In the study, researchers analyzed five years of flu virus samples from around the world. They found that most flu strains followed a circuitous global route that began in Asia, travelled first to Australia, Europe or North America, and then made their way to South America before dying out.
The knowledge could help scientists and public health officials better predict emerging flu strains and make yearly flu vaccines more effective.
Derek Smith of Cambridge University in Britain, one of the study's co-authors, told the Wall Street Journal that the finding suggests that "we should be paying more attention to East and Southeast Asia."
Each year, flu viruses infect between 5 and 15 percent of the world's population, making 3 to 5 million people severely ill and killing up to 500,000.
The flu vaccine helps protect the approximately 300 million people who receive it each year, but it can be difficult to keep up with the quickly evolving strains of the virus.
To produce the vaccine, the World Health Organization Global Influenza Surveillance Network collects samples of the strains circulating in more than 80 countries, then meets each year in February to choose the strains that will be included in the next season's vaccine.
But they can't always identify all the right strains. This past season, for example, two strains that weren't included in the vaccine hit hard, making it the worst flu season in several years.
One of the main challenges health researchers face involves understanding where new flu strains originate and how they circle the globe. In general, there have been three competing theories: that strains migrate between the northern and southern hemispheres following the seasons, that they originate in the tropics, and that they come mainly out of China.
In the new study, researchers used a method called antigenic cartography to analyze more than 13,000 WHO samples of strains of the most common form of the flu, called Type A(H3N2).
They found that new strains emerged in East and Southeast Asia, travelled to Europe, Australia and North America at roughly the same time, and then later travelled from these continents to South America, where they died out.
There's a good reason that Asia could be the cradle of the world's flu viruses, the researchers explain: In temperate climates winter is flu season, but in tropical areas the rainy season is flu season. In Asia, many areas that aren't geographically very far apart have different rainy seasons, so the flu can migrate and circulate year-round, allowing new strains to evolve more easily.
"There is a lot of variability like this in East and Southeast Asia, so lots of opportunity for an epidemic in one country to seed an epidemic to another nearby country, like a baton passed by runners in a relay race," Smith said.
After that, global trade and migration patterns probably drive the flu's movements, according to the researchers. The strong ties between Asia and Europe, North America and Australia give the viruses ample opportunity to hitch a ride to those continents, while it takes longer for them to reach South America.
The study, which was funded by the National Institutes of Health, was published in this week's Science.
