A large randomized trial from Mass General Brigham has delivered a surprising twist in the search for COVID-19 treatments. Credit: Stock

Vitamin D didn’t blunt COVID’s initial impact—but it may hold a surprising clue in the fight against long COVID.

A large randomized study led by researchers at Mass General Brigham found that taking high doses of vitamin D3 did not lessen the severity of COVID-19 infections. However, the results suggest the supplement could play a role in long COVID outcomes. The findings were 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 VIVID Trial Design and Participants

Vitamin D has long been studied for its potential effects on immune health, though research related to COVID-19 has produced inconsistent results. To better understand whether supplementation could help, scientists launched the Vitamin D for COVID-19 (VIVID) Trial. The study carefully evaluated the impact of high-dose vitamin D3 supplements among people recently diagnosed with COVID-19, as well as members of their households.

The trial included participants from 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 receive either vitamin D3 or a placebo for four weeks. The supplementation regimen consisted of vitamin D3 (9,600 IU/day for two days followed by 3,200 IU/day).

The U.S. portion of the trial ran from December 2020 through September 2022. The Mongolia study took place between September 2021 and April 2022. On average, participants began taking vitamin D or a placebo about three days after receiving a positive COVID-19 test result.

Ensuring Balanced Study Groups

Lead authors Davaasambuu Ganmaa and Kaitlyn Cook worked alongside Manson and the research team to ensure the study groups were comparable. The researchers used stratified randomization and statistical weighting to balance factors that could influence COVID-19 outcomes (including age, sex, body mass index, race/ethnicity and COVID-19 vaccination status).

No Change in COVID-19 Severity or Transmission

After four weeks, the study found no difference between the vitamin D and placebo groups in terms of healthcare use or death. Healthcare utilization included hospitalizations, clinic visits either in person or virtual, and emergency department visits. Researchers also saw no meaningful difference in symptom severity.

In addition, vitamin D supplementation did not lower the likelihood that household contacts would become infected with COVID-19.

Possible Signal for Reduced Long COVID Symptoms

Although vitamin D did not improve short-term COVID-19 outcomes, researchers noticed a possible connection with long COVID when they looked at participants who consistently followed the supplement regimen. These individuals appeared somewhat less likely to report lingering symptoms eight weeks after infection.

Among those taking vitamin D, 21% reported at least one ongoing 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,” said Manson. “We hope to conduct further research in larger populations on whether long-term vitamin D supplementation reduces the risks and severity of long COVID.”

Reference: “A Randomized Trial of Vitamin D Supplementation and COVID-19 Clinical Outcomes and Long COVID: The Vitamin D for COVID-19 Trial” by Davaasambuu Ganmaa, Kaitlyn A Cook, Polyna Khudyakov, Dorjbal Enkhjargal, Tsolmon Bilegtsaikhan, Kenneth H Mayer, Allison Clar, Michael Rueschman, Raji Balasubramanian, Aditi Hazra, Howard D Sesso, Valerie E Stone, Patricia Copeland, Georgina Friedenberg, Douglas C Smith, Quanhong Lei, Todd Lee, Emily G McDonald, Tserenkhuu Enkhtsetseg, Erdenebaatar Sumiya and JoAnn E Manson, 12 March 2026, The Journal of Nutrition.
DOI: 10.1016/j.tjnut.2026.101398

Authorship: In addition to Manson and Ganmaa, Mass General Brigham authors include Allison Clar, Michael Rueschman, Aditi Hazra, Howard D. Sesso, Valerie E. Stone, Patricia Copeland and Georgina Friedenberg. Additional authors include Cook, Polyna Khudyakov, Dorjbal Enkhjargal, Tsolmon Bilegtsaikhan, Kenneth H. Mayer, Raji Balasubramanian, Douglas C. Smith, Quanhong Lei, Todd Lee, Emily G. McDonald, Tserenkhuu Enkhtsetseg, Erdenebaatar Sumiya, Yansanjav Narankhuu, Myagmarsuren Erdenetuya, Dalkh Tserendagva, Rikard Landberg, Niclas Roxhed, and Susanne Rautiainen.

Disclosures: Roxhed is a founder and shareholder of Capitainer AB, a company commercializing the blood collection devices used in this study. All other authors declare no conflicts of interest.

Funding: The study received anonymous foundation support and philanthropic support from Jon Sabes of Minneapolis, Minn. The authors also acknowledge support from the Tishcon Corporation, which donated the vitamin D and placebo study capsules; Takeda; and Capitainer cards. The authors have not declared a specific grant for this research from any funding agency in the public, commercial, or nonprofit sectors.

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