Vitamin D and Influenza
Vitamin D's role in respiratory infection prevention is one of its better-supported applications. It won't replace the flu vaccine, but for people who won't or can't get vaccinated, or as an adjunct, vitamin D at physiologic doses reduces flu and other acute respiratory infection risk in trials.
The Martineau IPD meta-analysis
Martineau 2017 (BMJ) pooled individual participant data from 25 RCTs (10,933 participants) of vitamin D vs placebo. Vitamin D reduced acute respiratory tract infections (all-cause) by ~12% overall, and by ~70% in participants with baseline 25(OH)D < 25 nmol/L. Daily/weekly dosing worked; large bolus doses did not. Updated 2021 IPD analysis (46 trials, 75,541 people) confirmed the daily/weekly benefit.
The Urashima Japanese school children trial
Urashima 2010 randomised 334 Japanese schoolchildren to 1,200 IU/day cholecalciferol vs placebo during a Dec–March flu season. Vitamin D reduced influenza A incidence by 42% (RR 0.58). Effect was more pronounced in children not previously taking vitamin D. This single trial had a large impact on the "vitamin D for flu" discussion.
Mechanism
Cathelicidin — an antimicrobial peptide upregulated by vitamin D — has broad activity against enveloped viruses including influenza. Vitamin D also modulates T-cell responses (favouring Treg over pro-inflammatory Th1/Th17), which may reduce the "cytokine storm" component of severe flu. The seasonal pattern of flu (winter peak, when 25(OH)D is lowest) has long been noted as consistent with a vitamin D role.
Practical guide
- Get the annual flu vaccine — its evidence base for reducing severe flu is far larger than vitamin D's.
- Maintain 25(OH)D ≥ 30 ng/mL year-round with 1,000–2,000 IU/day cholecalciferol.
- Daily dosing preferred; weekly is a reasonable alternative for compliance.
- Do not use one-off high bolus doses for flu — evidence supports daily/weekly, not bolus.
- Vitamin D is a general immune adequacy measure, not a treatment for active flu — for that, oseltamivir/baloxavir taken early is the evidence-based option.