* H1N1 virus spreading beyond schools alongside seasonal flu
* Vaccines to be ready near September, total supply unknown
(Adds WHO statement on vaccines, at-risk populations)
By Laura MacInnis and Stephanie Nebehay
GENEVA, July 24 (Reuters) - The H1N1 flu virus is starting to infect older people, and pregnant women and the obese are at highest risk, the World Health Organisation said on Friday.
In a statement, the United Nations agency said school-age children remain most affected by the newly discovered virus that has been spreading fast in schools and is gaining momentum in broad communities alongside seasonal flu.
"It remains a top priority to determine which groups of people are at highest risk of serious disease so steps to best protect them can be taken," it said, estimating that vaccine manufacturers should have H1N1 shots ready soon.
"Manufacturers are expected to have vaccines for use around September. A number of companies are working on the pandemic vaccine production and have different timelines," the statement on the WHO website read.
About 800 people have died from the new virus whose fast international transmission caused the WHO to declare in June that a flu pandemic is under way. But for most patients, H1N1 is causing mild and manageable symptoms.
"For the moment we haven’t seen any changes in the behaviour of the virus," WHO spokesman Gregory Hartl said earlier on Friday, while warning the virus could change as it circulates, especially in flu-conducive wintry conditions.
"We do have to be aware that there could be changes and we have to be prepared for those," he told a Geneva news briefing.
At least 50 governments have placed orders or are currently negotiating with pharmaceutical companies to secure supplies of H1N1 vaccines, which are still being developed. [ID:nN16442908]
The U.S. Food and Drug Administration is helping companies design ways to quickly test experimental versions of pandemic shots, and the European Medicines Agency is aiming to approve H1N1 vaccines before the onset of northern hemisphere winter, the traditional "flu season" in Europe. [ID:nLO128743]
The WHO is trying to ensure that health workers in poor countries can be vaccinated so hospitals can stay open if the flu becomes more debilitating as it spreads. Sanofi-Aventis (SASY.PA) and GlaxoSmithKline (GSK.L) have promised to donate 150 million doses to this aim to date.
Other leading flu vaccine makers include Novartis NOVN.VX, Baxter (BAX.N), and Solvay (SOLB.BR).
Clinical trials of H1N1 shots "will give a better idea of the number of doses required for a person to be immunised, as well as the quantity of active principle (antigen) needed in each vaccine dose," the WHO said.
Estimates of the global supply of vaccines will be based on how many jabs are needed to protect each person.
The H1N1 virus, first discovered in Mexico and the United States, is a never-before-seen combination of swine, bird and human flu strains, and initially infected mainly young people.
Concerns about the way it was spreading, and about deaths reported among healthy people in North America, caused the WHO to declare that a flu pandemic is under way. [ID:nLC321991] Some 160 countries have now reported infections.
Last week, the WHO described H1N1 as the fastest-moving pandemic ever seen. [ID:nLG309223] On Friday, it said infections were "still increasing substantially in many countries, even in countries that have already been affected for some time." (Additional reporting by Madeline Chambers in Berlin and Michael Kahn in London; editing by Robin Pomeroy) (For a factbox on weather and flu, click on: [ID:nLS126199)