Some 160 countries and territories have now reported laboratory-confirmed cases of the virus, but how it could potentially change over the coming weeks is still unknown, WHO spokesperson Gregory Hartl told reporters in Geneva.

Although five isolated cases of anti-viral resistance have been reported, no changes to the virus' behaviour have been detected for now, he added.

Mr. Hartl warned that A(H1N1) is expected to increase this winter in the Northern Hemisphere due to the colder weather, but he underscored that WHO cannot predict the death toll over the coming months.

The highest percentages of deaths from the pandemic are among adolescents and young adults, he said, mostly likely because it spreads more quickly in schools and institutions.

On vaccines against A(H1N1), the spokesperson said that each manufacturer is working at their own pace, with clinical trials only having been started recently.

But WHO still expects the first doses to be ready by early fall in the Northern Hemisphere, he added.

Two manufacturers have guaranteed that 150 million doses will be made available, and the agency is pressing other partners to secure more doses, Mr. Hartl noted.

WHO plans to send the bulk of them to the least developed countries, with health-care workers, who work on the frontlines and are the most exposed to the virus, to be given priority.

Last month, the agency raised the alert level for A(H1N1) to Phase 6, the highest on its pandemic alert scale.

However, it stressed that Phase 6 refers to the spread of the virus and not its severity, and that the upgrade means that sustained human-to-human transmission of the virus has spread beyond North America, where it was initially concentrated.#