Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren.

Randomized, placebo-controlled trial in Japanese schoolchildren aged 6 to 15 years recruited from 12 hospitals and 8 private practices. Participants included children with and without underlying disease, including asthma, and baseline characteristics were reported as similar between groups.

Oral vitamin D3 (Status D3), 1200 IU/day total, given as 3 tablets twice daily from 1 December 2008 to 31 March 2009. Each tablet contained 200 IU vitamin D3 or placebo.

Vitamin D3 supplementation reduced seasonal influenza A incidence versus placebo. Influenza A occurred in 18/167 children (10.8%) in the vitamin D3 group and 31/167 (18.6%) in the placebo group, corresponding to RR 0.58 (95% CI, 0.34 to 0.99; P = 0.04). Benefits were larger in children not taking other vitamin D supplements (RR 0.36; 95% CI, 0.17 to 0.79; P = 0.006) and in those who started nursery school at age 3 years or older (RR 0.36; 95% CI, 0.17 to 0.78; P = 0.005). Asthma attacks were also reduced among children with asthma (2 vs 12; RR 0.17; 95% CI, 0.04 to 0.73; P = 0.006), while influenza B and other secondary outcomes were not significantly different. No adverse events were reported.

Follow-up was limited to one winter season, so durability and reproducibility are uncertain. The trial was conducted in Japanese schoolchildren, which limits generalizability. Some subgroup findings were exploratory, and detailed arm-specific baseline characteristics were not fully reported.