The World Health Organization (WHO) on Thursday pressed drug makers to accelerate work on a vaccine against an influenza pandemic that could kill millions of people.

Some 50 representatives from drug companies, governments and vaccine licensing agencies are attending a closed-door meeting amid fears that the lethal bird flu virus endemic among poultry in Asia could mutate and spread among humans.

“The current outbreak of H5N1 (bird flu virus) is a stark warning that conditions are highly favorable for the emergence of a pandemic virus,” the agency said in a statement posted on its Web Site http://www.who.int.

“Production capacity for a pandemic vaccine will be vastly inadequate unless other companies engage in vaccine seed development and preparation of batches for clinical testing.”

Only two drug makers - Aventis-Pasteur of France and British manufacturer Chiron Corp - have been working on potential pandemic vaccines and licensing issues.

Both are producing clinical batches of a vaccine and testing is expected to begin on animals and humans early next year.

In the event of a pandemic, vaccines, antiviral agents and antibiotics to treat secondary infections will be in short supply and will be unequally distributed, the WHO said.

WHO spokesman Dick Thompson said the United Nations agency planned a media briefing on the meeting at 9 a.m. EST on Friday.

There was a window of opportunity to meet a pandemic with a vaccine, it said, noting that none had been available in the last two outbreaks in 1957 and 1968. The greatest influenza pandemic occurred in 1918-1919, killing an estimated 40 to 50 million people, according to WHO.

The U.S.-based Centers for Disease Control and Prevention (CDC) has projected that a pandemic today would likely result in between 2 million and 7.4 million deaths globally.

Previously, drug companies have cited thorny patent issues and fears of making huge investments in a vaccine that may not be needed, according to WHO, which provided the prototype strain used to develop the vaccines.

In the event of a pandemic, Europe, which is home to up to 75 percent of the global drug manufacturing capacity would be the key to producing sufficient quantities, it says.

The United States is currently hit by a vaccine shortage during its annual flu season because Chiron was barred from shipping after some doses were found to be contaminated.

In Thailand, where 12 people have died in the bird flu epidemic, authorities have said they are bracing for more outbreaks of the deadly virus as the weather cools and migratory wildfowl arrive. The virus has also killed 20 people in Vietnam.