Scientists have finally solved a long-standing mystery of where seasonal flu viruses originate and how they migrate throughout the world, new research shows. According to them, influenza viruses arise and evolve annually in East and Southeast Asia and migrate to Europe and America in the winter.

“For over 60 years, the global migration of influenza viruses has been a mystery,” said study author Colin Russell of the University of Cambridge in the United Kingdom.

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

The viruses take about nine months to reach Europe and North America and several months later, they reach South America because of South America’s isolation in terms of travel from East and Southeast Asia, Russell explained.

“It is a surprise to everyone that South America is the end of this seeding hierarchy.”

The good news is that once the viruses leave Asia, “they’re really on a pathway to an evolutionary graveyard,” because they seem to be less dangerous as they migrate from continent to continent,” Derek J. Smith a biologist at Britain’s University of Cambridge, who helped lead the team, said in an interview, according to the Associated Press.

Smith, who is with the World Health Organization’s Global Influenza Surveillance that meets every February to determine which strains of the flu virus to include in a vaccine for the following year, says knowing precisely where the flu virus is coming from should make it easier to develop an effective vaccine 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,” Smith said.

He underlines that the current flu vaccine works extremely well to protect 300 million people every year and people should continue to take it even though this year’s vaccine did not appear to be 100 percent safe. However, researchers hope the latest findings will lead to a flu vaccine that provides greater protection.

For this study, Russell and his colleagues analyzed 13,000 samples of flu virus collected by the World Health Organization’s Global Influenza Surveillance Network from six continents from 2002 to 2007. According to the WHO, yearly flu epidemics cause some 3 million to 5 million cases of severe illness, and 250,000 to 500,000 deaths every year.

The study, supported by the National Institutes of Health, the National Institute of Allergy and Infectious Disease, the Netherlands Influenza Vaccine Research Center, the Australian Department of Health and Aging and the United Kingdom Medical Research Council, is published in the April 18 issue of the journal Science.

In another study appearing today in the journal Nature, biologist Edward Holmes of Pennsylvania State University and his colleagues studied about 1,300 influenza virus samples gathered from New York state in the Northern Hemisphere and New Zealand in the Southern Hemisphere over a 12-year period.

Holmes’ results were very similar to Russell’s. His study was supported by the NIH, the National Institute of Allergy and Infectious Diseases, the National Institute of General Medical Sciences and the Royal Society of London.