Americans are facing a shortage of flu vaccine after British health authorities Tuesday unexpectedly yanked the license of the company that produces nearly half the U.S. supply.

Within hours of the British decision, U.S. health officials began rationing flu shots, urging doctors not to vaccinate healthy adults and children.

America is heading into the flu season with about 56 million doses of flu vaccine. In each of the past two years, 83 million people got the shots.

Top U.S. health officials pleaded with doctors and nurses Tuesday to redirect the vaccine to the people most at risk for influenza, a disease that kills about 35,000 people a year. The rationing is voluntary.

Government officials said at a Washington news conference that they were considering the unusual steps of diluting some vaccine for shots for healthy people, buying other foreign-made supplies if they can pass regulatory hurdles and even using some of the vaccine that British health officials just ruled off-limits.

“This is serious but we are on top of it,” Health and Human Services Secretary Tommy Thompson said.

The federal government’s top panel of infectious-disease experts met for several hours Tuesday, issuing a new priority list of who should get shots. They are children between 6 months and 23 months old; adults 65 and older; anyone with chronic diseases, pregnant women, nursing home residents and health caregivers.

There are 180 million people at risk.

Dr. Julie Gerberding, the director of the Centers for Disease Control and Prevention, said most of those people wouldn’t get vaccinated, so supplies should last. “We don’t have much leeway here,” she said.

It’s too early to tell how bad this upcoming flu season will be, Gerberding said.

Not everyone agrees.

“One probability is that we’ll see a return of Fujian (flu), which was bad last year,” said W. Paul Glezen, an epidemiologist at the Influenza Research Center at Baylor College of Medicine.

This is the third flu-vaccine shortage in five years – the eighth shortage of any kind of vaccine in the past three years. Fewer and fewer companies are making flu shots, influenza experts said Tuesday.

This year two companies – Aventis Pasteur and Chiron Corp. – are making about 99 percent of America’s flu shots. A third company makes nasal flu spray, useful for healthy people, but produces only about 1.5 million doses.

Tuesday’s shortage was triggered when British health authorities surprised Chiron and U.S. officials by suspending the license for Chiron’s plant in Liverpool, England, for three months. It’s the only place where Chiron makes its 47 million doses of flu vaccine.

In August, Chiron had reported that a few million doses of its vaccine were tainted with bacteria – identified by U.S. officials as serratia – that can cause urinary tract infections, wound infections and pneumonia. British officials investigated, and suspended the plant’s license early Tuesday because of “overall issues of the system and the process of our manufacturing facility,” Chiron Chief Executive Officer Howard Pien said.

The stock of Chiron, which is based in Emeryville, Calif., fell $7.44 a share to $37.98.

“We profoundly regret that we will be unable to meet public health needs this influenza season,” Pien said in a teleconference Tuesday. “We respect the regulatory authority’s judgment because it is based on concerns of safety.”

Pien said his company’s tests showed the contamination problem “was confined to a limited number of lots” but the firm didn’t dispute the British ruling. Thompson said U.S. authorities had known about the potential problem for several weeks but thought it would keep only 6 million to 8 million doses at most off the market.

The U.S. Food and Drug Administration, which licensed the Liverpool plant, made a major inspection of the plant a year ago and a “limited” one this summer, said Dr. Jesse Goodman, the FDA’s director of biologics evaluation and research. After the August contamination, FDA officials contacted the plant and were waiting for further test results by the company, he said

The U.S. government already has bought about 1.5 million doses of the Chiron vaccine, but they can’t be used unless the FDA says they aren’t contaminated, Gerberding said.

“The reason why this (shortage) is happening is the very limited number of vaccine manufacturers,” said Dr. John Treanor, a University of Rochester professor of medicine and a top flu-vaccine researcher. “This leaves the world very vulnerable to this kind of problem. If something goes wrong at the factory, you might have this kind of problem.”

In 2000, health officials initiated a similar voluntary rationing during a shortage. Because people complied, about 15 million doses weren’t used and were wasted, Baylor’s Glezen said.

“I think it’s a mistake to say we’re going to start rationing the vaccine,” Glezen said. “Then you end up with the vaccine in the refrigerator and not in somebody’s arm.”

Dr. Arnold Monto, a University of Michigan public health professor, said the government’s voluntary rationing made the best of a bad situation: “We have an obligation to those in need to get whatever they need. We may not be 100 percent successful, but it’s worth trying.”

WHO SHOULD GET VACCINATED?

These people should still get flu shots, according to new recommendations made Tuesday by the Centers for Disease Control and Prevention:

-Children ages 6 months to 23 months.

-Adults older than 65, and residents of nursing homes or long-term care facilities.

-If you are between ages 2 and 65 and have chronic medical conditions.

-Women who will be pregnant during the flu season.

-Health-care workers who handle patients.

The CDC suggested that people caring for children younger than 6 months vaccinate themselves with the nasal-spray vaccine.

For more information on influenza on the Web, go to www.cdc.gov/flu/about/disease.htm and www.nfid.org/library/influenza.