Mild winters raise risk of flu epidemics

Arizona State University mathematician Sherry Towers has found links between a warm winters with little flu to epidemics the following year. Credit: Arizona State University

Arizona State University mathematician Sherry Towers has found links between a warm winters with little flu to epidemics the following year. Credit: Arizona State University

In 2012/13 the US flu season started especially early for two strains for the first time since the government started tracking it in 1997. Sherry Towers at Arizona State University has put this down to the unusually warm winter the country saw in 2011/2012, after showing such a link can be seen in previous years. She hopes that her findings can help health services prepare as winters get yet milder with continuing climate change.

“Until now, it had not been noticed that the dynamics of the current season depend not only on the temperature of the current season and vaccine match, but also on what had occurred the year before,” Sherry told me. “If there has been a mild flu season during a mild winter, public health authorities know several months in advance that a severe season with early onset is much more likely to occur the next season. This allows them to expedite the manufacture and distribution of vaccines to the population.”

As a mathematician looking at how climate affects the spread of infectious diseases, Sherry follows influenza data collected by the US Centres for Disease Control (CDC) closely. The CDC tracks the various influenza virus types in circulation. These include letter and number combinations you might have heard, like H1N1 and H3N2, which together are classed as influenza A, plus the single type of influenza B virus. Scientists had previously shown that high temperatures reduce transmission of the virus, which alone would make mild flu seasons more common in warm winters. Though this suggests less of a threat from flu in a warmer world, this season’s data made her wonder if there could be a downside.

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