Voluntary Recall of Swine Flu Vaccine for Infants Nearly a million doses of swine flu vaccine for infants may have been slightly less potent than required but should work anyway, federal officials said Tuesday in announcing a recall of the shots. The maker of the vaccine, Sanofi-Aventis, voluntarily recalled 800,000 doses of low-dose, thimerosal-free vaccine in prefilled syringes intended for infants ages 6 months to 35 months, the Centers for Disease Control and Prevention said. Since most of the vaccine was released a month ago, it presumably has already been used, but the recall is intended to alert doctors to return any supplies they have left. Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases, said the vaccine was fully potent at release but dropped by about 12 percent in follow-up tests. Nonetheless, Dr. Schuchat said, "we think children who got the vaccine are fully protected, assuming they got the two shots we recommend for that age." "We don't think parents need to contact their doctors," she continued. The vaccine was the only thimerosal-free version licensed for children under 2 years old. Thimerosal is a mercury-based preservative that federal health officials consider harmless, but some parents are afraid of it because antivaccine activists blame it for autism and other ills. Low doses in multidose vials, which contain thimerosal to kill any bacteria or fungus accidentally introduced by needles piercing the rubber stopper, remain available for infants. Two-year-olds can use the nasal spray vaccine, which contains no thimerosal. About 95 million doses in various forms are now available, Dr. Schuchat said. More Articles in Health " A version of this article appeared in print on December 16, 2009, on page A28 of the New York edition.