Nov. 15, 2004

Small seasonal changes can lead to big flu outbreaks

McMaster researcher investigates why influenza epidemics happen in the winter

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Mathematician David Earn uses mathematical models to investigate and understand how infectious diseases move through populations.

Hamilton, ON - Flu season is on its way to homes across Canada. But the question of why influenza epidemics take place in the few cold months of winter remains unanswered. Is it the change in the weather? The return to school? Or increased viral production under winter conditions?

The answer to the increase of flu cases may be extremely minute seasonal changes, says David Earn of the Department of Mathematics & Statistics at McMaster University. Earn, who uses mathematical models to investigate and understand how infectious diseases move through populations, recently examined the question of why most people catch the flu in the winter.

“In this study, we found that it may be difficult or even impossible to pin down the cause of seasonality in influenza epidemics,” Earn explains. “Large fluctuations in the number of flu cases between winter and other seasons may be caused by very small changes in the number of people infected by a single infectious person. These small changes in ‘transmission rate’ are amplified by interactions between the evolving virus and the changing level of immunity that people have to specific strains.”

Because the influenza virus is constantly adapting, immunity to the flu is not a permanent condition as it is with other viral diseases like chicken pox. Incorporating loss of immunity into a simple model of disease spread, Earn and colleagues at Princeton University and Harvard University found that under many scenarios, small seasonal changes in transmission rates can lead to regular, annual epidemics. Their work appeared in today’s edition of the Proceedings of the National Academy of Sciences of the USA.

Concludes Earn, “The reason that more people catch the flu in the winter appears to be that small seasonal changes in flu transmission at the individual level are greatly amplified as the disease spreads through communities. The underlying cause of seasonal fluctuations in transmission may be too small to measure.”

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