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Page: << Prev | 1 | 2 | 3 | Next >> Russell said flu viruses rarely return to their place of origin and usually become extinct after a flu epidemic has run its course. "When these viruses leave East and Southeast Asia, they rarely return," he said. "The regions outside East and Southeast Asia are essentially the evolutionary graveyard of influenza virus."
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.
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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.
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