The Centers for Disease Control and Prevention said the shots from the French drug maker may have been slightly under-strength. Most of them are presumed to have been administered already and parents are not being urged to contact their doctors, The New York Times reported.

“We think children who got the vaccine are fully protected, assuming they got the two shots we recommend for that age,” said Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases.

The Centers for Disease Control and Prevention said the thimerosal-free vaccine, meant for children 6-to-36 months old, was fully potent when released but had dropped 12 percent during follow-up testing, the Times reported.

The recall of the prefilled syringes, which were released a month ago, is voluntary. Doctors are being told to return any supplies they have left.

Copyright 2009 by United Press International