Researchers are still trying to understand why some people develop Long COVID: lasting symptoms of the disease that can linger for weeks, months, or even years after an infection. People’s genetic makeup and their environment—including stress, sleep, and diet—likely play a role, and a new study explores whether vitamin D levels might, too. Previous studies have suggested that vitamin D’s role in reducing inflammation and boosting production of immune defenses might help ease COVID symptoms.
An international group of researchers led by Dr. JoAnn Manson of Brigham and Women’s Hospital and Harvard Medical School studied more than 1,700 people in the U.S. and Mongolia who were recently diagnosed with COVID. They randomly assigned them to receive either a high-dose regimen of vitamin D or a placebo. Those in the vitamin D group took 9,600 IU per day for two days, then 3,200 IU daily for four weeks. (The average person who takes a vitamin D supplement takes about 1,000 to 2,000 IU per day.)
People in the vitamin D group did not have statistically significant differences in the severity of their symptoms or in the rate of hospitalization for COVID. Manson says the results might reflect the fact that people started taking the vitamin D supplements later in their infection course, after diagnosis and symptoms began, and that the supplement takes time to affect the immune system. “It takes a while for vitamin D to stabilize in the blood, and to get blood levels up to a physiologically active point,” says Manson.
The study also hinted that people taking the supplements seemed to have a lower risk of developing Long COVID about eight weeks after their diagnosis. The results were not statistically significant but trended in that direction, and longer follow-up is needed. “There were suggestive benefits and promising signals, and these findings suggest that more research is warranted on this question,” says Manson.
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Manson is hoping to conduct additional studies in which people start taking vitamin D at more standard levels—about 2,000 IU a day—during the fall and winter when infections like flu, COVID, and other respiratory diseases rise, but before people get infected. She expects that strategy may lead to stronger benefits in reducing the severity of COVID symptoms and in lowering the risk of Long COVID. That’s based on her previous work studying vitamin D and its effect on lowering risk of autoimmune disease and the spread of advanced cancer.
“I would suspect that a dose of 2,000 IU taken weeks and weeks during the viral season in the fall and winter, and maybe boosting at the time of infection or first symptoms to 3,000 IU, would be promising in reducing Long COVID,” she speculates. “So you’d be at a steady state and would have reached an equilibrium in terms of having active vitamin D in your system to help reduce the severity of symptoms at the time of acute infection. Then if you add a little more—another 1,000 IUs a day—it might reduce Long COVID.”
That strategy would have to be tested and validated in a rigorous study, but Manson says it’s a reasonable hypothesis to explore further given what’s now known about vitamin D and its ability to support the immune system. If future studies bear out these findings, she says vitamin D supplements could become a useful tool in helping people better control the symptoms of other viral infections as well. At such doses, vitamin D is relatively safe, and in her studies, Manson didn’t identify any serious adverse events related to the supplement. Extremely high doses of vitamin D taken over long periods of time can lead to abnormal buildup of calcium and kidney stones, as well as irregular heartbeat, but the doses that could potentially be helpful in controlling viral infections are much lower, and people would only be taking them seasonally, when the risk of infections is higher. “This is the first trial looking at this question,” she says, “and further research is warranted.”