More than three-quarters of a million doses of swine flu vaccine for infants and children were recalled Tuesday in the United States after routine tests showed they had lost potency.

Four lots of Sanofi Pasteur's injectable (A)H1N1 vaccine for children aged six months to three years old were recalled after tests conducted after shipping in November showed that the shots had lost strength, the Centers for Disease Control and Prevention (CDC) said in a health alert sent to doctors.

The recall involves some 800,000 doses of vaccine, some of which could well have been administered to young children.

The drop in potency was minimal and children who have been innoculated with vaccine from the recalled lots would not need to be revaccinated, the CDC said.

But children less than 10 years old who have only received one dose of vaccine so far should still receive a second, booster dose, the CDC cautioned.

Clinical trials have shown that children under 10 need two doses of swine flu vaccine, while older children and healthy adults show a robust immune response with one dose.

The recalled vaccines all met manufacturer and US federal government standards for purity, potency and safety at the time of shipping. The drop in potency was found by Sanofi Pasteur in routine testing conducted after the vaccines had been distributed to health care providers.

Sanofi Pasteur is still trying to work out what caused antigen levels in the vaccines to fall.

The recall only affects injectable vaccines for infants and children under three years; no potency drop has been found in vaccines for older children and adults.