The shots, made by Sanofi Pasteur, were distributed across the country last month and most have already been used, according to the Centers for Disease Control and Prevention. The 800,000 pre-filled syringes that were recalled are for young children, ages 6 months to nearly 3 years.
Dr. Anne Schuchat, a CDC flu expert, stressed that parents don't need to do anything or to worry if their child got one -- or even two -- of the recalled shots. The vaccine is safe and effective, she said.
The issue is the vaccine's strength. Tests done before the shots were shipped showed that the vaccines were strong enough. But tests done weeks later indicated the strength had fallen slightly below required levels.
Why the potency dropped isn't clear. "That's the $64,000 question," said Len Lavenda, a Sanofi Pasteur spokesman.
Young children are supposed to get two doses, spaced about a month apart. Health officials don't think children need to get vaccinated again, even if they got two doses from the recalled lots, said Schuchat.
Swine flu vaccine has been available since early October, and since then manufacturers have released about 95 million doses for distribution in the United States.
The recalled shots were made by Sanofi Pasteur, the vaccines division of France-based Sanofi-Aventis Group. The vaccine all tested fine when it was shipped out earlier this fall. But last week, testing of one lot showed that the potency had fallen about 12 percent below the government standard, Lavenda said.
The company found three other lots with diminished strength. It notified government health officials and did a voluntary recall, asking doctors to return any unused doses. The vaccine has been in high demand and the company doesn't expect to see much come back, Lavenda added.
Officials with the Food and Drug Administration, the CDC and the company all said they believe the strength of the recalled doses is still high enough to protect children against the virus. No potency problem has been detected in the same vaccine packaged in other types of syringes or vials, Lavenda said.
Experts have a theory that the problem is specific to the children's pre-filled syringes. For some reason, the antigen -- the key vaccine ingredient -- may be sticking to the walls of those syringes, said Dr. Jesse Goodman, the FDA's deputy commissioner for science and public health.
Another manufacturer, Novartis, in February recalled five lots of seasonal flu vaccine packed in pre-filled syringes under similar circumstances.
Sanofi Pasteur bills itself as the No. 1 manufacturer of flu vaccines in the world. It makes flu vaccine at sites in France and in Pennsylvania.
Swine flu was first identified in April. During the first seven months of the pandemic, it has sickened about 50 million Americans and killed about 10,000, according to CDC estimates.