Interest in vitamin D supplements has spiked during the pandemic as many people have sought ways to improve their immune health. According to Nielsen data from December, sales for vitamin D supplements increased 41.5 percent year over year. But although more research has emerged suggesting a possible connection between vitamin D deficiencies and cases of covid-19, experts say there is not yet sufficient evidence to support taking supplements to prevent or treat the disease caused by the coronavirus.

“We don’t want to come to premature conclusions or kind of pin our hopes on the wrong mechanism,” said Hana Akselrod, an infectious-disease physician at George Washington University. “On the research side, we want to be as specific as possible about what actually gets people better or protects them from infection.”

Here is what we know so far about vitamin D and covid-19, and why experts are urging people to approach using supplements with caution in the absence of clear data from clinical trials.

Vitamin D deficiencies are “not rare,” said Sabyasachi Sen, a professor of endocrinology and medicine at GWU. According to the National Institutes of Health, groups that may not be getting enough vitamin D include older adults, people who are obese and those with darker skin, such as African Americans and other minorities — populations that have been disproportionately affected by the coronavirus.

In addition to protecting bone health, vitamin D plays an important role in the immune system, Sen said. It is believed to enhance the function of cells, such as T cells, that protect the body from pathogens and may help modulate inflammatory responses if the body is under attack. Lower levels of vitamin D have also been associated with an increased susceptibility to infection, he added.

“Now, what is unknown is whether it’s a cause and effect rather than an association,” he said. And that’s the question researchers studying the effects of vitamin D levels on covid-19 outcomes are aiming to answer.

Studies conducted in the United States and other countries during the pandemic have found correlations between vitamin D deficiencies and both coronavirus infections and more severe symptoms requiring hospitalization.

A study looking at the health records of 489 people in Chicago found that patients who had a vitamin D deficiency in the year before they were tested for covid-19 were 77 percent more likely to test positive than those with normal levels, according to results published in September in JAMA Network Open.

Similarly, a peer-reviewed observational study in Italy looking back at a small group of patients who were hospitalized with acute respiratory failure due to covid-19 found that 81 percent were vitamin D deficient. The researchers noted that patients with severe deficiencies “had a significantly higher mortality risk.”

“We do know that people who have lower blood levels of vitamin D tend to have a higher risk of being infected with covid and having severe covid illness,” said JoAnn Manson, chief of the Division of Preventive Medicine at Brigham and Women’s Hospital and a professor at Harvard Medical School. “But as we say in epidemiology, ‘Correlation doesn’t prove causation.’ We don’t know for sure that the low vitamin D level is causing an increased risk of covid.”

People with vitamin D deficiency are often dealing with other health factors that could affect their chances of contracting the coronavirus and falling seriously ill, said Natasha Chida, an infectious-disease expert and assistant professor of medicine at Johns Hopkins University. Chida added that vitamin D levels typically drop when people are sick with diseases such as covid-19.

“Unless you take into account all those factors and separate all those out and look at just vitamin D, . . . it’s really hard to make any inferences about what vitamin D is doing here,” she said.

There is also no solid evidence that vitamin D supplementation will lower the risk of becoming infected or developing a serious case of covid-19.

Research into whether vitamin D supplements may be beneficial for covid-19 patients has returned mixed results. A small randomized clinical study involving 76 patients hospitalized with covid-19 in Spain reported that early treatments of calcifediol, an activated form of vitamin D that is different from over-the-counter supplements, appeared to reduce the disease’s severity. But a double-blind randomized controlled trial in Brazil with 240 participants, which has not yet been peer-reviewed, found that a single, large dose of vitamin D did not reduce the length of hospital stays or mortality rates among patients with severe covid-19 infections compared with the placebo group.

“There is some biologic plausibility that there could be some benefit,” Chida said. “It’s just that despite years of research into the use of vitamin D in respiratory tract infections, there still hasn’t really been a clear, slam-dunk answer that there’s benefit.”

Chida and other experts, however, are hopeful that an answer may be forthcoming. About 70 clinical trials focused on vitamin D and covid-19 have been registered in a database run by the U.S. National Library of Medicine.

Manson said she recently launched a national trial that will evaluate whether high-dose vitamin D is effective for reducing the severity of illness in people newly diagnosed with covid-19 and whether it can prevent infection in those individuals’ household members.

David Meltzer, the lead author of the Chicago study, said he is also working on two trials in Chicago to see whether vitamin D supplementation can lower the risk of covid-19.

“Whether taking more vitamin D, which presumably would raise levels in your blood, would necessarily lead to a decrease in the risk of catching covid, I think there are lots of good reasons to believe that that’s possible,” said Meltzer, chief of the Section of Hospital Medicine at the University of Chicago. “But we don’t have the randomized trials yet to prove it.”

In the meantime, experts encouraged people who know they are vitamin D deficient to continue treatment. For those who are considering starting to take supplements, first talk to a health-care provider.

“People should be wary of taking mega doses of vitamins or unproven interventions specifically for covid-19, because we don’t have good quality data yet to suggest that this is of any help,” said Akselrod, the GWU infectious-diseases physician.

The recommended daily allowance of vitamin D for most adults is 600 international units (IU), or 800 IU for those 71 and older, according to the NIH Office of Dietary Supplements. People should not take more than 4,000 IU per day, which is considered the daily upper limit for vitamin D. Going over that amount could lead to an increased chance of developing side effects, including a higher level of calcium in the blood, which may cause kidney stones, among other problems.

Aside from supplements, people can improve their daily intake through diet and regular outdoor activity, experts said. Certain foods, such as fatty fish or fortified dairy products and cereals, can be good sources of vitamin D, Manson said. Getting outside for 15 or 20 minutes a day could also be helpful for vitamin D levels and overall health, Akselrod added.

“There are all of the positive confluences around nutrition and outdoor exercise that aren’t just limited to the number of how many units of vitamin D you get every day,” Akselrod said. “And on top of that, people absolutely need to continue all the other safety precautions, like masking and safe distancing and avoiding gatherings, because we’re in the most dangerous phase of the pandemic yet.”