Could vitamin D reduce severe COVID-19?

In early March I discussed past research showing a link between vitamin D deficiency and increased susceptibility to seasonal respiratory illnesses. While that research, published in 2017, did not address the new coronavirus, the insights about immunity likely apply to a wide range of viral illnesses, possibly including COVID-19.

Now, a new study suggests researchers should look more closely at vitamin D’s role in COVID-19 cases.

The Louisiana State University study in April was too small for its data to be independently useful, but the paper’s introductory discussion gave much food for thought. The authors considered two facts: First, African Americans constitute more than 32 percent of Louisiana’s population but a majority of its deaths (56 percent, as of mid-May) from COVID-19. Second, in a Boston study, 147 people in a single homeless shelter tested positive for SARS-CoV-2—yet none was seriously ill.

While many have plausibly suggested the high death rate among African Americans reflects their often-limited access to quality medical care, the homeless shelter study seems to argue there could be more to the picture. Homeless people normally have neither a healthy diet nor reliable medical care, yet the homeless in the Boston study contracted the coronavirus without developing severe disease. The LSU authors hypothesize that time outside in the sunlight might have protected the homeless patients from vitamin D deficiency, and thus from severe COVID-19.

Because of how our bodies produce vitamin D, that makes it harder for people with dark skin—whether African American, Asian, or other ethnicities—to get enough of it. (For the curious: Ultraviolet light catalyzes a reaction just under the skin surface, converting an inactive compound into vitamin D. Melanin, the molecule responsible for making skin darker, blocks much of that light.) It’s possible this could also help explain why the Louisiana data showed African Americans faring so much worse—their vitamin D levels are often lower.

Data from England show people of Asian Indian heritage dying more frequently from COVID-19. Yet according to a government survey, Indian households in the U.K. earn more, on average, than whites, making poverty an unlikely reason for them to fare worse against the virus.

Finding a correlation between vitamin D deficiency and severe COVID-19 would not absolutely prove one caused the other. Also, the above research does not reach the level of a good prospective study, where one large group of people receives vitamin D and another doesn’t, and where researchers follow both groups to see how many get sick, and how badly. But that process takes months.

Meanwhile, low-dose vitamin D is unlikely to cause harm. While researchers work to gather higher-quality data, I’m taking seriously the Mayo Clinic’s dosing advice of 600 international units of vitamin D per day—and taking a supplement.

—This story has been corrected to remove references to two research articles that linked severe COVID-19 with vitamin D deficiency. The two pre-print articles, describing supposed studies in Indonesia and the Philippines, were retracted amid doubts about their authenticity.