Yes, modestly: a BMJ meta-analysis of 25 trials and nearly 11,000 people found vitamin D supplementation cut the risk of at least one respiratory infection (odds ratio 0.88; about 33 treated to prevent one infection).
The effect concentrates in people starting out deficient, and daily or weekly dosing beats bolus doses. It is a nudge, not a shield - it does not replace sleep, vaccination, or hand hygiene. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Does vitamin D help your immune system?
Yes, modestly - this is one supplement-immunity claim with real trial data behind it. The landmark BMJ individual-participant meta-analysis pooled 25 randomized trials with nearly 11,000 people and found vitamin D supplementation significantly reduced the proportion experiencing at least one acute respiratory infection - adjusted odds ratio 0.88, meaning about 33 people treated to prevent one infection. The effect concentrates where you would expect: the protective signal is strongest in people who start out deficient, and daily or weekly dosing beats big occasional bolus doses. The honest calibration: this is a meaningful nudge, not a shield - a 12% relative reduction leaves most infections untouched, and it does not replace sleep, vaccination, or hand hygiene. Who should act: people with dark winters, indoor lives, darker skin in northern latitudes, or known low levels - a blood test settles it and typical repletion doses are safe and cheap. The move the evidence supports: daily low-dose through the dark months, especially if your level has ever tested low - not mega-doses, which the data does not reward.