The World Health Organization has called an unprecedented summit meeting of flu vaccine makers and nations to expand 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 already have agreed to attend the summit on Nov. 11 in Geneva, Switzerland, said Klaus Stohr, influenza chief of the United Nations’ health agency.

With increasing signs that bird flu is becoming established in Asia and several worrisome human cases that can’t be linked directly to exposure to infected poultry, it’s only a matter of time until such a virus adapts itself to spread more easily from person to person and cause a severe worldwide outbreak, he said.

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

The world’s total capacity for flu vaccine now is only 300 million doses, and it would take at least six months to develop a new vaccine to fight a pandemic. The WHO wants to get “all issues on the table,” monetary and scientific, that prevent getting more vaccine more quickly, he 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. But tens of millions die in a pandemic, which occurs every 20 to 30 years, when a flu strain changes so dramatically that people have little immunity from previous flu bouts.

There were three pandemics in the 20th century; all spread worldwide within a year of being detected.

The worst was the Spanish flu in 1918-19, when as many as 50 million people worldwide were thought to have died, nearly half of them young, healthy adults. More than 500,000 died in the United States.

The 1957-58 Asian flu caused about 70,000 deaths in the United States, followed by the 1968-69 Hong Kong flu, which caused about 34,000 U.S. deaths.

The current vaccine shortage in the United States, caused by loss of one of the country’s two major flu shot suppliers, 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 Dr. 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 noted. The National Institutes of Health has given Aventis Pasteur and Chiron Corp. contracts to produce prototype bird flu vaccines that are expected to be ready for human tests late this year. Aventis already 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 shut down last month, causing the U.S. vaccine shortage.

If a pandemic occurred and a vaccine wasn’t ready, antiviral drugs could play a key role in slowing its spread, said Dr. Frederick 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.

The 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, he said.

Bird flu actually describes three deadly strains of avian influenza, which have wiped out millions of chickens in Asia. So far, they have not spread easily from person to person but have been very deadly to those who have become infected. They’re named and numbered for the two “H” and “N” proteins on the surface of the virus.

The first strain, H5N1, appeared in Hong Kong in 1997, causing 18 human infections and six deaths.