The supply of influenza vaccine for the coming winter was abruptly cut in half yesterday when one of only two companies making flu shots for use in the United States said it would not be able to sell 48 million doses here because some of it may be contaminated.

Within hours of the announcement, federal officials said they would appeal to healthy adults to forgo flu shots this year so that the now-diminished supply can go to the young, the old and the infirm, who are at higher risk for complications from the viral illness.

Ironically, the change of course comes at a time when the U.S. Centers for Disease Control and Prevention (CDC) had just expanded the target population for flu shots and was in the midst of a campaign urging more people than ever to get them.

Before yesterday's announcement, this season's supply was supposed to be about 100 million doses, up from 87 million last winter. Influenza kills about 36,000 people in the United States each year, and as many as 500,000 worldwide.

Chiron Corp., which makes flu vaccine at a plant in Liverpool, England, said in August it would deliver most of its product about a month late because some lots had been found to be contaminated with bacteria. Yesterday, however, virtually the entire order was canceled after the British agency that regulates drug manufacturing decided over the weekend to halt all vaccine shipments from the plant for three months while it investigates.

The CDC immediately convened a telephone meeting of experts that advise it on vaccine policy. By mid-afternoon the panel had drawn up a list of groups that should get first claim on the 54 million doses expected to be available. The list excludes nearly all healthy adults.

Speaking at a hastily called news conference at the headquarters of the Department of Health and Human Services, officials said the plan will work only if people at low risk for flu complications do not ask for the vaccine -- and if doctors do not give it to them if they do.

"Health care providers need to try as best as possible to comply with the recommendations . . . to give preference to people who need it most, and to defer vaccine for the healthy population," said Anthony S. Fauci, head of the National Institute of Allergy and Infectious Diseases and an adviser to HHS Secretary Tommy G. Thompson.

"A big rush on vaccine is absolutely what we don't want to see happen," said Julie Louise Gerberding, director of the CDC. An early outbreak of influenza last year led to a run on flu vaccine and fears that it would be an unusually deadly season -- which it did not turn out to be. Gerberding warned against panic this year, too.

"We all need to take a deep breath. This is not an emergency," she said.

Thompson said there are no plans for the government to seize the existing supply of vaccine. Unlike the case with vaccines against childhood illnesses, the CDC owns and controls less than 10 percent of the flu vaccine used each year.

"I think that would cause more disruption than the current system" of voluntary rationing, Thompson said. "It's getting pretty late in the game for trying to find ways legally to distribute the vaccine. I don't think that would be practical."

Leading the list of those who should get the vaccine now are children 6 to 23 months old. This is the first year the government has targeted large numbers of children for flu shots, a decision made because of mounting evidence that influenza causes more death and severe illness in infants and toddlers than previously believed. The CDC estimates that 60 children in that age group died of flu last winter in the United States.

Other high-priority groups include everyone 65 years and older; people between 2 and 65 who have chronic illnesses, such as diabetes or asthma; pregnant women; nursing home residents; children taking aspirin; health care workers; and people in close physical contact with babies less than 6 months old.

The reduced supply of flu vaccine will not be enough to provide shots to everyone in those groups, but it may cover the fraction -- well below 100 percent -- that has sought them in recent years. "Based on our usual patterns, we'll be close" to having enough vaccine, said Gerberding.

In addition to the 54 million doses of the injectable vaccine made by Aventis Pasteur, the U.S. public will have access to as many as 2 million doses of a nasal vaccine, FluMist, made by MedImmune Inc. of Gaithersburg. FluMist, which is more expensive than the traditional vaccine, is made from a "live" weakened version of virus. Under Food and Drug Administration guidelines, however, that product can only be given to healthy people aged 5 to 49, making it inappropriate for most people in the target groups.

Giving a half-dose of vaccine may be one option for stretching the supply, Fauci said. Studies in 2000 showed that a half-dose provided acceptable immunity in healthy adults with that year's vaccine. Whether that is true with this year's formulation, which is different, is unknown. Also unknown is whether half-doses would work in the high-priority groups, which include many people with somewhat weakened immune systems. Studies to answer those questions would take at least four weeks, officials said.

"That is something we will at least consider," Fauci said.

Jesse Goodman, head of the FDA office that oversees vaccines, is going to London this week with other officials to review the findings of the Medicines and Healthcare Products Regulatory Agency -- Britain's equivalent of the FDA -- which suspended the plant's license to sell flu vaccine.

Goodman would not speculate on how his agency might persuade its British counterpart to release some of the vaccine, should tests show it to be safe.

In August, Chiron told that agency it had found some lots of vaccine contaminated with Serratia, a genus of "gram-negative" bacteria that can cause severe, and occasionally fatal, infections in human beings. The company expected to discard about 8 million doses and deliver the balance about a month late after further testing and analysis of plant operations.

Thompson said that more than a month ago his agency started planning what it might do if the entire Chiron supply was lost. He spoke twice yesterday to British Health Minister John Reid, but was still awaiting details on what led to that government's latest decision.

Thompson said "more than a million" doses of the Chiron vaccine are is already in the United States. He would not describe the exact size of that lot or say under what conditions it might be used.

Syringes are loaded with influenza vaccine at a Chiron facility in Liverpool, England.