Where Flu Hides Out During the Warm Season

A new genetic analysis has revealed that flu strains do not die off during the summer months, but rather they elope warm regions, and move to where conditions can ensure their survival.

All around the world, the autumn months are associated with the onset of the flu season. Regardless of when autumn comes – times differ between hemispheres – you can be sure that flu outbreaks accompany it.

Researchers have now determined that flu strains are not eradicated during vaccination campaigns, as initially thought. Rather, they tend to move southwards, if the outbreak occurred in the Northern Hemisphere.

Things go the other way around if the strains affect the Southern Hemisphere. Temperate regions are the most affected by flu outbreaks.

Previous studies had indicated that strains which wrecked havoc one year die out completely, and get replaced by other strains “imported” from other parts of the world the next flu season.

But the new data seems to suggest the existence of a “hidden chain of sickness,” which is capable of enduring throughout the summer. It provides the seeds for that autumn's epidemic.

The new investigation was conducted by experts at the University of Michigan, the Howard Hughes Medical Institute and Florida State University (FSU).

The genetic analysis was led by UM postdoctoral fellow Trevor Bedford, who also conducted the joint research team.,

“The prevailing view that has developed over the past three years or so is the out-of-tropics hypothesis, in which the strains that bring about each temperate flu season originate from China and Southeast Asia, where influenza A is less seasonal,” Bedford says.

But the new data seems to indicate that, at least in the case of the United States, seasonal flu strains come from South America, perhaps even further.

The pathogen migrates southwards in the spring, and returns in the autumn carrying mutations that allow it to remain impervious to last season's vaccines.

“We found that although China and Southeast Asia play the largest role in the influenza A migration network, temperate regions – particularly the US – also make important contributions,” Bedford adds.

The new investigation was funded by the Howard Hughes Medical Institute, the US National Institutes of Health (NIH) and the National Science Foundation (NSF).