Scientists tracking the most common type of flu say new bugs start out in China and Southeast Asia, then travel around the world, eventually reaching South America where they lose steam and die out. Researchers say the information is important because it will give them a head start on which flu strains to include in the formulation of an annual influenza vaccine. VOA's Jessica Berman reports.

Until now, scientists have disagreed about the global migration of the influenza A virus which, according to the World Health Organization, infects between three and five million people each year and is responsible for up to a half million deaths.

Some infectious disease experts believe the flu emerges after migrating between the northern and southern hemispheres. Others feel it evolves after circulating throughout the tropics or China.

But according to researchers writing this week in the journal Science, the annual influenza A virus originates in China and Southeast Asiaand then it follows a predictable path around the globe.

Lead author Colin Russell of the University of Cambridge in Britain says there's little variation in that path.

"Once viruses leave East and Southeast Asia, they rarely return," said Colin Russell. "And thus regions outside of East and Southeast Asia are essentially the evolutionary graveyards of influenza viruses."

According to the analysis, the dead end for most annual flu viruses is Latin America. Experts say strains that seem to be emerging in Latin America don't pick up any traction.

Study co-author Derek Smith is with the World Health Organization's Global Influenza Surveillance Network which meets every year in February to try to determine which strains of the flu virus to include in a vaccine for the following year.

Smith says knowing precisely where the flu virus is coming from should make it easier to develop an effective vaccine more quickly and easily.

"Because we can now pinpoint, at least over the last five years, where the source of these H3 viruses has been, this allows us to focus on new strains that are emerging in East and Southeast Asia and to put less focus let's say on new strains that are emerging in South America," said Derek Smith.

Smith emphasizes that the current flu vaccine works extremely well to protect 300 million people every year and people should continue to take it. But researchers hope the latest findings will lead to a flu vaccine that provides greater protection.