The Food and Drug Administration has issued a warning letter to vaccine maker Sanofi Pasteur after Sanofi reported that some batches of its influenza vaccine failed sterility tests in April.

That report prompted an FDA inspection of Sanofi´s Swiftwater plant, which is the largest producer of influenza vaccine in the United States. The warning letter is an advisory action that contains no sanctions or penalties.

Sanofi Pasteur officials say they have located and fixed the source of the problem — which they identify as a defect in one part of a filtration system — and expect to produce 50 million doses of influenza vaccine in time for the upcoming 2006-07 flu season, as scheduled.

"We believe we have discovered the root cause," said Dr. David Johnson, director of scientific and medical affairs for Sanofi Pasteur.

None of the affected material was used in making vaccine, according to both the FDA and Sanofi Pasteur. The FDA said it issued the warning letter because the source of the contamination has not been determined, meaning the agency cannot be sure the problem has been corrected.

Sanofi Pasteur´s Dr. Johnson, however, said that Sanofi knows what went wrong.

"It was a defect in one filtration system," he said. "We found where and how this happened. ... We have taken corrective action.

"This is an example of why the process calls for frequent testing for sterility," Johnson added. "That´s why we do the sterility tests. ... Fortunately we found this very early."

The problem had no connection to Sanofi´s efforts to develop a vaccine to combat avian flu, also known as bird flu or pandemic flu, Johnson stressed.

Sanofi Pasteur has cooperated fully with the FDA, said Dr. Karen Midthun, deputy director of the FDA´s Center for Biologics Evaluation and Research. Sanofi takes the FDA´s concerns "very seriously," Johnson said.

Sanofi Pasteur, one of the world´s leading producers of vaccines, is the largest private employer in Monroe County, with a workforce of about 1,500. Sanofi plans to open a new state-of-the-art flu vaccine production building, which will double the company´s production capacity, by 2008.

Sanofi Pasteur issued a statement Monday that reads in part:

"Sanofi Pasteur is committed to providing our customers with vaccines of the highest purity, potency, and safety. Consistent with our commitment, we take the FDA´s recent Warning Letter very seriously. It is important to note that none of the issues raised by the FDA compromises the safety, purity, potency, or supply of any of our products in the market."