Researchers at Mass General Brigham announced findings from a major clinical trial on March 15, 2026. The study investigated whether high doses of vitamin D3 could alter the course of COVID-19 infections. While the data showed no reduction in acute severity, a potential link to long-term outcomes emerged. This development marks a significant pivot in post-pandemic health research regarding nutritional interventions.
Findings from the large study led by Mass General Brigham suggest scientists should continue investigating whether vitamin D supplements could influence long COVID. The research showed that taking high doses of vitamin D3 did not make COVID-19 infections less severe, but it may have a possible connection to long COVID outcomes. The results were published in The Journal of Nutrition.
There has 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. Senior author JoAnn Manson, MD, DrPH, of the Mass General Brigham Department of Medicine stated the team did not find that high-dose vitamin D reduced COVID severity. She noted they observed a promising signal for long COVID that merits additional research.
Vitamin D is often linked to immune health, though studies examining its role in COVID-19 have produced mixed results. To help clarify its potential benefits, researchers launched the Vitamin D for COVID-19 Trial. The study evaluated whether high dose vitamin D3 supplementation could influence outcomes in people recently diagnosed with COVID-19 and in members of their households.
The trial included participants in both the United States and Mongolia. In total, 1,747 adults who had recently tested positive for COVID-19 and 277 household contacts were randomly assigned to take either vitamin D3 or a placebo daily for four weeks. The supplementation protocol used vitamin D3 (9,600 IU per day for two days followed by 3,200 IU per day).
The United States portion of the trial ran from December 2020 through September 2022, while the Mongolia study took place from September 2021 to April 2022. On average, participants began taking vitamin D supplements or placebo about three days after receiving their positive COVID-19 test. Ensuring balanced study groups involved stratified randomization and statistical weighting to balance factors known to affect COVID-19 outcomes.
Over the four week study period, the researchers found no meaningful difference between the vitamin D and placebo groups in terms of healthcare utilization or death. Healthcare utilization included hospital stays, clinic visits either in person or virtual, and emergency room visits. Symptom severity was also similar in both groups.
High dose vitamin D supplementation also did not lower the chance that household contacts would become infected with COVID-19. When researchers analyzed participants who consistently followed the vitamin D regimen, they noticed a potential signal related to long COVID. These individuals appeared somewhat less likely to report persistent symptoms eight weeks after infection compared with those who received placebo.
Among participants taking vitamin D, 21% reported at least one lingering symptom, compared with 25% in the placebo group. The difference was considered borderline statistically significant. 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 and global healthcare systems.
The authors hope to conduct further research in larger populations on whether long-term vitamin D supplementation reduces the risks and severity of long COVID. The study received anonymous foundation support and philanthropic support from Jon Sabes of Minneapolis. The authors acknowledge support from the Tishcon Corporation, which donated the vitamin D and placebo study capsules.
