"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 Associated Press reports.

The manufacturer: Sanofi Pasteur.

The number of doses: About 800,000.

Reuters adds that:

There is no safety concern, and children vaccinated from the lots already will not need to be re-vaccinated, the U.S. Centers for Disease Control and Prevention said.

The affected vaccines are pre-filled syringes for children under 3, the CDC said.

According to the AP: "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."

Check NPR's health blog — Shots — for more as the story develops.