* 800,000 doses of children’s vaccine recalled

* CDC says no safety concerns (Adds quotes, details from letter)

WASHINGTON, Dec 15 (Reuters) - Vaccine maker Sanofi-Aventis SA SASY.PA> SNY.N is recalling four lots of its pediatric H1N1 swine flu vaccine because it is not as potent as it should be, U.S. officials said on Tuesday.

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 age 3, the CDC said in a message to state and local health officers, public information officers and doctors’ groups. A CDC spokesman said about 800,000 doses of vaccine from the lots had been distributed to providers.

The CDC said the four lots were discovered as part of a quality assurance testing program and that the company had also notified the U.S. Food and Drug Administration.

“While the antigen content of these lots is now below the specification limit for the product, CDC and FDA are in agreement that the small decrease in antigen content is unlikely to result in a clinically significant reduction in immune response among persons who have received the vaccine,” the CDC said.

“For children 6 months of age and older, vaccine is available in multidose vials. The vaccine in multidose vials is safe and effective vaccine for children,” it added.

States have been struggling to get H1N1 vaccine to people who want it because companies have had trouble making it quickly. Last week the CDC said 85 million doses of the vaccine had been made available for distribution so far.

The CDC estimates that swine flu has killed 10,000 Americans since it emerged in March and put 200,000 into the hospital. Children, young adults, people with medical conditions such as asthma, healthcare workers and caregivers for infants are all advised to get the vaccine.