GENEVA - Vaccines should be ready within months for H1N1 flu, which could mutate and become more severe in winter, the World Health Organization said on Friday.

WHO spokesman Gregory Hartl said about 800 people have died from the newly-discovered swine flu virus that has spread to some 160 countries worldwide. So far, the pandemic strain is causing mainly mild symptoms, he told a news briefing.

For the moment we haven't seen any changes in the behavior of the virus. What we are seeing still is a geographic expansion across countries, Hartl said, while warning that the flu could mutate with the onset of colder temperatures.

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

At least 50 governments worldwide have placed orders or are negotiating with pharmaceutical companies to secure supplies of vaccines against the H1N1 strain, which are still being developed and tested.

We expect the first doses to be available for human use in early autumn of the northern hemisphere, Hartl said.

The WHO is trying to ensure that health workers in the world's poorest countries can be vaccinated against the strain so that their hospitals and medical clinics can stay open.

Two manufacturers have promised to donate 150 million doses and the Geneva-based United Nations agency is negotiating with other producers for further doses which would be earmarked for the least developed countries, he said.

Hartl did not name the donor companies. Leading vaccines makers include Sanofi-Aventis, Novartis, Baxter, GlaxoSmithKline and Solvay.

It is still unclear if one or two jabs will be required for protection against the virus -- a never-before-seen combination of swine, bird and human flu strains. Its emergence and international transmission caused the WHO to declare in June that a full pandemic is under way.

While influenza viruses typically spread fastest in winter, when cold and dry conditions help the virus survive outside of the body, Hartl said people have been catching H1N1 flu even in warm climates because they have no natural immunity against it.

Last week, the WHO described H1N1 as the fastest-moving pandemic ever seen and said it was pointless to count every case. [ID:nLG309223] Most of the people infected recover fully without drugs, though pregnant women and those with other health problems have been vulnerable to more serious effects.

Public health officials fear the H1N1 strain could mix with other viruses such as the deadly H5N1 bird flu or become widely resistant to the antivirals Tamiflu, made by Roche and Gilead Sciences, and Relenza, made by GlaxoSmithKline under license from Biota.

Hartl said that five patients have been identified with infections with Tamiflu-resistant H1N1, but said those were isolated and did not require a change to the WHO's guidance on how infections should be treated.

WHO Director-General Margaret Chan has said patients with mild symptoms should avoid seeking medical care unless they have key warning signs. These include long-lasting high fever in adults and a lack of alertness in children.

(Additional reporting by Jonathan Lynn; editing by Robin Pomeroy)

Glaxosmithkline Medicine