"It is really surprising that there has not been more pandemic flu activity in many European countries," says Arnold Monto, an epidemiologist at the University of Michigan, Ann Arbor.

In France, flu cases rose in early September, then stayed at about 160 per 100,000 people until late October, when numbers started rising again. The delayed rise was puzzling, says Jean-Sebastien Casalegno of the French national flu lab at the University of Lyon.

"We think that when you get one infection, it turns on your antiviral defences, and excludes the other viruses," says Ab Osterhaus at the University of Rotterdam in the Netherlands."