Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data

Key finding

Individual-participant meta-analysis of 25 RCTs (n=10,933) found vitamin D supplementation reduced risk of acute respiratory tract infection, with greatest benefit in baseline-deficient participants and daily/weekly (not bolus) dosing.

Critique

Individual-participant meta-analysis across 25 RCTs is a strong design — it re-analyses raw subject data rather than just pooling summary statistics, which handles heterogeneity better than conventional meta-analysis. The overall effect is modest (NNT ~33) and the pooled estimate hides substantial variation between trials in dose, population, seasonal timing, and baseline vitamin D status. A cleaner read of the data is that vitamin D meaningfully helps people who are deficient; evidence of benefit in already-replete individuals is weaker.

What would be more convincing

A single large pre-registered RCT restricting enrollment to baseline 25(OH)D below 20 ng/mL, using daily (not bolus) dosing, would resolve the heterogeneity question and settle the 'does deficiency matter' framing that the current evidence only hints at.

Reviewed 2026-04-21 · Opinion based on verifiable facts in the published paper.