This paper was revised on January 24, 2019
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 across 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 or October by 0.8 points on a 10-point scale. A second, complementary study employs a separate data set to study flu incidence in New York State counties. The results are strongly in accord. Remarkably, the national results are driven almost entirely by the severe H1N1 epidemic in fall 2009. That year the flu epidemic was intense, and it began early, so that September-October sunlight could play a major protective role. We also compare sunlight protection to protection produced by vitamin D supplementation in randomized trials. The sunlight effect was far greater. A plausible explanation is that exposure to sunlight is far broader, and sufficient to provide herd immunity.