Even so, other interesting connections have begun to bear out the vitamin-D hypothesis. In April, a small study from the Philippines showed a correlation between vitamin-D levels and better outcomes for patients with the disease. In May, a study led by Vadim Backman, a professor at Northwestern University, found a similar correlation between severe vitamin-D deficiency and death rates among people with COVID-19. His research team found that, in general, people in countries with high COVID-19 mortality rates had lower levels of vitamin D compared with patients in countries that were not as severely affected.
There are also plausible mechanisms to suggest that these correlations might not be entirely due to chance. Vitamin D is most famous for its role in bone health, but it has an important role in the immune system, specifically in the maturation of some white blood cells. Having low levels of vitamin D has long been known to leave people susceptible to various infectious diseases, including upper respiratory tract infections.
In addition to fortifying the immune system, vitamin D might assist in preventing it from becoming overactive. Backman and his team reported that people with adequate vitamin-D levels were less likely to experience a cytokine storm, a misdirected inflammatory response that ultimately kills many people with COVID-19. The researchers posit that vitamin D may decrease this risk by helping to appropriately calibrate the inflammatory response.
Although all of these correlations and mechanisms have not yet borne out in clinical trials (which are underway), some researchers say that in the meantime, the default should be for everyone to take small amounts of vitamin D. Health officials in the United Kingdom have already begun recommending that people do so for the remainder of the pandemic. Rose Kenny, a gerontologist at University of Dublin, told me she believes that all public-health entities should be recommending the same. “I feel very passionately about this,” she said. Given the low risk of harm, even a small benefit would be worthwhile, she said. “It’s a bit of a no-brainer.”
This sounds reasonable, especially considering that everyone is advised to stay at home as much as possible, meaning less exposure to sunlight. But such advice should come with caveats. For one, as with any pre-hormone someone ingests, taking excessive vitamin D can be dangerous. And most people will probably see no benefit from taking vitamin D, unless they’re deficient to begin with. Any benefit would most likely be in mitigating the severity of disease—not preventing it. That means taking vitamin D is no excuse to ease up on social distancing and masks (sorry).
Looking to a supplement to combat COVID-19 also risks distracting from the heart of the problem. It’s true that darker-skinned people are more likely to have a vitamin-D deficiency, in part because melanin in the skin mitigates the absorption of sunlight needed to convert the hormone to its active form. But racial disparities in COVID-19 infections have been far more dramatic than this effect could even potentially account for. Black Americans constitute 13 percent of the United States population, but 24 percent of deaths (where race is known). In England, black people are more than four times more likely to die of COVID-19 than white people are. The differences in disease burden we see around the world will not be explained by any single compound, nor will they be addressed by one.