Sunlight and Protection Against Influenza

Recent medical literature suggests that vitamin D supplementation protects against acute respiratory tract infection. Humans exposed to sunlight produce vitamin D directly. This paper investigates how differences in sunlight, as measured over several years within states and during the same calendar month, affect influenza incidence. We find that sunlight strongly protects against influenza. This relationship is driven by sunlight in late summer and early fall, when there are sufficient quantities of both sunlight and influenza activity. A 10% increase in relative sunlight decreases the influenza index in September by 3 points on a 10-point scale. This effect is far greater than the effect of vitamin D supplementation in randomized trials, a differential due to broad exposure to sunlight, hence herd immunity. We also find suggestive evidence, consistent with herd immunity theory, that the protective sunlight effect is strongest with a middle level of population density.