An estimated 800,000 doses of the H1N1 vaccine for children have been recalled because they lost their potency, the US officials say.
US manufacturer Sanofi Pasteur recalled several vaccine lots after stability tests revealed that the pediatric syringes had lost their potency after being shipped.
The doses were for children between aged between 6 to 35 months old, the Centers for Disease Control and Prevention (CDC) reported.
There were no safety concerns, the CDC emphasized, as all the doses passed tests for purity and safety.
The tests showed that the doses contained the recommended 7.5 micrograms of antigen, which is needed to stimulate immunity.
Subsequent testing revealed that some of the virus component had degraded, weakening the vaccine.
Children who had already been administered the vaccine did not need to be re-immunized, the CDC said. But those who had received just one dose should get a second shot, as is required in the case of all children below 10 years, it added.
Last week, the CDC said that swine flu has claimed an estimated 10,000 lives in the United States, more than double the estimated death toll in November.
The H1N1 virus infected about 50 million people, or one in six Americans, the November data showed.
The fatalities include about 1,100 children and 7,500 younger adults, who continue to be the most vulnerable to the H1N1 virus.