New flu strains originate in East and Southeast Asia, then make their way around the world before they go extinct in South America, according to two studies released yesterday.
This conclusion will enable public health agencies to focus their surveillance on Asia and will increase researchers' ability to quickly develop effective influenza vaccines.
"Asia is the place we should be watching. We didn't know that," said Edward Holmes, a biologist at Penn State University and the author of one of the studies. The papers appeared in the latest issues of Nature and Science.
Flu surveillance extends around the world today, but Holmes said the new findings would change that focus to Asia.
Both studies looked at the influenza genome and analyzed mutations from different regions. Scientists then constructed a model that explained how the virus traveled around the world.
"This is very important work," said Steven Salzberg, a computational biologist at the University of Maryland. He has also studied flu mutations and geography.
Until now, the global course of influenza has remained mysterious. The illness strikes year-round in the tropics but only during the winter months in temperate regions. Some scientists thought that in temperate areas, the virus lay dormant, reactivating with cold weather. Others suspected Asia as the starting point but lacked conclusive proof.
"There's never been any concrete evidence to say that the virus came from East and Southeast Asia," said Derek Smith, a professor of infectious disease informatics at Cambridge University in England. Smith was a lead author on the Science study. He and his colleagues analyzed 13,000 flu samples from around the world.
Once a new strain of flu evolves in Asia, it takes six to nine months to each North America. The earlier researchers can identify a strain, the sooner they can begin working on a vaccine.
No one knows why flu strains die off in temperate regions. Some researchers think the virus may thrive only in climates that are either cold and dry, or hot and wet. Others suspect that increased density in winter months - people stay inside because of the cold - enables the virus to infect more hosts. But no one knows. "If I knew that, I'd be more famous than I am," Holmes said.
Flu kills up to half a million people a year worldwide, according to the World Health Organization. Different strains circulate each winter, so scientists must continually develop new vaccines.
The researchers looked at the H3N2 and H1N1 subtypes. These are human strains, different from the H5N1 strain of avian flu, which is deadly to many birds.