Health officials are recalling hundreds of thousands of doses of swine flu vaccine after tests indicated they may not be potent enough to protect against the virus.

The Centers for Disease Control and Prevention notified doctors about the recall Tuesday. The recall involves about 800,000 doses made by Sanofi Pasteur. The doses are pre-filled syringes intended for young children, ages 6 months to almost three years.

Health officials recommend children those ages get two doses, spaced about a month apart.

Health officials say it's not clear how many doses have already been given, but they don't think children need to be re-vaccinated. The lots passed potency tests when they were first shipped, but tests indicate the potency waned after.