A few years back, respiratory physician Adrian Martineau of Queen Mary University of London and colleagues analyzed data from about 11,000 participants in 25 trials that tested the effect of vitamin D supplementation on the risk of contracting an acute respiratory tract infection such as influenza. They determined that taking a daily or weekly dose of vitamin D was protective against infections and safe overall. People with the lowest starting vitamin D levels benefitted the most from supplementation.

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When David Meltzer, an internist and economist at the University of Chicago, saw that analysis earlier this year, he decided to look into a possible connection between vitamin D and COVID-19. There was good reason to do so. Groups who are often low in vitamin D—such as African Americans, who tend to have darker skin in which higher melanin levels limit UV rays from fueling vitamin D production, and the elderly—were being hit especially hard by the disease. Meltzer and his colleagues went back through the medical records of people who had been tested for COVID-19 in Chicago between March 3 and April 10 and measured the relationship between vitamin D levels in the past year and positive tests. “The people who were vitamin D deficient were dramatically more likely to have COVID than the people who weren’t vitamin D deficient,” says Meltzer, who, along with his coauthors, published the results as a preprint on medRxiv in May.

Another study from Israel, published July 23 in The FEBS Journal, likewise found a link between low levels of vitamin D and increased COVID-19 susceptibility.