Seasonal Flu Outbreaks Start in Asia: Study

By Steven Reinberg
HealthDay Reporter
Thursday, April 17, 2008; 4:00 AM

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WEDNESDAY, April 16 (HealthDay News) -- Each year, new strains of virus that produce seasonal flu epidemics start in East Asia and Southeast Asia and then spread around the rest of the world, researchers report.

And by focusing on new flu strains emerging in Asia, scientists may be able to improve their forecast of seasonal flu strains and develop better vaccines, the researchers said.

"For over 60 years, the global migration pattern of influenza has been a mystery," lead researcher Colin Russell, of the University of Cambridge in England, said during a Wednesday teleconference.

Conventional wisdom has held that flu viruses migrate between the northern and southern hemispheres after the flu season. Other theories contend that the viruses surface in the tropics and circulate continuously, or start out in China, Russell said.

"We found solid evidence that influenza H3N2 viruses [the most common viruses] have migrated out of what we call the East and Southeast Asian circulation network, which includes tropical, subtropical and temperate countries," Russell said.

Virus strains begin in East and Southeast Asia and take about nine months to reach Europe and North America. They arrive in South America several months later because of South America's isolation in terms of travel from East and Southeast Asia, Russell explained.

Currently, the decision about which strains to include in the yearly flu vaccine are made almost a year before the flu hits the United States, co-author Derek Smith, also from the University of Cambridge, said during the teleconference. Knowing where flu patterns begin will be invaluable in helping to develop more effective vaccines earlier, he added.

"The ultimate goal is to increase our ability to predict the evolution of influenza virus, and this study is one step along that path," Smith said. "This may help us get a step ahead of the virus, because we now know where to look."

Russell said flu viruses rarely return to their place of origin and usually become extinct after a flu epidemic has run its course. "When these viruses leave East and Southeast Asia, they rarely return," he said. "The regions outside East and Southeast Asia are essentially the evolutionary graveyard of influenza virus."