Eideard

Every autumn, as predictably as falling leaves, flu season descends upon us. Every spring, just as predictably, the season comes to a close. This cyclical pattern, common in temperate regions, is well known, but the driving forces behind it have been in question.

Do existing strains die off each spring, only to be replaced each fall by new founding strains from other parts of the world, or does a “hidden chain of sickness” persist over the summer, seeding the next season’s epidemic?

A genetic analysis by University of Michigan postdoctoral fellow Trevor Bedford and colleagues at U-M, Howard Hughes Medical Institute and Florida State University reveals that in the United States, not all strains of influenza die off at the end of winter; some move southward to South America, and some migrate even farther.

“We found that although China and Southeast Asia play the largest role in the influenza A migration network, temperate regions — particularly the USA — also make important contributions,” Bedford said. Rather than dying off at the end of our flu season, many strains simply move on to more favorable environments.

Growing knowledge about patterns of flu migration eventually may make it possible to tailor vaccines to particular locations, Bedford said.

“We found, for instance, that South America gets almost all of its flu from North America. This would suggest that rather than giving South America the same vaccine that the rest of the world gets, you could construct a vaccine preferentially from the strains that were circulating in North America the previous season. As we gather more data from other regions, this could be done for the entire world.”

All the more reason to develop a universal flu vaccine – even if it needs to be tailored geographically.