Why Does Flu Season Happen in Winter?

    By Kevin E. Noonan --

It is always surprising to non-scientists when they discover how little we know about the answers to some fundamental questions.  Everyone knows, of course, the paradox that "we can land a man on the moon, but we can't prevent the common cold."  More esoterically, Dr. J.R. Powell at Yale University once published a paper with his mentor, Theodosius Dobzhansky, asking the simple question:  "How far do fruit flies fly?" about the common genetic lab animal, Drosophila.  But recently the answer to a more pertinent question was established by workers in New York:  why does flu season occur in winter?   

This is a question that has vexed researchers for decades, even as the phenomenon has been well established:  on either side of the equator, the flu strikes in wintertime, and in the tropics (which have no "winter") there isn't much flu or flu season at all.  Anecdotal explanations, such as crowding indoors amongst school children, were unsatisfactory in light of similar crowding in autumn with little influenza infection.  Efforts towards establishing an answer experimentally have been thwarted by the absence of a convenient laboratory animal with which to conduct a study, since the ideal candidate, the human population, is properly off-limits.

Which is where persistence, perspicacity, and serendipity come in.  Dr. Peter Palese (at left), a renowned microbiologist and flu researcher, has the answer, which is elegant in its simplicity, and he got it from a hint found in reports dating from the infamous 1918 "Spanish flu" pandemic that killed millions.  As reported today by Gina Kolata of The New York Times (herself a renowned popular science writer), Dr. Palese noticed that an American Medical Association report from 1919 about the flu epidemic at Camp Cody in New Mexico mentioned that all the guinea pigs (of all things) died shortly after the influenza outbreak, and that necropsy showed their deaths were the result of pneumonia.  Dr. Palese tested guinea pigs for influenza infection and found, remarkably, that they are susceptible to infection with the virus and could spread it from one animal to another (a key difference with mice, who get the flu but can't pass it to other mice). 

Having "found" his experimental animal, the experiments themselves were straightforward.  Dr. Palese and his colleagues tested the effects of environmental temperature and humidity on the frequency of influenza infection in these guinea pigs.  They found that viral transmission decreased as the temperature and humidity increased, and that the best conditions for transmission were when the air was cold (41°F) and dry (20% humidity) - just the conditions that occur in the temperate zones during winter.  This also confirms that, unlike cold viruses that spread from direct physical contact between individuals, the flu is spread through the air like a medieval miasma.  The virus is suspended in water droplets from coughs and sneezes and persists suspended in air longer under cold and dry conditions, according to Dr. Palese. 

It is not clear that this new information suggests anything about how to avoid the flu (Dr. Palese recommends getting a flu shot) other than providing a reason to ensure that indoor temperature and humidity is not allowed to plummet in winter in a misguided effort to save heating costs in public institutions or to avoid "mollycoddling" youngsters.  It does suggest that there may be more harm than good in avoiding dramatic changes in temperature as children go from outdoors to indoors in winter, since it is certainly likely that human transmission is influenced by population density, and crowded schools are a great place to transmit airborne viruses.  But besides these possible practical consequences, Dr. Palese's work can be seen as a satisfying answer to another nagging question about the world.  There is a benefit to answering these questions, the benefit of reducing even incrementally our vast ignorance about the natural world, that makes the answer, and the story behind the answer, sufficient in itself for celebration.

For those interested in the details, Dr. Palese's report was originally published in the October 19th issue of PLoS Pathogens.