In addition to the many studies published during 2020 and 2021, since December 2021, four large systematic meta-analyses1,2 have been published, looking at either vitamin D levels, supplementation or both.

In all cases, the data consistently show that low vitamin D levels raises your risk of COVID while higher baseline levels and/or supplementation lowers all risks by 1.5 to three times.

Therapeutic Role of Vitamin D in COVID

In the first of these four meta-analyses, "Prognostic and Therapeutic Role of Vitamin D in COVID-19: Systematic Review and Meta-Analysis,"3 published December 11, 2021, the researchers sought to determine "the association between vitamin D deficiency/insufficiency and susceptibility to COVID-19, its severity, mortality and role of vitamin D in its treatment."

A total of 72 observational studies, covering 1,976,099 patients, met the criteria for inclusion. Compared to those with sufficient vitamin D, the odds of developing COVID-19 among those with deficient or insufficient vitamin D levels were 1.46 times higher.

Their risk of severe disease was also 1.9 times higher, and their risk of death 2.07 times higher. One potential drawback was that all studies ranked "high" for risk of bias. The studies also differed in their definitions of severe illness and vitamin D deficiency/insufficiency, making absolute comparisons difficult. That said, the authors concluded:4

"Multiple observational studies involving nearly two million adults suggest vitamin D deficiency/insufficiency increases susceptibility to COVID-19 and severe COVID-19, although with a high risk of bias and heterogeneity. Association with mortality was less robust."