Vitamin D has long been studied for its role in immune health, and early in the pandemic many researchers wondered whether supplements might help reduce the severity of COVID-19.
A large randomized clinical trial now offers one of the clearest answers so far: high-dose vitamin D supplements did not reduce how sick people became during COVID infections. However, researchers observed a possible signal that vitamin D may be linked to a slightly lower risk of long COVID symptoms.
The findings come from the Vitamin D for COVID-19 (VIVID) Trial, led by researchers at Mass General Brigham and published in The Journal of Nutrition.
“There’s been tremendous interest in whether vitamin D supplements can be of benefit in COVID, and this is one of the largest and most rigorous randomized trials on the subject,” said senior author JoAnn Manson, MD, DrPH, of the Mass General Brigham Department of Medicine. “While we didn’t find that high-dose vitamin D reduced COVID severity or hospitalizations, we observed a promising signal for long COVID that merits additional research.”
The study enrolled 1,747 adults who had recently tested positive for COVID-19 and 277 members of their households. Participants were randomly assigned to receive either vitamin D3 supplements or placebo pills for four weeks.
Those assigned to vitamin D received a relatively high dose: 9,600 international units daily for the first two days, followed by 3,200 IU per day for the rest of the study period. Participants began taking the supplements a median of three days after testing positive for COVID.
Researchers then tracked a range of outcomes, including symptom severity, health care visits, hospitalizations and whether household members later became infected.
The results showed no meaningful difference between the vitamin D and placebo groups in how severe people’s infections became. Rates of hospitalizations, clinic visits and other medical care were similar in both groups.
Vitamin D supplementation also did not appear to reduce the likelihood that household contacts would contract COVID.
However, researchers did see a potential difference when they looked at longer-term symptoms. Among participants who closely followed the vitamin regimen, 21% reported at least one persistent symptom eight weeks after infection, compared with 25% in the placebo group.
The difference was small and only borderline statistically significant, meaning it could have occurred by chance. Still, the researchers say the signal is intriguing and warrants further investigation.
“Long COVID, which can include symptoms of fatigue, shortness of breath, brain fog, other cognitive challenges and more, continues to significantly impact people’s lives,” Manson said. “We hope to conduct further research in larger populations on whether long-term vitamin D supplementation reduces the risks and severity of long COVID.”
Vitamin D has been widely studied for its potential role in immune function, inflammation and respiratory infections. Previous research has produced mixed results, and the VIVID trial was designed to provide more rigorous evidence by testing supplementation in a randomized, placebo-controlled setting.
Because the study involved relatively high supplement doses and focused specifically on people recently diagnosed with COVID, the findings do not necessarily apply to routine vitamin D supplementation in the general population.
Still, the results add to a growing body of research examining how nutrition and micronutrients may influence immune responses and recovery after viral infections.
Researchers say further studies will be needed to determine whether vitamin D could play a role in preventing or reducing long COVID symptoms, and whether similar results would appear in larger or longer-term trials.
The study received philanthropic funding and support from several organizations, including donations of vitamin D and placebo capsules from supplement manufacturers. Researchers reported no major conflicts of interest among the study authors.