The World Health Organization has called an unprecedented meeting of flu vaccine makers and nations to accelerate plans for dealing with the growing threat of a flu pandemic.

Sixteen vaccine companies and health officials from the United States and other large countries have agreed to attend the summit in Geneva on Nov. 11, said Klaus Stohr, influenza chief of the United Nations' health agency.

There are signs that bird flu is becoming established in Asia, and several worrisome human cases cannot be linked directly to exposure to infected poultry. Stohr said it is only a matter of time until such a virus adapts itself to spread more easily from person to person and causes a severe worldwide outbreak.

"We believe that we are closer to the next pandemic than we ever were," Stohr said in an interview before a speech at an American Society for Microbiology meeting in Washington.

The total global capacity for flu vaccine is 300 million doses, and it would take at least six months to develop a new vaccine to fight a pandemic. WHO wants to get "all issues on the table," monetary and scientific, that make it impossible to get more vaccine more quickly, Stohr said.

"If we continue as we are now, there will be no vaccine available, let alone antivirals, when the next pandemic starts," Stohr said. "We have a window of opportunity now to prepare ourselves."

Flu kills about 36,000 people in the United States and a million worldwide each year by conservative estimates, Stohr said. Tens of millions die in pandemics, which occur every 20 to 30 years when a flu strain changes so dramatically that people have little immunity from previous flu bouts.

The current vaccine shortage in the United States, caused by the loss of supplies from one of the country's two major manufacturers, reveals how vulnerable the world is and serves as a dress rehearsal for the kind of rationing and emergency measures that would be needed in a pandemic, said Wendy Keitel of Baylor College of Medicine in Houston.

"The ability to respond with the production of billions of doses of vaccine is quite limited," Keitel said. "We need to think through these problems now. Ninety percent of vaccines are produced in 10 countries that have 10 percent of the world's population."

The United States is the only nation that has commissioned work on potential pandemic bird flu vaccines, Stohr said. The National Institutes of Health has awarded Aventis Pasteur and Chiron Corp. contracts to produce prototype bird flu vaccines that are scheduled to be ready for human tests late this year. Aventis has made 8,000 doses at its plant in Swiftwater, Pa.; Chiron is making its doses at a factory in Europe, not the one in Britain that regulators closed last month, causing the U.S. vaccine shortage.

If a pandemic occurred and a vaccine was not ready, antiviral drugs could play a key role in slowing its spread, said Frederick G. Hayden, a University of Virginia virus expert who has researched and consulted on many flu vaccines and drugs, including oseltamivir, or Tamiflu, which showed some activity against bird flu in lab experiments. It, too, is in short supply.

"It's hard to get explicit numbers, but the production capacity worldwide is very limited," making it difficult to develop an international stockpile that could be used in a pandemic, Hayden said.

WHO has 120,000 packages of the drug, Stohr said, and the United States is stockpiling several million doses.

"That will not go very far," he said, but if targeted to a region where a pandemic was breaking out, "we might be able to buy time" and limit its spread while a vaccine was being readied.

Bird flu actually describes three deadly strains of avian influenza, which have resulted in the loss of millions of chickens in Asia. They have not spread easily among people. They are named and numbered for the two "H" and "N" proteins on the surface of the viruses.

The first strain, H5N1, appeared in Hong Kong in 1997, causing 18 human infections and six deaths. This year it has caused 44 human cases and 32 deaths throughout Asia, Stohr said.

A second strain, H9N2, appeared in 1999 in Hong Kong and China, and caused two human cases in Hong Kong last year. A third strain, H7N7 appeared in 2003 in the Netherlands.