It could take years for the World Health Organization 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 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 , Novartis , Baxter , AstraZeneca and CSL are among the firms now scrambling to develop and sell H1N1 flu shots, yielding them billions of dollars in government orders. 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.

Hartl said there was no sign yet that the pandemic strain had mutated into a more dangerous or more mild form than the one first identified in Mexico and the United States.

"So far the virus has remained quite homogenous," he said.

In its latest snapshot of the spreading virus, also released on Friday, the WHO said there has been an unusually early start of flu-like illness in the northern hemisphere this autumn.

Influenza viruses thrive in colder climates and normally pack the biggest punch in winter.

In recent weeks some countries in Europe seen higher than normal respiratory disease activity and Japan's flu pattern is above its, especially in big cities. The United States, Mexico and Canada have also had higher than normal illness rates for the time of year, the WHO said in the statement.

Flu transmission has stayed steady in tropical parts of the Americas and Asia, with "high intensity respiratory diseases activity" reported in Colombia, Cuba and El Salvador.

But in the southern hemisphere, flu infections have waned with the end of the winter season, the WHO said, describing subsided transmission in Chile, Argentina and New Zealand and falling rates of illness in South Africa and Australia.