The elderly and others most at risk from the flu should be the first vaccinated this fall because of lingering uncertainty about vaccine supply, federal health officials said Wednesday.

"We want the flu shot in their arms first," Dr. Julie Gerberding, head of the Centers for Disease Control and Prevention (search), told members of Congress.

The government is hoping to avoid the flu vaccine shortages (search) of the past winter, when a problem with an overseas manufacturing plant caused the nation's supply to be abruptly cut in half.

This year, the Food and Drug Administration is working closely with that manufacturer, Chiron Corp., in hopes that it can sell flu shots here again. To further boost supplies, it also is working with GlaxoSmithKline in its bid to become the nation's fourth vaccine supplier.

But it's too soon to tell if those efforts will pay off, Gerberding said. So federal health officials are planning for a worst-case scenario, urging that the people at highest risk of death or other complications from the flu receive the first available vaccinations. If plenty of vaccine is produced after all, the government will tell healthier people to roll up their sleeves.

People will "have to pay attention, because the message will evolve," Gerberding told a subcommittee of the House Energy and Commerce Committee (search).

High-risk people include those 65 or older, nursing home residents, anyone with a chronic condition such as heart or lung disease, babies and toddlers ages 6 months to 23 months, pregnant women and health care workers who look after high-risk patients.

Those are the same people who were supposed to be given first priority last fall, after British regulators shut down Chiron because of contamination concerns, thus blocking shipment of 48 million doses to the United States.

As it happened, it was a mild flu season, and the government was able to stretch remaining supplies. Ultimately, 57 million people got flu shots last year, Gerberding said; about 3 million shots went unused.

In addition to the worst-case scenario, in which the nation gets only 53 million doses of vaccine, the CDC is also preparing for two other possibilities:

--Best case, a record 98 million doses, if all the companies are cleared to sell and there are no production problems with the notoriously hard-to-brew vaccine.

--A middle scenario, with 75 million to 83 million doses considered most likely.

The nation has never administered more than 83 million flu shots in a single year, even though 185 million people are considered high-risk. That's because many people who need a shot either don't realize it or don't want it. About 63 percent of the elderly got their shots last year, for example, almost as many as in years when supply is ample, the CDC said.

Sanofi-Pasteur remains the nation's leading flu shot provider, with an estimated 50 million to 60 million doses planned for this fall.

Chiron could provide about 25 million shots but first must complete changes at its factory in Liverpool, England, to ensure the vaccine's sterility.

Chiron has "made substantial progress," said FDA vaccine chief Dr. Jesse Goodman, who visited the factory last week. But "it's too soon to tell" if the fixes will be complete in time for flu season.

Glaxo, which has long sold flu shots in other countries, is preparing to seek an FDA license. It could provide 10 million additional doses but only if it meets all FDA requirements in time, Goodman said.

Some 3 million doses of a nasal-spray vaccine for use only by healthy people, MedImmune Inc.'s FluMist, also are anticipated.

Under intense questioning by lawmakers, federal health officials said they are working to guard against future shortages. They plan to stockpile vaccine in new ways, are working more closely with regulators abroad so they're not surprised by problems at an overseas factory and are encouraging additional manufacturers to enter the U.S. market.