Influenza Introduction

 Influenza is more common in winter due in large measure to lower solar ultraviolet-B doses and vitamin D levels in winter.

There is strong evidence that vitamin D reduces the risk of developing and dying from influenza.  

The first clue was that influenza incidence rates peak in winter, a time when solar ultraviolet-B doses and, thus, vitamin D production rates, are lowest.  

Vitamin D strengthens the innate immune system by producing compounds such as cathelicidin and defensins, which have antimicrobial actions.  

A randomized controlled trial using 1200 IU/d of vitamin D for school children in Japan found a 64% reduction in type A influenza cases compared with controls taking a placebo1.  

The recent H1N1 “swine flu” was type A, and those in groups known to have lower vitamin D levels, such as pregnant women, had much higher rates of hospitalization than others.  

Vitamin D also reduces the risk of death from influenza by reducing the production of proinflammatory cytokines and fighting the bacteria that lead to pneumonia.

That vitamin D might reduce the risk of influenza was proposed by Cannell et al2 based on a consideration of the seasonal variation of influenza infection rate, which peaks in winter, a time when solar ultraviolet-B (UVB) doses are low to absent at high latitudes and serum 25-hydroxyvitamin D [25(OH)D] levels are lowest.

In the early 1980s, Edgar Hope-Simpson3 had discussed the global seasonality of influenza activity, recognizing that it was related to the seasonal variation of solar radiation, but ruling out UV rays, which he considered lethal to the virus, since UV doses did not explain the seasonal variations in the tropics.

Page last edited: 04 June 2011


  1. Urashima, M. Segawa, T. Okazaki, M. Kurihara, M. Wada, Y. Ida, H. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010 May; 91 (5): 1255-60.
  2. Cannell, J. J. Vieth, R. Umhau, J. C. Holick, M. F. Grant, W. B. Madronich, S. Garland, C. F. Giovannucci, E. Epidemic influenza and vitamin D. Epidemiol Infect. 2006 Dec; 134 (6): 1129-40.
  3. Hope-Simpson, R. E. The role of season in the epidemiology of influenza. J Hyg (Lond). 1981 Feb; 86 (1): 35-47.