Some researchers, like Robert Edgar Hope Simpson, link solar radiation to the seasonality of influenza. Recent evidence suggests this ‘seasonal stimulus’ may be seasonal impairments of the antimicrobial peptide (AMPs) systems crucial to innate immunity, also called ‘first line of defense’, and it works like the heuristic detection of the anti-virus program of your computer. The innate immunity is regulated by levels of vitamin D and those are low in times of winter. This mode of action suggests that vitamin D may be regarded as the ‘antibiotic vitamin’.
Your innate immunity acts by rapidly and irreversibly damaging the lipoprotein membranes of microbial targets, including enveloped viruses, like influenza.
 only very recently. Because humans obtain most vitamin D from sun exposure and not from diet, a varying percentage of the population is vitamin D deficient, at any time, during any season, at any latitude, although the percentage is higher in the winter, in the aged, in the obese, in the sun-deprived, in the dark-skinned, and in more poleward populations. However, seasonal variation of vitamin D levels even occur around the equator and widespread vitamin D deficiency can occur at equatorial latitudes, probably due to sun avoidance, rainy seasons, and air pollution. Also, pregnant woman are at risk for influenza because they have low levels of vitamin D.
This data suggests that the virus may be circulating during the entire year but that it only is able to infect when levels of vitamin D are low. Maintaining high levels of vitamin D in winter are therefore crucial in thwarting a possible lethal infection of influenza. But even at 26°N there is about four times more UVB during the summer than during the winter which suggests that you need to take your daily intake of vitamin D as a supplement because sunlight alone cannot create adequate levels.
 Hope-Simpson et al: A new concept of the epidemic process of influenza A virus in Epidemiology and Infection - 1987
 Cannell et al: Epidemic influenza and vitamin D in Epidemiology and Infection - 2006
 Wang et al: Cutting edge: 1,25-dihydroxyvitamin D3 is a direct inducer of antimicrobial peptide gene expression in Journal of Immunology - 2004
 Cannell et al: Diagnosis and treatment of vitamin D deficiency in Expert Opinion on Pharmacotherapy - 2008
 Grant et al: Pregnant women are at increased risk for severe A influenza because they have low serum 25-hydroxyvitamin D levels in Critical Care Medicin - 2010
 Juzeniene et al: The seasonality of pandemic and non-pandemic influenzas: the roles of solar radiation and vitamin D in International Journal of Infectious Diseases - 2010