Up to two billion people could be infected by swine flu if the current outbreak turns into a pandemic lasting two years, the World Health Organisation said yesterday.

WHO flu chief Keiji Fukuda said the historical record of flu pandemics indicates one-third of the world's population gets infected in such outbreaks. Independent experts agreed that the estimate was possible.

In Mexico, the hardest hit country so far, high schools and universities opened for the first time in two weeks as the government's top health official insisted the epidemic is on the decline. All students were checked for swine flu symptoms and some were sent home.

"If we do move into a pandemic, then our expectation is that we will see a large number of people infected worldwide," Fukuda said. "If you look at past pandemics, it would be a reasonable estimate to say perhaps a third of the world's population would get infected with this virus."

World has changed

With the current total population of more than six billion, that would mean an infection total of two billion, he said, but added that the world has changed since pandemics of earlier generations, and experts are unable to predict if the impact will be greater or smaller.

Chris Smith, at flu virologist at Cambridge University in England, said the two-billion estimate was possible."That doesn't sound too outlandish to me for the simple reason that this is a very infectious virus," Smith told The Associated Press. "You're talking about a virus that no one in the population has seen before and, therefore, everyone is immunologically vulnerable. Therefore, it's highly likely that once it starts to spread, people will catch it.