With flu season just around the corner, New York medical experts were distressed to learn yesterday that British health officials blocked delivery of almost 50 million doses of flu vaccine that were to be shipped to America in the next several weeks. Authorities revoked the license of the Liverpool based pharmaceutical giant Chiron, which supplies America with vaccine for approximately half of its flu shots.

Specifics surrounding the suspension of Chiron’s license remained unclear, but wire-service reports suggested that British health officials had concerns both about the safety of the flu vaccine and about factory conditions.

“If we don’t figure out a way to get any more flu vaccine and if the flu season is as bad as it’s been in the past few years, we’re going to have a major problem,” an expert at Weill Medical College of Cornell University said. An assistant professor of public health and medicine at the school, Nathaniel Hupert, who is also an assistant attending physician at New York Presbyterian Hospital, said the demand for vaccinations would probably exceed the supply.

“For the first time in recent memory, we will have to engage as a society in voluntary rationing of the flu vaccine,” Dr. Hupert said. Public-health organizations, community groups, and medical providers would decide who would be denied the vaccine.

In a statement released late yesterday, the city health commissioner, Thomas Frieden, said: “The full implications of this announcement are being determined, as we do not yet know the extent to which area physicians and hospitals, which order their own vaccine, depend on Chiron vaccine supplies. However, it will undoubtedly result in a decrease and a delay in the availability of vaccine in the city.”

A spokesman for the Centers for Disease Control, Llelwyn Grant, said the CDC “is going to have to be very strategic, assuring that those most at risk are able to be vaccinated.”

After emergency meetings held yesterday in Atlanta and Washington, D.C., bringing together officials of the CDC and members of the Advisory Committee on Immunization Practices, both organizations were said to have agreed on priority groups to receive vaccination: children 6 to 23 months of age; adults over 65; people with chronic medical conditions; women who are pregnant during flu season; residents of nursing homes and long-term-care facilities; health-care workers with direct patient-care responsibility, and caregivers who have contact with children 6 months and younger.

A CDC spokeswoman, Bonnie Hebert, said the agency is encouraging healthy people 65 and under not to get flu vaccines this year.

Typically, flu-shot operations start in mid-October to prepare for the November flu season. Each year, between 30,000 and 40,000 people die of the flu in America and more than 100,000 are infected.

It is difficult for public-health officials to generalize about the coming flu season, because each year’s flu strain is different. The chairman of medicine at the NYU School of Medicine, Martin Blaser, who is president-elect of the Infectious Diseases Society of America, said, “The most predictable thing about the flu is its unpredictability.”

The CDC’s Mr. Grant said there are currently 54 million doses of the regular flu vaccine and 1.1 million doses of a live vaccine, in the form of FluMist, available. In addition, the CDC has about 2.5 million emergency doses on hand. But Dr. Hupert noted that the “priority groups” the CDC is targeting, those most at risk for the flu, make up about 185 million Americans. Ms. Hebert, though, said it’s likely that only half of the at-risk individuals will seek vaccination.

Last year also saw a shortage of flu vaccine, because pharmaceutical companies underestimated the demand for shots. In his statement, Dr. Frieden said, “this demonstrates for the second consecutive year the need to improve our current system of ordering and producing the flu vaccine.”

The director at the Center for Public Health Preparedness at Columbia’s Mailman School of Public Health, Stephen Morse, said New York City is particularly vulnerable to flu.

“Everyone in the United States is at risk, but it may well be that a new strain of the flu will first come in through a major port city, like New York,” Dr. Morse, who is professor of epidemiology at the school, said. Similarly, Dr. Hupert said: “Thanks to great features of our town like subways, areas where lots of people congregate, New Yorkers are especially at risk for diseases like influenza.”

Absent the vaccine, the only thing New Yorkers can do, he said, is purchase handkerchiefs and anti-microbial wipes. “Do exactly what your grandmother told you to do,” he said.