WASHINGTON — Scientists have pinpointed the path that flu takes as it sweeps the globe every year, starting with the birth of new strains in Asia and ending when the virus burns out in South America.
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," said team leader Derek Smith of Britain's University of Cambridge.
Even if sick Americans fly flu back to Asia in January, people there would be mostly immune to the old strain and already circulating new ones, said study lead author Colin Russell of the University of Cambridge.
Why those routes? Travel and trade, Russell said. 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.