A large randomized clinical trial by researchers at Mass General Brigham tested high-dose vitamin D3 in adults with newly diagnosed COVID-19 infection.[1][2][3] Enrolled 1747 patients in the USA and Mongolia and 277 of their household contacts who received either vitamin D3 (9600 IU/day for 2 days, then 3200 IU/day) or placebo for 4 weeks.[1][2][3] The median time from a positive test to the start of supplementation was 3 days.[1][2] High-dose vitamin D3 did not reduce the severity of COVID-19 infection, hospital visits, hospitalizations, or mortality compared with placebo.[1][2][3] They also did not reduce the rate of infection among household contacts.[1] However, the study revealed a trend that consistent use of vitamin D3 may reduce the incidence of long-term COVID after 8 weeks (OR 0.78; 95% CI: 0.59-1.03), which warrants further research.[3] The results were published in The Journal of Nutrition.[1][2]