Sanofi-Pasteur had reported that some batches of its influenza vaccine failed sterility tests in April, prompting an FDA inspection of the plant in Swiftwater, Pa.
None of the affected material was used in making vaccine and no further problems have occurred, FDA said.
But the agency said it was issuing the warning letter because the source of the contamination has not been determined, meaning it cannot be sure the problem has been corrected.
The letter is an advisory action and the plant can continue making vaccine, said Dr. Karen Midthun, deputy director of the FDA's Center for Biologics Evaluation and Research.
The company has been cooperating fully with the FDA, Midthun said. She said the contamination affected only a small proportion of the vaccine.