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09 oct. 2009 •18:31

Pandemic flu could become seasonal-like virus in 2-3 years

* Spread slows with rising immunity from exposure, vaccines * "No indication" of slowdown so far, nor any big mutations

By Laura MacInnis

GENEVA, Oct 9 (Reuters) - It could take years for the World Health Organisation to downgrade the H1N1 flu from a pandemic to seasonal-like virus, the U.N. agency said on Friday.

The WHO moved its six-point pandemic alert level to the top rung in June [ID:nLC321991] in response to the spread of the new virus widely known as swine flu, which has killed at least 4,500 people, especially in North America.

WHO spokesman Gregory Hartl said that health warning would stay in place until people can better fend off infection from the H1N1 strain.

"At some point in the future, there would be a recognition of the fact that if it's no longer circulating on a sustainable basis in communities. Then you would lower the pandemic level," he said, while stressing: "There is absolutely no indication yet of that happening."

In previous pandemics, Hartl said, it has taken time for worrisome flu strains to become less contagious. The slowdown generally comes from people having some prior exposure to the virus or gaining protection from a vaccine.

"Eventually a pandemic virus becomes more like a seasonal virus and that normally will take something like two to three years," Hartl said. "Once enough people either have been vaccinated or have contracted the virus, then it becomes more difficult to spread. It starts acting like a seasonal flu."

National health authorities conduct regular monitoring of flu viruses and research on the circulating strains is used by pharmaceutical companies who sell seasonal flu shots, which normally contain a mixture of a few viruses.

GlaxoSmithKline (GSK.L), Novartis (NOVN.VX), Baxter (BAX.N), AstraZeneca (AZN.L) and CSL (CSL.AX) are among the firms now scrambling to develop and sell H1N1 flu shots, yielding them billions of dollars in government orders. [ID:nL7604472] China began the world's first mass vaccination programme in late September and Australia and the United States have also launched campaigns targeting children and health workers first.