Background: Vitamin D has mainly been described in the literature beyond its skeletal
functions, including an influence on the immune responses against infections. Observational and
interventional studies have represented evidence that Vitamin D deficiency may cause increased risk
of seasonal influenza and pulmonary tract infection.
Methods: A structured search of EMBASE, Medline, and Web of Science databases was fulfilled to
extract all eligible articles published prior to September 2017.
Results: In this review, our goal is to define the possible mechanisms that link influenza-mediated
immune responses to Vitamin D. Herein, we first briefly describe the role of Vitamin D in the
immune responses and then elucidate three immunological processes that connect Vitamin D to
influenza infection. Finally, we describe randomized controlled trials and observational studies
exploring the effect of Vitamin D supplementation on seasonal influenza infections and vaccinations.
Conclusion: Our literature review suggests that treatment of influenza-infected individuals with
Vitamin D supplements or cathelicidin-derived agents may provide appreciable protection against
natural influenza infection. Moreover, Vitamin D given at appropriate doses may facilitate protection
against seasonal flu.