BANGKOK: The highly dangerous H5N1 bird flu virus remains a major candidate for the next flu pandemic, even though relatively few people have been infected in the more than 10 years that it has been around.

Professor Albert Osterhaus, head of virology at Erasmus Medical Centre in Rotterdam, said: 'What we're seeing today is unprecedented in history. The virus has spread in Europe, Asia and Africa. It's still spreading, and it's not going down. It's not under control.'

Prof Osterhaus, who led the team that identified the Severe Acute Respiratory Syndrome (Sars) virus in 2003, is a speaker at the four-day international symposium on respiratory viral infections in Bangkok.

More than 60 per cent of the over 400 people who have been infected have died, so an H5N1 pandemic could be lethal, he said.

This is why so many countries are stockpiling medicine and vaccines against a possible outbreak.

All it needs is a slight change to make human-to-human transmission possible. Today, the vast majority of infections are from bird to man.

Dr Ruben Donis from the Centres for Disease Control and Prevention in the United States said the virus is constantly throwing up new subtypes, some more potent than others.

If a mutation starts jumping from person to person, and is not contained within four weeks, then 'it's out of the box', Prof Osterhaus said.

Unlike Sars, where those infected could be isolated to prevent the spread of the bug, this is not possible in a flu virus. A Sars victim becomes infectious days after the first symptoms appear. But a flu victim is infectious before showing any outward signs of illness.

Prof Osterhaus said: 'Incubation is so short, the onset is so rapid and you're infectious so early on that it's an illusion to think you can contain it.'

He said efforts to prevent the seasonal flu, which comes every year, have failed. If the pandemic flu spreads as easily, how is it possible to stop it, he asked.

Stopping all air travel might slow down its spread, but not halt its progress. He said: 'There was no air travel in 1918. But the Spanish flu, which killed over 40 million people, still managed to spread.'

So the best course is to keep alert for emerging flu bugs and to be prepared by having vaccines and antiviral medicine on hand.

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