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 In between, influenza catches a ride to North America and Europe about six to nine months after a new strain emerges in Asia, a pattern that promises to help health authorities better prepare each winter's flu vaccine.
Already, monitoring is being beefed up in parts of East and Southeast Asia "as fast as we can" in hopes of more accurately spotting strains poised to jump continents, said Dr. Michael Shaw, a flu specialist with the U.S. Centers for Disease Control and Prevention.
Now, "we know the part of the world to look in, and the probable time of the year to look," he said.
The good news: Once they leave Asia, new flu strains don't seem to get more dangerous as they migrate from continent to continent, an international research team reports in Friday's edition of the journal Science.
"Once the viruses leave that region, they're really on a pathway to an evolutionary graveyard," Derek Smith of Britain's University of Cambridge, who helped lead the team, said in an interview.
Influenza evolves so quickly that slightly different strains circulate each year. Specialists have long suspected that China was the world's main incubator, and used outbreaks in Asia to guide the following year's vaccine recipe.
But the new work shows flu's annual evolution is far trickier than previously thought.
Every year, a World Health Organization network painstakingly collects nose or throat swabs of flu patients in over 80 countries, to identify circulating strains. The researchers studied 13,000 samples of the most common type of flu
-- the H3N2 version of Type A influenza -- collected since 2002. They tracked changes in a protein on the virus' coat, called hemagglutinin, that are small but enough to let flu evade the immune system and sicken people
-- in other words, new strains.
The first surprise: Flu may be a winter problem in much of the world, but H3N2 virus is constantly circulating somewhere in East and Southeast Asia
 .
It's not that flu infects year-round in any one city or country. In tropical zones, it prefers the rainy season; in more temperate zones, it thrives in chillier weather. Densely populated East and Southeast Asia has just the mix of the two to experience outbreaks in different parts of the region throughout the year, said study lead author Colin Russell of the University of Cambridge.
Bangkok and Kuala Lumpur, for example, are just 700 miles apart but experience flu outbreaks six months apart. Enough of those local outbreaks overlap, thanks to heavy travel throughout the region, that the region becomes a continuous incubator for new flu strains, the study concludes.
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"We actually see ... viruses passing from epidemic to epidemic
like relay runners passing a baton," Russell said.
Like following a trail of footprints, the researchers then used a
tool called "antigenic cartography" to map how newly emerged strains
moved to other continents -- reaching Australia, North America and
Europe six to nine months later.
The final surprise: Several months after that, the strains
reached South America and died out.
"In the temperate regions, the viruses go extinct after each
epidemic," Russell said.
Even if, say, sick Americans fly flu back to Asia in January,
people there by now are pretty immune to the old strain and already
circulating new ones, he explained. "The evolution in Asia has moved
on."
Why those routes? Travel and trade, says Russell: There is far
less direct air travel between Asia and South America than Asia and
North America, for example. By the time the virus made it to South
America, the rest of the planet already had been exposed.
Africa might be a last stopover, too, Russell cautioned. There
simply is too little tracking of influenza in Africa to be able to
tell.
The work dovetails with a major study of influenza's genes
published Wednesday in the journal Nature, which examined how the
virus ebbs and flows in the Northern and Southern Hemispheres. That
research also pointed to single world source of new flu strains -- although it couldn't narrow the location
-- that spreads to temperate
regions in the winter before dying out by summer.
"The geography's important in the context of vaccination," said
Edward Holmes of Pennsylvania State University, who led the Nature
genetic study. In picking the yearly vaccine's recipe, "sometimes we
get that wrong. Part of it is not looking in the right population.
We need to look in Southeast Asia every season."
Most countries in Asia already have some flu surveillance, but
Myanmar, Cambodia and Laos in particular need more, said CDC's Shaw.
The flu mapping research was funded by the U.S. National
Institutes of Health.
[Associated
Press; By LAURAN NEERGAARD]
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