The announcement that the company that makes nearly half the flu vaccine used in the United States will not supply any vaccine for the coming flu season left U.S. officials scrambling to pull together a flu vaccination program.

Chiron Corp. said British regulators had forbidden them to sell its flu vaccine after checking out sterility problems at its plant in Liverpool. The U.S. had expected to get 46 million to 48 million doses from Chiron.

That would leave the United States with at least an estimated 54 million doses of vaccine made by other companies - far short of the 100 million health officials had expected to have on hand.

“This is serious but we are on top of it,” Health and Human Services Secretary Tommy Thompson told a news conference Tuesday.

The news hit just as the United States was starting its annual flu vaccine program, one that health officials had hoped would be its biggest yet. Last year’s flu season hit early and unusually hard and doctors hoped that would encourage a usually apathetic population to get the vaccine.

On top of that, health officials had added babies aged 6 months to 23 months to the list of people who should get the vaccine, and they need two doses each to be fully protected.

Influenza kills 36,000 people in the United States and 500,000 worldwide in an average season.

Dr. Julie Gerberding, head of the U.S. Centers for Disease Control and Prevention, urged Americans to save the flu vaccine for people who needed it the most. That includes infants, people over 65, patients in nursing homes, people with chronic illnesses, pregnant women, and those caring for or in contact with these groups.

Realistically, Gerberding said, only a percentage of those who should get the vaccine every year usually do.

“We estimate that the 50-some million doses that we have on hand now will come close to meeting the demand of this group,” Gerberding told a news conference. “One problem is the number of doses and another problem is where is it. We will be working on it.”

“We currently anticipate having approximately 54 million doses of influenza vaccine from Aventis and about another 1 million to 2 million doses of FluMist nasal spray,” HHS said in a statement.

HHS said it was looking at possible ways to make more doses in time for this year. “This includes working with Aventis on its ability to provide more vaccine,” HHS said.

Thompson said HHS would also look at the possibility of diluting the vaccine to stretch it out for certain groups.