Contamination in a British factory that supplies flu vaccine to the United States is expected to cause some delays but no shortage of flu shots this year, federal health officials said on Friday.

“This is not a crisis,” Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention, said in a conference call. “This is not likely to impact our ability to protect people.”

Gerberding’s comments came one day after Chiron Corp. said it was delaying shipment of 50 million doses of its Fluvirin influenza vaccine because it had discovered some batches in its Liverpool plant did not meet sterility standards.

The California-based company had been expected to supply about half the estimated 100 million doses ordered for the 2004-2005 flu season in the United States. It expects to deliver up to 48 million doses in October, a month later than expected.

Gerberding noted that many clinics would be able to administer flu shots as planned because about half the nation’s supply was coming from another manufacturer, Aventis Pasteur, which has reported no production problems.

Aventis, a unit of European pharmaceuticals group Sanofi-Aventis, said on Friday it was considering increasing production above the more than 52 million doses of Fluzone vaccine it intends to ship to the U.S. market.

Although 180 million Americans are encouraged each year to get a flu shot, only about half actually do. The CDC recommends that people, especially the elderly, health care workers and others in high-risk groups get vaccinated in October and November, before the flu season typically peaks.

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Despite assurances that enough vaccine will be on hand, the problems at Chiron have stirred memories of the troubling vaccine-production glitches that marked the 2000-2001 flu season.

The United States could be vulnerable to an unexpectedly severe flu epidemic if the disease strikes earlier in the year as it did during the 2003-2004 season, which was considered one of the worst in recent memory.

Exact figures for adult fatalities are not available, but the disease killed more than 150 adolescents and children.

Influenza typically kills about 36,000 people and hospitalizes 114,000 in the United States every year, according to U.S. officials.

Heavy demand for vaccine last year prompted federal authorities to order 17 million more doses this year. Chiron, which produced 38 million doses last year, was expected to pick up much of that excess demand.

The company is conducting tests to determine the source of the contamination and still expects to deliver most of its promised vaccine. Shares of Chiron fell more than 8 percent to $43.65 on the Nasdaq stock exchange on Friday.