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Nearly five million doses of swine flu vaccination have been recalled by drugmaker MedImmune, it has been announced.
The Food and Drug Administration (FDA) said the move affects 4.6 million doses, of which the majority have been used already.
It added that the vaccine was strong enough when it was distributed throughout October and November.
Norman Baylor, director of the FDA’s vaccine research office confirmed that the decrease in strength was unlikely to have any effect on patients who have already used the vaccine. He added: “We are not recommending revaccination.”
The agency is looking into the problem but said it was not uncommon for vaccines to lose strength over time.
MedImmune’s vaccine has a recommended shelf life of about four months. The company has about 3,000 doses in its warehouses but does not know how many remain in the field, according to the FDA.
Vaccine maker Sanofi Pasteur recalled hundreds of thousands of swine flu shots for children because tests indicated those doses lost some strength.
Maryland-based MedImmune, a subsidiary of London-based AstraZeneca PLC, voluntarily recalled 13 lots of its vaccine, “due to a slight decrease in potency” discovered through routine quality control testing, said spokesman Tor Constantino.
“It’s not a safety concern. People who have received doses from the affected lots do not need to be revaccinated. The doses were well within potency specification,” he said.
Swine flu vaccine has been available since early October, and since then manufacturers have released over 111 million doses for distribution in the U.S. MedImmune makes the only nasal spray version, which can be used by healthy people aged two to 49.
