The bird flu virus is far more lethal than the SARS virus that struck Asia last year and could unleash a pandemic that could kill as many as 50 million people, a World Health Organization official said Monday.

A WHO estimate last week that H5N1 could infect up to 30 percent of the world's population and kill between two and seven million people was a conservative estimate, said Shigeru Omi, regional director of WHO's Western Pacific Regional Office.

"The maximum range is more ... maybe 20 to 50 million people," Omi said in a speech in Hong Kong.

"It will be incomparable to SARS," he said, referring to the Severe Acute Respiratory Syndrome epidemic that killed 800 people around the world in 2003.

While SARS had a mortality rate of around 15 percent, the deadly H5N1 strain of bird flu kills up to a third of the people it infects. H5N1 has proven to be versatile and is now able to latch itself onto more hosts, Omi said.

"It has gone through huge genetic changes and become more pathogenic. It has affected not only birds, but cats, pigs and tigers ... ducks are now playing a more important role," Omi said.

The virus has killed 32 people in Thailand and Vietnam this year and millions of chickens, ducks and other birds have been culled across Asia.

POULTRY SLAUGHTERING BAN

Almost all the human bird flu victims in Hong Kong, Thailand and Vietnam fell ill after direct contact with sick chickens.

With the illness now endemic in poultry farms, experts fear it will only be a matter of time before the disease mutates into a form that can leap between humans and sweep through populations with no immunity. Pigs are seen as a likely next step.

Infected ducks now display no symptoms of the disease but shed huge amounts of the virus in their feces, a source of concern because ducks and chickens are often kept together in Asia and this could give rise to cross infection.

Two U.S. companies and a Japanese firm are working on a vaccine against H5N1 and clinical trials on its efficacy and safety have begun, Omi said. But he cautioned people against thinking that vaccines were a cure-all.

"Vaccines are very useful in reducing the scale of a pandemic but it is not a magic blitz in averting a pandemic," Omi said.

Due to commercial reasons, mass production of vaccines would only start after a pandemic begins, which means it would only reach the public after a time-lag of at least five to six months.

Hong Kong said Monday it may ban shopkeepers from slaughtering poultry in its battle against the virus.

Hong Kong scientists have been fighting to end the widespread practice of killing live chickens in markets since 1997, when the H5N1 virus first spread to humans and killed six people in the territory.

A food department spokeswoman said the government might set up a central abattoir or restrict slaughtering to a few areas.

  Strong opposition from the poultry industry has prevented the government from stopping stall holders in markets from selling live chickens and ducks and slaughtering them in front of customers.

Hong Kong people like their food fresh and often shop in markets where they can pick the birds they want and have them killed on the spot. Many of the chickens are from mainland China.