"This is not a safety issue," Norman Baylor, director of the Office of Vaccines Research and Review at the Food and Drug Administration, said in a conference call. "All lots passed pre-release testing for safety, purity and potency. There has just been a slight decrease in potency."
Gaithersburg, Md., still has 3,000 doses of the vaccine affected by the recall, but no one knows how many are in warehouses around the country.
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This is the second recall of a swine flu vaccine. Just a week ago, Sanofi-Aventis recalled 800,000 doses of vaccine for children because it too had lost some potency.
It is unusual for there to be two recalls of a seasonal flu vaccine in a single season, Baylor said. But, he added, "These are biologicals made from living organisms, and it is not abnormal to see a vaccine decrease in potency over time."
That is why the vaccine has a licensed shelf life of 18 weeks and why the FDA requires companies to measure potency routinely. MedImmune tests its products weekly for potency, he said, and the decline was observed during routine testing.
The recall, he emphasized, was a precautionary measure. People who have received a vaccine from those lots should be fully protected and there is no need to be revaccinated.
"We believe there is no negative impact on vaccine safety and effectiveness," he said.