The effect of different doses of vitamin D for prevention of wintertime respiratory tract infections (RTI) in pre-school children has been assessed in a randomised trial.¹ A recent meta-analysis of 25 trials in adults and children found that vitamin D had a modest effect of preventing acute RTIs among all participants (adjusted odds ratio 0.88, 95% CI 0.81 to 0.96; number-needed-to-treat 33), with a more pronounced effect in those with low vitamin D status (baseline circulating 25-hydroxyvitamin D …