A new study in PLoS Currents: Influenza finds that mild winters are often followed by serious flu outbreaks in the following year, which suggests that climate change could bring harsher flu outbreaks.
For the study, Arizona State University researchers examined government data for each flu season between 1997 and 2012. They found that when a winter had above-average temperatures, the following flu season was more severe 72% of the time, lead author Sherry Towers says.
Although "correlation doesn't mean causation," Towers says her findings provide "compelling evidence" to support the link between mild winters and subsequent severe flu seasons.
Research suggests that the flu virus is less likely to spread during a mild winter because the virus dies more quickly in warm, humid air. Although an average flu season peaks in late January or early February, Towers' team found that flu seasons that follow a warm winter were 80% more likely to peak before Jan. 1.
For example, the flu season last winter—"the fourth warmest on record," according to Towers—did not begin until late February, according to CDC. This year's flu had spread to 41 states before the end of 2012.
If global warming brings more mild winters, U.S. residents may be in for harsher, more deadly flu seasons, the study suggests.
However, Towers admits that "[t]he dynamics that cause a severe flu season are so multifactorial." She notes that flu season's severity can be affected by whether a flu vaccine for a particular season is a good match for the predominant flu strain.
Strains are not identical from season to season as the virus "changes its coat," according to Stephen Baum, a professor of microbiology and immunology at Albert Einstein College of Medicine. However, the change is usually slight, so individuals who caught the flu one year may not catch it the next year, or may brush off a mild case of it as a "bad cold."
Baum told HealthDay that Towers' study does not prove that a mild winter directly causes a more miserable flu season, but it is "a possibility" (Norton, HealthDay, 2/1).
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