Americans should brace themselves for earlier and more severe flu seasons, say scientists, thanks to the effects of climate change.
The team studied waves of influenza and climate patterns in the US from the 1997-1998 season to the present, using data from the Centers for Disease Control.
They found a pattern for both the A and B strains of flu, with warm winters generally followed by heavy flu seasons.
"It appears that fewer people contract influenza during warm winters, and this causes a major portion of the population to remain vulnerable into the next season, causing an early and strong emergence," says Sherry Towers of Arizona State University.
"And when a flu season begins exceptionally early, much of the population has not had a chance to get vaccinated, potentially making that flu season even worse."
The current flu season, which is still affecting many parts of the country, began 'early and fiercely', says the team. It followed a relatively light 2011-2012 season, which saw the lowest peak of flu since tracking efforts went into effect, and which coincided with the fourth warmest winter on record.
If global warming continues, warm winters will become more common, and the impact of flu will likely be more heavily felt, say the researchers.
However, says mathematical epidemiologist Gerardo Chowell-Puente, "The expedited manufacture and distribution of vaccines and aggressive vaccination programs could significantly diminish the severity of future influenza epidemics."