BioEd Online

The winter flu season could be the result of our reduced exposure to sunlight, according to a review scheduled to be published in print in Epidemiology and Infection¹ this December.

The seasonal tide of influenza has baffled epidemiologists for ages. The flu is always at its worst in the months following the winter solstice, and pandemics tend to hit at this time too, but nobody is certain why.

If that puzzle could be unpicked, the information could be used to save lives; in an average year, about one million people die worldwide from influenza-related illnesses. In a flu pandemic, such as occurred in 1918, millions of people can die.

In 1981, Edgar Hope-Simpson, a British epidemiologist, suggested that influenza's seasonality might be linked to solar radiation. Very few epidemiologists took the idea seriously. But in the years since, evidence has continually surfaced that vitamin D, created when solar radiation strikes the skin, plays an important role in our immune system.

Now, 25 years later, a review of the evidence suggests that Hope-Simpson's theory of an influenza seasonal stimulus be rigorously tested.

Dark days

"Winter brings a host of confounding factors associated with the influenza," says Edwin Kilbourne, professor emeritus at New York Medical College. Low relative humidity favours influenza virus aerosols, and indoor crowding facilitates transmission. Autumn brings young children together in schools, and Christmas travel mixes up previously separated populations.

"But there's more to it then just environmental conditions," says Kilbourne. "Controlled experiments with my laboratory mice have shown that even after careful control of environmental factors, including crowding and humidity, there remains a seasonal factor that slightly, but significantly, affects transmission of influenza."

John Cannell of Atascadero State Hospital, California lead author on the new review and executive director of the non-profit organization the Vitamin D Council thinks vitamin D might be the missing link.

He points to one study, conducted in St Petersburg and repeated in Krasnodar, Russia, showing that young men inoculated with live attenuated influenza virus were eight times more likely to develop fever and evidence of an immune response just after the winter solstice then they were during the summer months.

Another showed that children in India with vitamin D levels of less than 10 nanograms per millilitre were 11 times more likely to have respiratory infections than those with higher levels of the vitamin.

And a series of studies from the 1930s showed that cod-liver oil, which is rich in vitamin D, can reduce viral infections by 50% in adults taking a daily dose over 4 months.

Vitamin boost

"Within a few minutes of summer sunbathing, our bodies make about 20,000 international units [500 micrograms] of vitamin D," notes Cannell. "We have evolved a system that makes lots of the vitamin very quickly. There is no doubt in my mind that this happened for a good reason." Vitamin D is known to regulate calcium in the blood and maintain bones, but Cannell thinks its role in immunity is just as important.

Kilbourne isn't convinced. "My mind is open on this, but persuasive evidence has just not been presented," he says. His seasonally flu-ridden mice, he notes, were in the basement away from any sunlight.

"The paper raises more questions than it answers," agrees James Cherry, a specialist in paediatric infectious disease at the David Geffen School of Medicine, University of California, Los Angeles. "However, the hypothesis should be easy to prove or disprove with a controlled blind study," he adds. Cannell is keen for that study to happen.

"It is premature for doctors to recommend vitamin D as a treatment or preventative against flu," Cannell says, but he himself is taking no chances. Cannell takes 125 micrograms of vitamin D a day during winter much more than the US government's recommended dose. He advises doctors to be on the lookout for vitamin D deficiency and treat it aggressively.

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