Since the onset of the COVID-19 pandemic, dozens, if not more, studies analyzing associations between vitamin D concentrations and COVID-19 outcomes have mostly concluded that vitamin D may have a positive influence on COVID-19 prognoses, including severity of symptoms, duration of hospitalization, mortality rates, and more.
 
Three new large-scale clinical trials have all concluded that deficiencies in vitamin D, a nutrient involved in a wide array of mechanisms related to immune function, significantly elevate the risk of more severe or fatal COVID-19 outcomes. While many characterize vitamin D’s role in acute respiratory infections as patchy, large-scale observational data largely has yielded positive findings, especially in groups known to be vitamin D deficient. Several national public health agencies in Europe and other continents have issued advisories encouraging the use of vitamin D supplements in the wake of COVID-19.
 
One study, published in the Endocrine Society’s Journal of Clinical Endocrinology and Metabolism, found that 82.2% of 200 COVID-19 patients have a vitamin D deficiency, according to measurements of serum 25-hydroxyvitaminD (25OHD) levels in hospitalized patients with COVID-19, which were cross-referenced with 197 population-based controls. Of the 216 patients, mean vitamin D levels were 13.8 ± 7.2 ng/mL, compared with 20.9 ± 7.4 ng/mL in controls. Of note, vitamin D-deficient COVID-19 patients had a greater prevalence of hypertension and cardiovascular diseases, raised serum ferritin and troponin levels, and a longer length of hospital stay compared with those who had sufficient vitamin D levels. However, there were no associations between vitamin D status and severity – additionally, because the data was observational, no causal relationship could be established.
 
Another clinical trial published in the American Journal of Clinical Nutrition looked specifically at the usage of vitamin D supplements as a parameter for links to COVID-19 outcomes. Scientists analyzed the results of 8,297 adults registered with the UK Biobank, who had COVID-19 test results measured from March to June 2020. The use of vitamin D supplements, circulating vitamin D levels, and main covariates were measured at baseline. After adjusting for covariates, the researchers found that habitual use of vitamin D supplements was tied to a 34% reduced risk of COVID-19 infection.
 
Interestingly, the association between the use of vitamin D supplements and COVID-19 infection risk didn’t vary based on different levels of circulating or genetically predicted vitamin D. The authors of the study predicted that because circulating vitamin D levels were recorded between 2006 and 2010, and because these levels are largely effected by diet and season changes, the baseline circulating vitamin D levels weren’t reliable real-time measurements to be tied to COVID-19 outcomes.
 
A third trial, published in JAMA, concluded that for Black people, having a vitamin D level that is higher than what has been considered sufficient could reduce the risk of developing COVID-19. Vitamin D deficiency disproportionately effects Black people, as melanin reduces the body’s ability to naturally synthesize vitamin D from sunlight exposure, and for these reasons, it has been hypothesized that vitamin D deficiency could by partially to blame for the disproportionate rates and severity of COVID-19 infections among Black populations. The objective of this present study was to examine whether COVID-19 test results are associated with differences in vitamin D levels of 30 ng/mL, 30 to <40 ng/mL, or 40 and above. The 4,638 individuals involved in this study had vitamin D levels tested at least a year before COVID-19 testing, including 2,288 Black individuals, 1,999 White individuals, and 351 individuals of another race/ethnicity. Only in Black individuals was vitamin D level associated with risk of COVID-19 infection, and the authors of the study found significant benefit to those who had 40 ng/mL or greater vitamin D levels.
 
“In this single-center retrospective cohort study, COVID-19 risk increased among Black individuals with vitamin D level less than 40 ng/mL compared with those with 40 ng/mL or greater and decreased with increasing levels among individuals with levels greater than 30 ng/mL,” the authors concluded, indicating that future clinical trials should evaluate the potential benefit of vitamin D levels greater than 40 ng/mL in Black individuals.