Meta analysis of 11 placebo-controlled studies of 5660 patients. Vitamin D showed a protective effect against RTI (OR 0.64 [0.49-0.84]).
The protective effect was larger in studies using once-daily dosing compared to bolus doses (OR = 0.51 vs OR = 0.86,
p = 0.01). There was some evidence that results may have been influenced by publication bias.
Bergman et al., 1/1/2013, peer-reviewed, 4 authors.