WASHINGTON — Drugmaker MedImmune is recalling nearly 5 million doses of swine flu vaccine because the nasal spray appears to lose strength over time, federal health officials announced yesterday.

The vaccine recall is the second this month caused by declining potency and comes as public health officials urge millions of Americans to get vaccinated against swine flu.

The action affects more than 4.6 million doses, but the vast majority have already been used, according to the Food and Drug Administration. Agency officials said the vaccine was strong enough when it was distributed in October and November.

"The slight decrease in potency is not expected to have any effect on the protective effect of the vaccine," said Norman Baylor, director of the FDA's vaccine research office. "We are not recommending revaccination."

The agency is looking into the problem but said it's 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.

Last week, vaccine maker Sanofi Pasteur recalled hundreds of thousands of swine flu shots for children because tests indicated those doses lost some strength. Most of those doses had already been used, too.

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 more than 111 million doses for distribution in the U.S. MedImmune makes the only nasal spray version, which can be used by healthy people ages 2 to 49.