Chances of containment are limited because the potentially catastrophic infection may not be detected until it has already spread to several countries, like the SARS virus in 2003. Avian flu vaccines developed in advance will have little impact on the pandemic virus.

It will take scientists four to six months to develop a vaccine that protects against the pandemic virus, by which time thousands could have died. There is little likelihood a vaccine will even reach the country where the pandemic starts.

That is the scenario outlined on Tuesday by Dr Hitoshi Oshitani, the man who was on the frontline in the battle against SARS and now leads the fight against avian flu in Asia.

"SARS in retrospect was an easy virus to contain," said Oshitani, the World Health Organization's Asian communicable diseases expert.

"The pandemic virus is much more difficult, maybe impossible, to contain once it starts," he told Reuters at a WHO conference in Noumea, capital of the French Pacific territory of New Caledonia. "The geographic spread is historically unprecedented."

Oshitani said nobody knew when a pandemic would occur, it could be within weeks or years, but all the conditions were in place, save one -- a virus that transmitted from human to human.

The contagious H5N1 virus, which has killed 64 people in four Asian countries since it was first detected in 2003, might not be the one to trigger the pandemic, he said. Instead a genetically different strain could develop that passes between humans.

While bird flu cases continued to spread throughout Asia, with Indonesia this week placed on alert after reporting four deaths, Oshitani said the winter months of December, January and February would see an acceleration in cases, and the more human cases the greater risk that the virus would mutate.

Vietnam, Indonesia and Cambodia were most vulnerable due to the large domestic poultry populations, he said.

MASSIVE, RAPID CAMPAIGN

When a pandemic is first detected, health authorities will need to carry out a massive anti-viral inoculation campaign within two to three weeks to have any chance of containment, said Oshitani.

"Theoretically it is possible to contain the virus if we have early signs of a pandemic detected at the source," he said.

Scientists estimate that between 300,000 and one million people will immediately need anti-virals, but there are only limited stocks. WHO will receive one million doses by the end of 2005 and a further two million by mid-2006.

Even when an avian flu vaccine is fully developed, production limitations will mean there will not be enough vaccine.

"Right now we have a timeframe of four to six months to develop and produce a certain quantity of vaccine and that may not be fast enough," said Oshitani.

Last week French drug maker Sanofi-Aventis won a $100 million contract to supply the United States a vaccine against H5N1. The United States has also awarded a $2.8 million contract to Britain's GlaxoSmithKline for 84,300 courses of an antiviral. The purchases are part of a U.S. plan to buy vaccine for 20 million people and antivirals for another 20 million.

Oshitani said the early vaccines were unlikely to protect against the pandemic virus. "The vaccine should match the pandemic strain. So a vaccine developed for the virus in Vietnam now may not protect you from another virus," he said.

But Oshitani fears that once a pandemic occurs, the world's rich nations may dominate vaccine supply.

"The distribution of a vaccine will be a major issue when a pandemic starts. There is no mechanism for distribution," he said. Asked whether poorer Asian nations such as Cambodia and Vietnam would get a vaccine, Oshitani said "probably not".

Avian flu has moved west from Asia and into Russia, with many fearing migratory wild birds will spread the virus to Europe and possibly the United States via Alaska.

But Oshitani casts doubt on the impact migratory birds are having on the spread of avian flu, saying different sub-types of the H5N1 virus are in Asia and Russia.

"There are so many uncertainties about the pandemic. We don't know how it will start. We don't know exactly how it is spreading," he said.

Oshitani said that the successful containment measures used against SARS, such as quarantining those infected and cross-border checks, would fail against an avian pandemic, as people spreading bird flu may not show early symptoms.