Drugmaker MedImmune is recalling nearly 5 million doses of H1N1 flu vaccine because the nasal spray appears to lose strength over time, federal health officials announced Tuesday.

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 H1N1 flu shots for children because tests indicated those doses lost some strength. Most of those doses had already been used, too.

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.

The H1N1 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 ages 2 to 49.

Only in recent weeks have state authorities lifted restrictions on who can get vaccinated. Previously the vaccines were reserved for high-risk patients, including pregnant women and schoolchildren.

In a telephone news conference on Tuesday, Dr. Anne Schuchat of the federal Centers for Disease Control and Prevention estimated that 60 million Americans have received H1N1 flu vaccine, and said intense monitoring for side effects has not turned up any safety concerns.

"We are not seeing any worrisome signs," she said.

The vaccine supply has increased so much in recent weeks that she urged parents of children 10 and younger to get them a second dose, because studies show this age group needs two for optimal protection.

Flu activity has slowed, but "it's not gone," Schuchat said. "None of us know what the weeks and months ahead will bring in terms of influenza activity, and it's very important not to become complacent."

The first wave of the H1N1 flu pandemic began in April, when the strain was discovered. A larger wave started in the late summer and is declining. Infections are now widespread in 11 states, down from 48 in late October.

A new Harvard poll released Tuesday finds that concern about H1N1 flu has waned along with the number of new cases being reported.

Only 40 percent of people now say they are concerned that they or a family member will get sick from swine flu during the next year. That's down from an earlier poll in September, when 52 percent said they were worried about swine flu.

The new poll found that most parents who sought vaccine for their children were able to get it. However, more than one-third of respondents said they did not plan to seek it for their kids.

The Harvard School of Public Health polled more than 1,600 adults on Dec. 16 and 17.

Also on Tuesday, a 13-year-old dog in suburban New York was confirmed as the first known case of swine flu in a dog. The pet apparently caught the virus from his owner. The CDC's Schuchat said that animals can carry and spread flu viruses, but such cases are rare and people should not be afraid to enjoy their pets. A few cats and ferrets have also been diagnosed with swine flu.