity have proven to be key markers of risk for severity of COVID-19 and mortality,^1,2 and residents of care homes have been proven to be at high risk. The Office for National Statistics has recorded 16 111 deaths related to COVID-19 in care home residents in England up to 20 November 2020.³ In the first wave of the pandemic, 46% of all excess deaths in England and Wales up to 7 August occurred in care homes.⁴ Older age is associated with increasing prevalence of vitamin D deficiency, which can affect up to 40% of care home residents.⁵ There is considerable overlap between the non-modifiable risk factors for severe SARS-CoV-2 infection and those associated with deficiency of vitamin D. For example, age, ethnicity, diabetes, and chronic pulmonary and cardiac diseases; in addition, there is the observed trend towards greater severity of disease in northern latitudes. While these could imply an association between reduced vitamin D levels and susceptibility to SARS-CoV-2 infection this may simply be an ecological fallacy.⁶ Therefore, it is important to understand the strength of evidence provided by epidemiological and observational studies of COVID-19, and compare it with what is known from clinical trials of the impact of vitamin D supplementation on acute respiratory infections, including those due to SARS-CoV-2.