"Earlier genetic studies had looked at H3N2 in a global context and concluded that new strains came from the tropics," said Dr. Bahl. "However, in those studies, a lot of key genetic data from the tropics was missing." This made it difficult to draw a firm conclusion about the origin of new flu strains, he said.
The researchers found that in temperate regions where flu seasons are relatively short, many new H3N2 virus strains arise every year, but they rarely persist from one season to the next. However, in Hong Kong and Southeast Asia, where flu seasons occur for longer periods of time, strains do persist between seasons.
Keeping these patterns in mind, the investigators traced the geographical movement of the strains to determine whether new flu strains actually originate in tropical regions. Instead, they found that influenza strains frequently migrate back and forth between tropical and temperate regions, and that the tropical regions were not necessarily the source of new strains.
In fact, none of the seven temperate and tropical regions they examined was the source of all new H3N2 flu strains in a given year. The migration pattern was more complex. Virus strains moved from one region to several others each year, and flu outbreaks were traced back to more than one source. And although the virus that migrated between Southeast Asia and Hong Kong persisted over time, its persistence was caused by the introduction of virus from the temperate regions. Therefore, the tropical regions did not maintain a source for the annual H3N2 influenza epidemics. Further, in contrast to annual flu epidemics in temperate climates, relatively low lev
|Contact: Nalini Padmanabhan|
NIH/National Institute of Allergy and Infectious Diseases