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.
The researchers 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 group immunity.
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