Does vitamin D prevent colds and flu?

Vitamin D supplementation has a small but real protective effect against respiratory infections overall, though the benefit is modest and uneven.

A 2025 updated meta-analysis of 43 randomised trials found a statistically significant reduction in acute respiratory infection risk with vitamin D supplementation 2, consistent with an earlier 2021 meta-analysis of 37 trials that found an odds ratio of 0.92 — roughly an 8% lower odds of infection 2. The 2017 individual-participant-data analysis (25 trials, ~11,000 people) found the benefit was concentrated in people who were deficient at baseline and who took daily or weekly doses rather than large infrequent boluses 3. People with adequate vitamin D levels at the start showed little to no benefit.

For older adults specifically, a large randomised trial (VITAL, ~15,000 people) testing 2,000 IU/day found no significant reduction in upper respiratory infections 5. A 2026 Cochrane review in children under five is the most recent word on that age group 4.

The short version: vitamin D is unlikely to prevent colds and flu if your levels are already sufficient. The modest benefit seen across trials is largely driven by people correcting a genuine deficiency.