A large randomized clinical trial has found that high-dose vitamin D3 supplementation does not significantly reduce the severity of acute COVID-19 infection or healthcare utilization, although it may have a modest effect on long COVID symptoms. The findings have been published in The Journal of Nutrition.
Vitamin D has long been investigated for its immunomodulatory effects, including its role in both innate and adaptive immune responses. While early observational studies suggested a potential protective effect against respiratory infections, evidence regarding its role in COVID-19 has remained inconsistent. To address this, researchers conducted the Vitamin D for COVID-19 (VIVID) Trial, a multicenter, randomized, placebo-controlled study designed to evaluate whether high-dose vitamin D3 could improve clinical outcomes in individuals recently diagnosed with COVID-19 and reduce transmission among household contacts.
The trial enrolled 1,747 adults with confirmed COVID-19 and 277 household contacts from the United States and Mongolia. Participants were randomly assigned to receive either high-dose vitamin D3 or placebo for four weeks. The dosing regimen included an initial loading dose of 9,600 IU daily for two days, followed by 3,200 IU daily. The U.S. arm of the study was conducted between December 2020 and September 2022, while the Mongolia arm took place between September 2021 and April 2022. On average, treatment was initiated three days after diagnosis, reflecting a real-world outpatient setting.
To ensure balance between groups, researchers used stratified randomization and statistical weighting to account for key variables, including age, sex, body mass index, race or ethnicity, and vaccination status. The results showed no statistically significant differences between the vitamin D and placebo groups in healthcare utilization outcomes, such as hospitalizations, outpatient visits, emergency care, or mortality during the four-week follow-up period. Symptom severity also did not differ meaningfully between the groups. In addition, vitamin D supplementation did not reduce the risk of SARS-CoV-2 transmission among household contacts.
However, exploratory analyses suggested a potential benefit in reducing persistent symptoms associated with long COVID. At eight weeks following infection, 21% of participants in the vitamin D group reported at least one ongoing symptom, compared with 25% in the placebo group. Although this difference approached statistical significance, it did not meet conventional thresholds and should be interpreted with caution. Long COVID encompasses a range of post-acute sequelae, including fatigue, shortness of breath, and cognitive impairment, which continue to pose a significant clinical burden.
The current findings indicate that high-dose vitamin D3 supplementation does not provide clinically meaningful benefits in reducing acute COVID-19 severity or transmission risk. However, the observed trend toward a reduction in long COVID symptoms highlights a potential area for further investigation. Researchers emphasize that larger studies with longer follow-up are needed to determine whether vitamin D may have a role in reducing long-term outcomes following COVID-19 infection.
References
- Ganmaa D, Cook KA, Khudyakov P, Enkhjargal D, Bilegtsaikhan T, Mayer KH, et al. A Randomized Trial of Vitamin D Supplementation and COVID-19 Clinical Outcomes and Long COVID: The Vitamin D for COVID-19 Trial. The Journal of Nutrition. 2026 Mar 12;0(0). doi:10.1016/j.tjnut.2026.101398 PubMed PMID: 41826107.