Updated

A top federal health official, lamenting "a very fragile vaccine production system," urged healthy people Wednesday to defer getting their influenza shots (search) so medication will be available for those most at risk.

"We really need a long-term solution so we don't end up in this year-to-year situation where we don't have a reliable supply," Dr. Julie Gerberding (search) said, after the supply of vaccine to the United States was abruptly cut in half.

Gerberding, head of the federal Centers for Disease Control and Prevention (search) in Atlanta, and other government officials appealed for voluntary rationing in the wake of a major supply interruption.

British regulators unexpectedly shut down a major flu-shot supplier Tuesday, citing manufacturing problems at the Chiron Corp. factory in England where roughly 46 million doses destined for the United States had been made.

That means only about 54 million flu shots will be available this year from a competing firm, and the U.S. government decided quickly that most healthy adults should delay or skip them to leave enough vaccine for the elderly and other high-risk patients.

Vaccine should be reserved for babies and toddlers ages 6-23 months; people 65 or older; anyone with a chronic condition such as heart or lung disease; pregnant women; nursing home residents; children on aspirin therapy; health care workers who care for high-risk groups; and anyone who cares for or lives with babies younger than 6 months.

For everyone else, "Take a deep breath. This is not an emergency," Gerberding said Tuesday. "We don't want people to rush out and look for a vaccine today."

The government has urged voluntary rationing before, during a shortage in 2000. This year, however, will mark a record shortage just before flu season begins.

"We will need the help of the public," said Health and Human Services Secretary Tommy Thompson.

Gerberding, appearing Wednesday on CBS News' "The Early Show," said that if officials "prioritize" the disbursement of available flu vaccine, "we will make it possible for people to get vaccine if they really need it."

Chiron's problem began in August, when it discovered contamination in a small amount of vaccine that delayed its U.S. shipments. Still, top U.S. health officials assured the public less than two weeks ago that close monitoring showed the rest of Chiron's supply was fine, and plenty of vaccine would be available.

Tuesday, British regulators disagreed and suspended Chiron's license for three months, officially prohibiting export of the Fluvirin brand that Chiron manufactures in Liverpool. The sanction means more than a delay, Chiron officials said. The company will ship no Fluvirin anywhere this year.

The move took U.S. regulators by surprise. Food and Drug Administration officials headed to Britain Tuesday night to investigate but wouldn't say if they would ask British regulators for a special release of shots for use here if the flu season proves a bad one.

Chiron had brought more than 1 million doses to this country before its license was suspended but hasn't released the batch, Thompson said. He would not say if those doses were potentially usable.

Thompson asked the maker of the remaining 54 million flu shots to try to make more. Aventis Pasteur plans to try, but can't increase production until it meets existing orders in November.

High-risk patients depend on flu shots because the injections are made of killed influenza virus. Other people have another option: About 1 million doses of an inhaled flu vaccine, MedImmune Inc.'s FluMist, will be available for healthy 5- to 49-year-olds. It's made from live but weakened influenza virus.

A flu treatment called Tamiflu also can protect against infection if swallowed daily during an outbreak. Manufacturer Roche Pharmaceuticals said Tuesday it would step up production in anticipation of greater demand this winter.

Flu vaccine is made using chicken eggs and takes months to brew, meaning manufacturers cannot suddenly produce more. Yet vaccine shortages and delays have plagued the country for several years, and Tuesday's debacle prompted scientists to urge that the system be modernized.

"This points up the vulnerability of our influenza vaccine supply," said Dr. William Schaffner of Vanderbilt University, a government vaccine adviser.

Congress allocated $50 million in the 2004 budget to begin making such changes, half the amount federal health officials had requested. Thompson urged Tuesday that lawmakers provide $100 million next year.

The government is taking other steps to ease the shortage:

--CDC is working with Aventis to alter its flu-shot distribution so that shipments also go to parts of the country that had depended on Chiron's supplies.

--FDA and NIH are studying whether Aventis' vaccine could be diluted to get two doses out of each original shot. A small NIH study several years ago suggested doing so could provide enough protection for healthy people, said Dr. Anthony Fauci, infectious disease chief for the National Institutes of Health.