Health authorities in Hong Kong want to double their stocks of Tamiflu, a drug known to be effective in fighting deadly bird flu, which has killed nine people in Vietnam in recent weeks.

“Now we have 1.7 million doses of Tamiflu *, we want to double that by the middle of the year,” Thomas Tsang, a consultant with the Centre of Health Protection, told a news conference.

Health experts have warned that the H5N1 bird flu virus could trigger a pandemic this winter if it mutates enough to be passed efficiently between humans, who have no immunity to it.

The virus, which is carried by wild birds, has a mortality rate of up to 75 percent in humans and a pandemic could kill up to 50 million people around the world.

Asia’s death toll from the virus is now 41. Vietnam has had 29 fatalities since the first outbreak hit the region’s poultry industry in December 2003, nine of them in a fresh outbreak in the past three weeks.

Twelve people died in Thailand last year.

Most of the victims are believed to have caught the virus from infected poultry but doctors fear it could mutate into one that is easily passed between people.

The World Health Organization (WHO) has singled out Tamiflu as its drug of choice to protect against bird flu and in case of a human flu pandemic but Tsang said the drug, made by Roche AG and also known as oseltamivir, was in short supply.

“The world’s stockpile may not be enough for an epidemic,” he said.

Tamiflu belongs to a drug class known as neuraminidase inhibitors, which block the action of viral enzymes. It was previously proven effective in managing an outbreak of the H7N7 avian strain in the Netherlands in 2003, which infected about 1,000 people.

The WHO says the world is overdue a pandemic that could kill millions and the H5N1 virus is a likely source.

Flu pandemics - global epidemics of new strains of disease that kill an usually higher number of people - come on average every 27 years. The last one was in 1968.