Since COVID-19’s emergence in late 2019, the disease has claimed the lives of nearly 5 million people worldwide. Scientists predict that SARS-CoV-2, the virus that causes COVID-19, will likely achieve endemic status in the coming years, so identification of strategies that bolster viral immunity and reduce negative outcomes is essential. Findings from a recent study suggest that optimal blood levels of vitamin D could markedly reduce the risk of death from COVID-19.

Vitamin D regulates the expression of thousands of genes in the human body via its interaction with specialized cellular proteins called vitamin D receptors. Nearly all cell types involved in the body’s immune response (monocytes/macrophages, T cells, B cells, natural killer cells, and dendritic cells) possess vitamin D receptors. Consequently, vitamin D plays essential roles in the regulation of both innate and adaptive immune systems.

The Endocrine Society defines optimal blood levels of vitamin D as 40 to 60 nanograms per milliliter (ng/mL). Unfortunately, approximately 40 percent of people living in the United States are vitamin D deficient, based on Endocrine Society standards. Deficiency is associated with poor bone health in adults and children as well as severe outcomes in COVID-19. Certain populations are at greater risk for deficiency, especially older adults, dark-skinned people (such as those of African or Hispanic descent), and people who have obesity, low education, or diabetes.

The authors of the study analyzed data from one population-based study (which measured long-term vitamin D status in more than 400 million people worldwide) and seven clinical studies (which measured vitamin D levels post-infection). They used a machine learning model to compute the expected death rates among the patients based on characteristics such as the patients' age, sex, median vitamin D levels, and whether they had diabetes. The timeframe for the data collection preceded the advent of vaccines against COVID-19.

The analysis revealed that people who had optimal vitamin D levels preceding SARS-CoV-2 infection were highly unlikely to die from their illness, suggesting that vitamin D reduces the risk of death from COVID-19. However, these findings were based on observational data, so interpretation of the findings must take into account the higher death rates associated with age and comorbidities (coexisting illnesses). Learn more about the role of vitamin D in COVID-19 in this episode featuring Dr. Roger Seheult.

• Link to full publication (PDF).

• Learn more about vitamin D in our overview article.