Study: US flu strains may head south in spring

May 27, 2010 (CIDRAP News) – The United States is a more important hub for the global circulation of seasonal flu strains than previously thought, a finding that may have practical public health implications for the best use of antivirals and vaccines, researchers reported today.

Over the past few years, other researchers have tagged China and Southeast Asia as tropical regions where seasonal flu strains originate. When a research group from the University of Michigan and Florida State University used mathematical modeling techniques incorporating gene sequence analysis to test the hypothesis, they found that the more temperate United States region also makes important contributions to the migration of seasonal flu, which they found often travels to South America at the end of the flu season.

The authors based their study on genetic analysis of influenza A H3N2 viruses collected from around the world between 1998 and 2009, which allowed them to identify relationships between the viruses. The modeling method allowed them to account for evolutionary processes and migration rates. Their findings appear in today's issue of Public Library of Science (PLoS) Pathogens.

The findings demystify what happens to seasonal flu viruses in the United States when they appear to die out at the end of each flu season. Instead, the researchers suggest they move on to more favorable environments in South America and even beyond.

Virus migration patterns that the group estimated correlate with air travel patterns between regions of the world. They noted that South America is relatively isolated in the global aviation network, though connections to North America are more plentiful. "We observe that most influenza in South America arrives from the USA," they wrote.

When the team traced the genealogical history of the viruses, they found that the "trunk" of its family tree mainly resided in China (34%) and Southeast Asia (32%), with a significant portion in the United States (24%). They wrote that the findings show the last decade of meaningful virus evolution occurred primarily in those three regions.

Trevor Bedford, PhD, a postdoctoral fellow at the University of Michigan and lead author of the study, told CIDRAP News that though human patterns could be responsible for the strong connection between US and South American flu strains, other factors could be in play, as well.

For example, he said absolute humidity may affect seasonality. In cold winter months, lower humidity may allow the virus to be more transmissible. South America's winter season has a more Southern Hemisphere pattern, with cooler, drier conditions from roughly March through October.

Though the group did not study influenza A H1N1 and influenza B migration patterns, Bedford said they suspect they follow a similar pattern," with East and Southeast Asia playing the strongest role in the migration network but with temperate regions still making occasional, but significant, contributions."

Patterns of flu virus migration could have important public health implications, the group wrote, adding that response activities in the United States can have a global impact. For example, cautious use of antiviral medications in the United States could help prevent the development of drug-resistant strains that could spread to the rest of the world. Also, vaccination programs outside of East and Southeast Asia could curb the worldwide spread of the disease.

A sharper understanding of flu migration patterns could open up the possibility of tailoring vaccines to specific locations, Bedford said in a press release from the University of Michigan. "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," he said.

"As we gather more data from other regions, this could be done for the entire world."

Bedford T, Cobey S, Beerli P, et al. Global migration dynamics underlie evolution and persistence of human influenza A (H3N2). PLoS Pathog 2010 May 27;6(5):e1000918 [Full text]

See also:

May 27 University of Michigan press release

Apr 16, 2008, CIDRAP News story "Study: New seasonal flu strains launch from Asia"

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