Gaithersburg-based MedImmune announced Tuesday evening a voluntary recall of early batches of its nasal spray vaccine against swine flu after tests revealed that the doses lost strength within months of being shipped.

The recall affects 4.7 million doses of FluMist, though most were distributed in October and early November when tests showed that the vaccine was at full strength, said officials at the Food and Drug Administration. Because of that, people who got vaccinated should be protected from the H1N1 virus and do not need to worry about getting vaccinated again, officials said.

The FDA was unable to say how many people received a less-potent dose. And it is unclear how many people might have been affected in Maryland. But state and federal officials said most of the vaccine that had been distributed probably has been used. About 3,000 doses of the vaccine are in a warehouse, although more may be sitting in doctors' offices and health clinics nationwide.

The recall was done as a precautionary measure because of concerns about potency, not safety, officials reiterated.

The diminished strength of the vaccine was detected during routine tests last week. Such tests are done at least 15 times per month for every batch of vaccine, said Sidoney Atse, a MedImmune spokeswoman.

People who were vaccinated "have nothing to worry about," she said. "They are protected. These doses are still well within the potency specifications at the time we distributed it."

Maryland health officials got word of the recall late Tuesday and were unsure how much of the vaccine had been distributed in the state or how much might still be on hand. Officials plan to spend Wednesday checking the lot numbers of the affected batches and alerting health care providers who received the vaccine, said Frances Phillips, the state's deputy secretary for public health services.

"We feel pretty confident this number is quite low," she said. "Back when this vaccine was received, because there was such a demand for it, it got used up every day. We know that because the providers came back and asked us for more every week."

She said staff at doctors' offices should check to see if they have doses of the vaccine from earlier batches. Those should be set aside and not given out.

"The point of this recall is not about what happened in the past; it is about making sure the vaccine that was in these affected lots is not administered in the future," she said.

The recall comes just a week after vaccine maker Sanofi Pasteur voluntarily recalled 800,000 doses of an injectable form of the vaccine made for children, also because of issues of strength. About 10,000 doses of that version of the vaccine had been distributed in Maryland.

Vaccine recalls are "not common, but they do occur," said Norman Baylor, director of the FDA's Office of Vaccines Research and Review. Last year there was a recall for potency issues with a seasonal flu vaccine, he said.

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