'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.' The 13 lots recalled involved 4.7 million doses of the intranasal vaccine, which is based on a live, weakened virus, but officials believe most of them were administered in October and November when the product would have still been at full potency. The company, headquartered in Gaithersburg, Md., still has 3,000 doses of the vaccine affected by the recall, but no one knows how many are still in other warehouses around the country. 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 conceded. 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 only 18 weeks and why the FDA requires companies to measure potency routinely. MedImmune tests its products every week to check for potency, he said, and the decline was observed during this routine testing. The recall decision, he emphasized, was simply a precautionary measure. People who have already received a vaccine from those lots should be fully protected and there is no need for them to be revaccinated. 'We believe there is no negative impact on vaccine safety and effectiveness,' he said. 