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Influenza's Aversion to Sunlight

Posted by plosmedicine on 31 Mar 2009 at 00:06 GMT

Author: John Cannell
Position: Staff psychiatrist
Institution: Atascadero State Hospital
Submitted Date: February 27, 2007
Published Date: February 28, 2007
This comment was originally posted as a “Reader Response” on the publication date indicated above. All Reader Responses are now available as comments.

I am impressed with the video models accompanying Colizza et al’s paper. According to their models, influenza is almost completely averse to solar radiation. Hope-Simpson first pointed out this aversion, noting:

“Outbreaks are globally ubiquitous and epidemic loci move smoothly to and fro across the surface of the earth almost every year in a sinuous curve that runs parallel with the midsummer curve of vertical solar radiation, but lags about six months behind it... Latitude alone broadly determines the timing of the epidemics in the annual cycle, a relationship that suggests a rather direct effect of some component of solar radiation acting positively or negatively upon the virus, the human host, or their interaction.”(1)

Influenza's aversion to sunlight is most parsimoniously explained by seasonal changes in innate immunity, especially the profound seasonality of the most immune-enhancing of the human steroid hormones, vitamin D.(2) Aloia and Li-Ng will soon confirm that theory.(3) Presenting data from a blinded interventional trial, their work will provide evidence that susceptibility to influenza is a function of vitamin D intake and that vitamin D is Hope-Simpson’s long lost “seasonal stimulus.”

George Bernard Shaw once said, “Progress is impossible without change, and those who cannot change their minds cannot change anything.” While some preparedness planners will rush to provide vaccines and antivirals, some of us will reread Hope-Simpson’s classic 1992 book(4) on influenza, where he said. “Understanding the mechanism of action (of the seasonal stimulus) may be of critical value in designing prophylaxis against the disease.” (Hope Simpson, 1992, p. 235)


1. Hope-Simpson RE. The role of season in the epidemiology of influenza. Journal of Hygiene 1981; 86: 35-47.

2. Cannell JJ, et al. Epidemic influenza and vitamin D. Epidemiology and Infection 2006; 134: 1129-1140.

3. Aloia J, Li-Ng, M. Vitamin D: colds and flu. Epidemiology and Infection, in press.

4. Hope-Simpson RE. The transmission of epidemic influenza. New York: Plenum Press, 1992.

Competing interests declared: Dr. Cannell heads the educational organization, the Vitamin D Council.