H1N1 pandemic flu has spread to some 160 countries and killed about 800 people, and needs to be watched carefully in case it mutates and becomes more severe in winter, the World Health Organisation said today.

"For the moment we haven't seen any changes in the behaviour of the virus. What we are seeing still is a geographic expansion across countries," WHO spokesman Gregory Hartl told a news briefing in Geneva.

The new virus, commonly known as swine flu, has been infecting people worldwide because no one has natural immunity to it. Like all influenza viruses, it may circulate more widely in colder weather and could also mutate in winter, he said.

"We do have to be aware that there could be changes and we have to be prepared for those," Hartl said.

He said the first vaccine doses for the disease should be ready in several months. "We expect the first doses to be available for human use in early autumn of the northern hemisphere," he said.

But it was not yet clear whether people would require a single or double injection for immunity, as clinical trials have just begun, he added.

The WHO so far has promises of 150 million doses from two manufacturers for developing countries and is negotiating with other producers for further doses which will be earmarked for the least developed countries, he said.

Hartl did not name the companies, but leading flu vaccine makers include Sanofi-Aventis, Novartis, Baxter, GlaxoSmithKline and Solvay.

The WHO, a United Nations agency, declared an H1N1 influenza pandemic on June 11. It said last week it was the fastest-moving pandemic ever and now pointless to count every case.