Freakenomics: What Makes Flu Seasonal?

October 16, 2009 , 11:41 am

Just as flu season gathers force here in the northern hemisphere, it’s petering out in the southern half of the globe. No matter where you are, you’re more susceptible to the flu in the winter months. Even if, let’s say, some research physicians expose you to live flu virus in the middle of summer, you’re still less likely to get sick than if the same doctors hit you with the same virus in the dead of winter. Why? One big risk factor for flu infection is a lack of vitamin D. We naturally produce vitamin D when we’re exposed to sunlight, and as the days shorten in the winter, we produce less and less of it. That led these researchers to think that flu epidemics could be a sign of widespread vitamin D deficiency, due in part to decreased solar radiation. So it seems darkness makes the flu go ’round. (HT: Jason Tetro)


http://freakonomics.blogs.nytimes.com/2009/10/16/what-makes-flu-seasonal/|What Makes Flu Seasonal?>

Epidemiol Infect. 2006 Dec;134(6):1129-40. Epub 2006 Sep 7

Epidemic influenza and vitamin D Cannell JJ, Vieth R, Umhau JC, Holick MF, Grant WB, Madronich S, Garland CF, Giovannucci E.
Atascadero State Hospital, 10333 El Camino Real, Atascadero, CA 93422, USA. jcannell@dmhash.state.ca.us

In 1981, R. Edgar Hope-Simpson proposed that a 'seasonal stimulus' intimately associated with solar radiation explained the remarkable seasonality of epidemic influenza. Solar radiation triggers robust seasonal vitamin D production in the skin; vitamin D deficiency is common in the winter, and activated vitamin D, 1,25(OH)2D, a steroid hormone, has profound effects on human immunity. 1,25(OH)2D acts as an immune system modulator, preventing excessive expression of inflammatory cytokines and increasing the 'oxidative burst' potential of macrophages. Perhaps most importantly, it dramatically stimulates the expression of potent anti-microbial peptides, which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract where they play a major role in protecting the lung from infection. Volunteers inoculated with live attenuated influenza virus are more likely to develop fever and serological evidence of an immune response in the winter. Vitamin D deficiency predisposes children to respiratory infections. Ultraviolet radiation (either from artificial sources or from sunlight) reduces the incidence of viral respiratory infections, as does cod liver oil (which contains vitamin D). An interventional study showed that vitamin D reduces the incidence of respiratory infections in children. We conclude that vitamin D, or lack of it, may be Hope-Simpson's 'seasonal stimulus'.