In the past, many theories have been put forward to explain the seasonal flu flux, but explanations such as cold air and the tendency of people to group together "remain astonishingly superficial and full of inconsistencies," said Dr. Scott Dowell, director of the Global Disease Protection Program at the Centers for Disease Control and Prevention in Atlanta.
Theories about a chill causing the disease's prevalence is upended by evidence from tropical locations, where flu remains common and follows a similar seasonal pattern to its cold-climate counterpart. The grouping theory is debunked by the fact that certain groups of people are stuck in small spaces together year round, with no greater likelihood of contracting flu than anyone else.
Now, the Harvard-University-led team is investigating whether inadequate sun exposure during the winter may open people up to infection, since exposure to ultraviolet B radiation (UVB) radiation from the sun causes vitamin D production in the skin. If the lack of vitamin D and increased flu cases in the winter are connected, it could have a significant impact on public health, as an average of 36,000 people die from flu in the United States every winter, primarily the elderly or the very young.
R. Edgar Hope-Simpson published the first paper that identified a link between flu epidemics and the winter solstice -- usually indentified as the start of winter and the shortest day of the year -- in 1981, despite having no formal training in the field of epidemiology. Simpson noted that flu infections spiked just before and after the winter solstice, and theorized that solar radiation might cause a sort of "seasonal stimulus" in the virus, the host or both, although he could not identify the stimulus.
Simpson's work was largely ignored, according to Dr. John Cannell, a psychiatrist at the Atascadero State Hospital in California. However, Cannel and his Harvard colleagues suggest the stimulus to which Simpson referred may be vitamin D. Cannell began investigating the possibility when a flu outbreak hit Atascadero in April of 2005 and all the wards surrounding his were infected, Cannell's patients were not. All of his patients, he said, were taking high daily doses of vitamin D.
During the winter, people are outdoors less often and the skin has less opportunity to produce vitamin D, and the atmosphere during that season is adept at blocking UVB radiation. This is why some health experts warn that Americans may not be getting sufficient vitamin D, especially with the resurgence of the vitamin-deficiency-related bone disorder known as rickets.
In the report -- published in the December issue of Epidemiology and Infection -- the researchers posit that the vitamin D stimulated by sunlight may, in turn, cause the body to produce the infection-fighting peptide cathelcidin. No studies been conducted that to show whether cathelcidin effects influenza, but previous studies in the March issue of Science have shown it attacks a range of fungi, viruses and bacteria, including the bacteria that causes tuberculosis.
The tropical evidence that upsets the chill theory does not preclude the vitamin D theory, as Cannell and colleagues point out, as studies show that vitamin D deficiencies have even been recorded in equatorial locations. Additionally, a 2003 analysis of flu cases found they were greatest during the rainy season, when there is a significant cloud cover and reduced sun exposure.
Despite the evidence offered by Cannell and colleagues, some members of the scientific community remain skeptical about the theory.
"They have manipulated the literature -- some of it very bad literature -- to prove their points," said Dr. James Cherry, a pediatric infectious disease specialist at UCLA's David Geffen School of Medicine. However, he added, "The hypothesis should be easy to prove or disprove with a controlled, blinded study."
Cannell, for his part, said he takes more than twice the recommended daily dose of vitamin D during winter months and reports he rarely gets sick.