The bird flu virus that rampaged across much of Asia this year is the most likely cause of the next human flu pandemic, which could hit up to 30 percent of the world's people, a top international expert said Thursday.

There was no question about whether another influenza pandemic would sweep through the world's more than six billion people, only a question of when, Dr Klaus Stohr told a news conference.

"There are estimates that would put the number of deaths in the range between 2 and 7 million and the number of people affected will go beyond the billions as 25 to 30 per cent will fall ill," he said.

"This virus is certainly the most likely one which will cause the next pandemic," Stohr said, referring to the H5N1 avian flu virus that has forced Asian governments to kill tens of millions of poultry in a vain attempt to wipe it out.

"We don't know if the next pandemic will happen in the next week or in the next years," said Stohr, who coordinates the World Health Organization (news - web sites)'s global influenza program.

"We have to be ready now," he said.

Two U.S. companies were working on producing a vaccine against the H5N1 virus, but one would not be available until March at the earliest, he said.

That meant people would be vulnerable in Asia through the winter and spring when the virus thrives best, Stohr added.

"If I were to make a suggestion as to where the virus will come from, it would be from the countries that are most affected by the disease in poultry," he said.

The worst hit countries have been Thailand, where the virus has killed at least 12 people, Vietnam, where 20 have died, and China, source of the Severe Acute Respiratory Syndrome (SARS (news - web sites)) that terrified the world two years ago.

MUTATION FEARED

Experts fear a repetition of the 1918-1919 flu pandemic thought to have killed more than 20 million people.

They fear the H5N1 virus, which has infected humans in close touch with infected birds but managed to move from person to person only after close and prolonged contact, could mutate into a form which would sweep through populations with no immunity.

That could happen if the bird flu virus got into an animal, most probably a pig, which can harbor the kind of influenza virus that affects people, mate and mutate.

There is not yet any sign that has happened and the prospective vaccine would be effective because the H5N1 virus had not mutated since April, said Stohr, who was attending a WHO conference of Asian health ministers in Bangkok.

Another WHO expert said the danger would not go away.

"There is evidence that low grade transmission continues to occur and for that reason we might fear that we will have a more long term, perhaps permanent situation," Bjorn Melgaard, head of the WHO Southeast Asia office, told a news conference.

  Migratory birds, which have begun their annual escape from the northern winter for warmer Asian climes, carry the virus without necessarily dying of it and that means it might never be eradicated, he said.

"Obviously, this virus has in various forms come back again and again and again so I do not think it can be eradicated," he said. "We have to live with it."