Seasonal Flu Outbreaks Start in Asia: Study

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Also, Russell said, flu viruses don't circulate continuously in any one region of the world. "They don't survive at the end of an epidemic in both temperate and tropical countries," he said. However, because East and Southeast Asia are made up of both temperate and tropical areas, flu virus is able to circulate year-round in those areas, he explained.

"It is this year-round circulation, combined with a substantial volume of air traffic amongst East and Southeast Asian countries, that allows East and Southeast Asia to serve as the source of influenza epidemics to the rest of the world," Russell said.

The findings are published in the April 18 issue of the journal Science.

For the study, researchers analyzed 13,000 samples of flu virus collected by the World Health Organization Global Influenza Surveillance Network from six continents from 2002 to 2007.

The researchers compared the differences between the strains in a surface protein called hemagglutinin. Hemagglutinin is the main target of the immune response to the flu, and even small changes can enable the virus to fool immune systems. The researchers also compared the genetic codes for hemagglutinin in a number of the flu strain samples.

Although there are sometimes mismatches between strains of circulating flu virus in the vaccine, the vaccine usually works very well and protects about 300 million people each year from getting the flu, Russell said. Even in years when the flu vaccine is a mismatch, getting vaccinated still offers protection, he said.

According to the World Health Organization, yearly flu epidemics cause some 3 million to 5 million cases of severe illness, and 250,000 to 500,000 deaths every year.

In a separate study published in the April 17 issue of the journal Nature, Edward Holmes, a professor of biology at Pennsylvania State University, and colleagues analyzed 1,302 samples of flu collected over 12 years from around the world.

Because of the limited sample size, Holmes said he can't tell where flu viruses start. But he agrees that East and Southeast Asia is the likely source. "That's a really big finding," he said.