Vitamin D supplementation neither protects against SARS-CoV-2 infection nor reduces the severity of COVID-19, two clinical trials suggest, contradicting a hypothesis that arose early in the pandemic.

The studies were launched in 2020 based on previous findings that vitamin D seems to boost the immune system. Queen Mary University researcher David Martineau, a coinvestigator of one of the clinical trials, told The Scientist at the time that he hoped the results would carry over to the coronavirus that had only recently swept the globe. But the results of the trials, both published Wednesday (September 7) in BMJ, may put that idea to rest.

One of them, which ran from November 2020 to June 2021 and included 34,601 adult participants from Norway, tested a low dose of vitamin D (in the form of a cod liver oil supplement) and did not find any association between supplementation and the number of SARS-CoV-2 infections (or any other acute respiratory infections, for that matter), or the severity of COVID-19 among those who caught it. The other trial—the one Martineau helped conduct—took place between December 2020 and June 2021 and involved 6,200 adults in the UK. That study measured the effects of both low and high doses of vitamin D, finding that neither was associated with a reduction in COVID-19 cases or other respiratory tract infections.

“The major takeaway is that for people in general, a vitamin D supplement did not prevent COVID-19, serious COVID-19 or symptomatic acute respiratory tract infections,” Oslo University Hospital microbiologist Arne Søraas, coauthor of the Norwegian study, tells CNN, adding that the UK study’s “result is supporting our findings and that is the key takeaway too: Neither study found any preventive effect after supplementation with vitamin D.”

Vitamin D has in the past been suggested as a preventive measure for myriad health conditions—such as cancer, cognitive decline, stroke, and more recently, bone fractures—but many of its potential use cases have been debunked in recent years.

See “Vitamin D on Trial

However, in an editorial on the two clinical trials also published September 7 in BMJ, Karolinska Institutet researcher Peter Bergman, who didn’t work on either trial, notes that vaccine uptake among participants may have masked the potential protective effects of vitamin D, since any measurable effect would have been smaller than that of the vaccines. For example, he notes that 1.2 percent of the UK study’s participants had been vaccinated at the start of the trial, while 89 percent had received at least one dose by its end. Still, he recommends that clinicians promote vaccines as the best preventive measure to their patients, and that they don’t offer vitamin D supplements to otherwise healthy people with normal levels in their system.

“As a medical doctor I see much speculation that very high doses of vitamin D could have beneficial effects for a range of different diseases,” Søraas tells CNN, “but I would encourage everybody to follow science-based government recommendations for all nutrients.”

Clarification (September 12): The subheading of this article originally stated that two clinical trials refuted the idea vitamin D prevents COVID-19. It has been updated to say the clinical trials don’t support vitamin D’s role.