In 2017 researchers set out to understand the role of vitamin D in management of acute respiratory disorders in this systemic review and meta-analysis. The authors included 25 eligible randomised controlled trials (total 11,321 subjects). Protective effects of daily or weekly vitamin D supplementation on the risk of development of uncomplicated acute respiratory tract infections was seen. The benefit was seen in all groups, however, was most pronounced in subjects with severe vitamin D deficiency at baseline.
Martineau A R, Jolliffe D A, Hooper R L, Greenberg L, Aloia J F, Bergman P et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data BMJ 2017; 356 :i6583 doi:10.1136/bmj.i6583