Some observational studies have suggested that low levels of vitamin D are associated with an increased risk for respiratory tract infections, but prospective, randomized data with adequate power to test that are lacking.
There are biological reasons to suspect that vitamin D might be protective in that it increases antimicrobial peptides in the respiratory epithelium.
A recent trial by Aglipay and colleagues was conducted in Toronto to determine whether daily oral supplementation with vitamin D at 2000 IU/day or a standard dose of 400 IU/day was effective in reducing viral upper respiratory tract infections (URIs). The children were randomly assigned in a 1:1 allocation during respiratory seasons from 2011 to 2015.
The children were 1-5 years old and were enrolled at eight primary care clinics that were part of a local research network. Children were only enrolled in the trial for one respiratory season, generally running from September through May. The vitamin D was administered as a single drop orally each day.