What it is. This meta-analysis pooled 43 randomized trials with 48,488 participants to test whether vitamin D supplementation reduces acute respiratory infections. It updated earlier work with a much larger evidence base.
What it found. Vitamin D produced a small but statistically significant reduction in the risk of having at least one acute respiratory infection. The overall odds ratio was 0.92, which means the effect was real but modest. The clearest pattern appeared with daily dosing, especially 400-1,000 IU per day for up to a year. Large intermittent bolus doses did not help.
Why Vitamin D is relevant. Vitamin D affects immune function, including antimicrobial peptides and inflammatory signaling. Deficiency is common in winter at northern latitudes, when respiratory infections also rise. That overlap makes vitamin D biologically plausible, but it also makes hype easy.
What surprised researchers. In this analysis, benefit was not limited only to people who were deficient at baseline. That does not mean baseline status is irrelevant; it means the protective signal was small and spread across groups rather than dramatic only in the deficient.
How strong is it? Randomized-trial meta-analysis is a strong design, but the effect size is small and trials varied in dose, duration, population and baseline vitamin D status. It does not support vitamin D as a stand-alone immune shield. It supports vitamin D as a sensible correction of a common low-intake problem, with possible modest infection benefit.
Practical take. Daily modest dosing is the pattern that looks best. Very large occasional doses are not the answer. For adults, common maintenance doses around 800-2,000 IU/day are often used, but individual needs depend on sun exposure, body size, diet, skin tone, age and blood levels.
WHO should pay attention. People in northern climates, people who cover most skin, older adults, people with darker skin at high latitudes, and those who rarely eat vitamin-D-rich foods are more likely to run low. Testing is useful when deficiency is suspected or dosing needs to be individualized. More is not automatically better; vitamin D is fat-soluble, and chronic excessive dosing can raise calcium too much.
Bottom line. Vitamin D is worth getting right, especially in winter, but expectations should stay realistic. It may reduce respiratory infections a little; it does not replace sleep, vaccination, hand hygiene or basic nutrition.