There is no question 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 in Thailand's capital Bangkok on Thursday.
"There are estimates that would put the number of deaths in the range between two and seven million and the number of people affected will go beyond the billions as 25-30% 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," Stohr, who coordinates the World Health Organisation's global influenza programme, said.
"We have to be ready now," he said.
Two US companies were working on producing a vaccine against the H5N1 virus, but one would not be available until March at the earliest, Stohr said.
Thailand is the worst-hit nation,
followed by Vietnam and China
That meant people would be vulnerable in Asia through the winter and spring when the virus thrives best, he 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 at least 20 have died; and China, source of the Severe Acute Respiratory Syndrome (Sars) that terrified the world two years ago.
Experts fear a repetition of the 1918-1919 flu pandemic that is thought to have killed more than 20 million people around the world.
They fear the H5N1 virus, which has infected humans in close contact with infected birds but managed to move from person to person only after close and prolonged contact, could mutate into a form that would sweep through populations with no
"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"
Dr Klaus Stohr,
WHO Global Influenza Programme
That could happen if the bird flu virus got into an animal, most probably a pig, which can harbour 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.
The average age of people who have been infected by the virus was 17, a fact that will help to determine those most at risk and who should receive the first vaccine doses available, he said.