The 2013 flu season is one of the worst on record—and, according to a new study, you might be able to blame climate change for that.
Researchers from Arizona State University analyzed statistics on lab-confirmed flu cases, collected by the Centers for Disease Control and Prevention (CDC), dating back to 1997, when the CDC first started collecting such information, and compared those to climatic data from 10 U.S. geographic regions over the same 16-year time period.
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"We saw a significant correlation between the temperature of the winter season before and the severity of the flu season that followed for all different types of flu," says Sherry Towers, PhD, lead author and research Professor at Arizona State University's Mathematical Computational Modeling Sciences Center. The study found that mild winters preceded a more-severe-than-average flu season 72 percent of the time, and that those flu seasons started an average of 11 days earlier. They were also 80 percent more likely to peak before January 1st. The study was published in the journal PLoS One.
Sound familiar? The winter of 2011 to 2012 was the fourth warmest in recorded history, based on data collected by the National Oceanic and Atmospheric Administration. The 2012 to 2013 flu season peaked in early December and, according to the CDC, 22,048 flu cases have been reported in the last three months, compared to just 849 cases during the same time period last year.
The flu's ability to spread depends heavily on temperature, says Towers. The virus doesn't survive well in warm, humid air, but thrives in cold temperatures, which is why warm winters usually trigger mild flu seasons. But, she says, healthy adults who get the flu can build up an immunity to similar strains that lasts a couple of years, so if you get the flu one winter, you'll be less likely to get it the next even if you haven't been vaccinated. (There isn't enough data to know whether the study's results can be attributed to people's not getting vaccinated during milder winters; there's some evidence that a vaccine can protect you for more than one flu season.) "So they're more vulnerable to the flu the next season, and when temperatures get back to normal, the conditions are ripe for an epidemic to take off early," she says.
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We can expect more of the same as climate change progresses, says Towers. "The expectation is that we will have more and more warm winters, as we did last winter," she says. But on average, abnormally warm winters are followed by winters with more back-to-normal temperatures—and winters accompanied by more severe flu seasons.
The take-home? Be vigilant about getting a flu vaccine, especially after a warm winter. "If you see that there's been a mild winter, you have several months' forewarning that there's a greater likelihood of a severe flu season," says Towers.