Seasonal flu may not spring from east Asia

FLU season is due any day in Europe and North America, but it may not spring from east Asia as many thought. Researchers may need to monitor flu evolution over more of the planet to match vaccines to next winter's flu.

In 2008 the first global genetic analysis of flu viruses found that flu's annual rampage through the northern, then southern hemisphere's winter is seeded from China and its neighbours. The virus strains' family trees suggested that flu always circulates locally in east Asia before emerging from this crucible for its global excursions. New work suggests the situation is not so simple.

If east Asia is the source, flu should be most genetically diverse there, says Justin Bahl of Duke-National University of Singapore. His team compared viruses collected between 2003 and 2006, from south-east Asia, including Hong Kong, China, and from Australia, Europe, Japan and New York. "We found much less diversity in east Asia than elsewhere," says Bahl. "That surprised us."

East Asian viruses seemed to have migrated there from all over the world, he says, and new varieties seemed to originate anywhere. "There is always a flu epidemic somewhere, and that seeds the annual temperate-zone flu seasons," says Bahl, but the breeding ground is not confined to east Asia (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1109314108).

The researchers who proposed the east Asian source are not convinced. Colin Russell at the University of Cambridge says the new study covers too little time and too few regions to detect a true pattern. Bahl says they will expand the study.

Flu's origins are important because the annual flu vaccine takes six months to make, so it is based on what virus epidemiologists think will be dominant when the vaccine is needed. Wider sampling may therefore be needed.