U.S. health regulators sent a warning letter to Chiron Corp. detailing manufacturing violations at a British flu vaccine plant and demanding more information on plans to fix problems that wiped out half of the U.S. flu shot supply this year.

In the letter to the company, Food and Drug Administration officials also asked for more details on efforts to find the source of contamination at the plant and the design of studies to investigate the problem, Chiron said.

The warning follows the FDA’s inspection of the Liverpool-based plant in October, which found unsafe conditions, including contamination with bacteria called Serratia that can cause infections in the blood, urinary tract and other areas.

The FDA has said there also seemed to be a problem filling vials at the facility, which was approved to produce all of Chiron’s flu doses intended for the United States.

U.S. officials inspected the plant after British regulators suspended the factory’s license because of the contamination, triggering a massive shortage of flu vaccine in the United States.

The Bush administration has come under fire by lawmakers for failing to ensure the United States would have enough flu vaccine for the current season. FDA inspectors had earlier documented high bacteria levels and other problems at Chiron’s Liverpool plant in June 2003.

Since the shortage, U.S. officials have scrambled to find enough doses to vaccinate the 100 million Americans they hoped would receive the shot this season. So far, 65 million are available this year.

Chiron said Friday it had submitted a plan to the FDA in November to fix the problem and was implementing it.

The Emeryville, California-based firm also said it has met with the FDA and plans to cooperate with the agency, which requires a response to the letter within 15 working days. If companies fail to correct such problems, the agency can seek injunctions, fines and other penalties.

It takes months to make flu vaccine and companies need to get started in February or March at the latest, soon after a special committee decides which strains of the virus to include for the upcoming season’s shots.

Chiron has said it must resume manufacturing by March 2005 for doses to reach the U.S. market in time for the next flu season. “Our plans remain the same,” company spokeswoman Alison Marquiss said.

The FDA’s letter comes days after British authorities extended the plant’s suspension for three more months starting Jan. 4, saying that would give the company more time to fix the problem. Chiron has said the UK action does not stem from any new safety problems.

Chiron and Aventis-Pasteur, a subsidiary of French pharmaceutical company Sanofi-Aventis, are the only two companies that make flu shots for the U.S. market. Maryland-based MedImmune also makes an inhaled vaccine called FluMist.

Other companies, including GlaxoSmithKline Plc, have said they will try to enter the U.S. market as early as next year.

Chiron’s shares were down 14 cents at $30.18 in midday trade on the Nasdaq.