Vitamin D and COVID-19
Early in the pandemic, vitamin D deficiency correlated strongly with worse COVID outcomes in observational cohorts. Randomised trials that followed produced a more modest picture — reasonable to correct deficiency, weak evidence that supplementation reduces severity in already-sufficient adults.
The observational signal
Dozens of cohort studies from 2020–2021 found deficient adults (25(OH)D < 20 ng/mL) had higher rates of hospitalisation, ICU admission, and death. The effect held after adjusting for age, BMI, and comorbidities in many cohorts but not all. The signal is real; the question is whether deficiency causes worse outcomes or simply flags generally poor health (obesity, chronic disease, indoor lifestyle) that itself drives severity.
Randomised trial results
- CORONAVIT (UK, 2022) — 6,200 adults; 800 IU/day or 3,200 IU/day vs no offer. No reduction in COVID-19 or acute respiratory infection.
- CovidVIT-D and successors — Spanish and Argentine RCTs of calcifediol (25(OH)D) in hospitalised patients suggested benefit; larger CARED-TRIAL was equivocal.
- Cochrane 2023 review — no evidence supplementation prevents infection or reduces mortality; low-certainty evidence for hospital length of stay.
Practical takeaway
- If your 25(OH)D is < 20 ng/mL, correct it — for many good reasons, not just COVID.
- If your 25(OH)D is > 30 ng/mL, adding more won't reduce COVID risk based on current trials.
- Vaccination and, when indicated, antivirals remain the evidence-based interventions for COVID.