The annual spikes in sore throats, fevers, coughs and chills from seasonal influenza can change depending on the seasons preceding them, according to recent findings.
Scientists looking at influenza patterns across the United States going back more than a decade found that warmer-than-average winters give way to earlier and more severe outbreaks. As the climate heats up and warmer winters become more frequent, researchers said, earlier influenza seasons may become more common. The team published their findings earlier this week in the journal PLOS Currents: Influenza.
The current influenza outbreak, which surged earlier and more strongly this year than in past seasons, may be an example of this in action, though health officials say it is still too soon to tell. The Centers for Disease Control and Prevention warned last December that the 2012-2013 influenza season was off to an early start. Though the illness is now on the decline, the CDC warns that Americans are not out of the woods yet and should still get vaccinated.
"It's a bit too early to say exactly when our season peaked," said Tom Skinner, a spokesman for the CDC. "Certainly, we're seeing a downward trend" in new infections. However, he added, "it's stacked up to be a moderate to severe season."
The mechanisms behind seasonal influenza outbreaks are unclear, but understanding these patterns could help health officials wrangle with a notoriously mercurial medical specter caused by a rapidly mutating virus. Annually, the illness has a multibillion-dollar economic price tag in terms of treatment and lost productivity.
Sherry Towers, a research professor at the Mathematical, Computational and Modeling Sciences Center at Arizona State University, led the recent study. She explained that warmer and more humid conditions tend to slow influenza transmission, leading to a drop in infections immediately following a warm winter.
A lull in vaccinations
The 2011-2012 winter was the fourth-warmest on record, which may have had a substantial impact on influenza last year. "Indeed, this last winter, it was the mildest influenza season on record," Towers said. With a drop in infections, it is likely that fewer people decided to get the influenza vaccine, creating a larger vulnerable population for both the A and B varieties of the virus going into the next influenza season.
These factors may be playing a role in the current 2012-2013 epidemic, though researchers are cautions about drawing direct links. "The reason why [the current influenza outbreak] getting so much attention is that it started really early, which really speaks to the susceptibility in the population," said Jeffrey Shaman, a professor at the Mailman School of Public Health at Columbia University who studies influenza and climate. Shaman was not involved in this study.
However, putting these climate findings to work is tricky. "This is just one of the things that can contribute to the severity or mildness of the flu season," Towers said.
Entirely new versions of the virus can emerge as it mixes and blends in its animal hosts. These vectors—for instance, birds—also face climate effects and can alter exposure patterns in humans depending on when and how they cross paths. In addition, vaccination rates can vary from season to season depending on the whims of the public, even though health officials warn that people can contract influenza every year because of how fast it changes. "The bottom line is that people over the age of six months need to get vaccinated each and every year," Skinner said.