The US Food and Drug Administration says the nasal spray vaccines are safe, but they seem to lose strength over time.
MedImmune has recalled nearly 5 million doses of swine flu vaccine because the nasal spray seems to lose potency over time, federal health officials announced on Tuesday.
It is the second vaccine recalled this month because of lower potency, and it comes as health officials urge millions of Americans to get vaccinated against swine flu.
The recall covers more than 4.6 million doses, but the vast majority have already been used, according to the FDA. Agency officials said the vaccine was strong enough when it was distributed in October and November.
"The slight drop in potency is not expected to have any effect on the vaccine's protective effect," said Norman Baylor, director of vaccine research at the FDA. "We don't recommend revaccination."
The agency is looking into the problem, but he said it is not unusual for vaccines to lose strength over time. MedImmune said the vaccine has a shelf life of four months.
Last week, Sanofi Pasteur recalled hundreds of thousands of doses of children's swine flu vaccine after tests showed that some doses had lost potency; most of those had also been used.
MedImmune, based in Maryland and owned by the London-based AstraZeneca, recalled 13 lots of the vaccine "because of a slight drop in potency found through routine quality control testing", a company spokesman said. "This is not a safety concern. People who received the affected doses don't need to be revaccinated, as the doses met potency specifications."
Swine flu vaccine has been available since early October, and manufacturers have since released more than 111 million doses for distribution in the US. MedImmune makes only the nasal spray, which can be used by healthy people aged 2 to 49.
In recent weeks, states have lifted the limits on who can get the vaccine; at first doses were reserved for people at risk, including pregnant women and schoolchildren.
At a telephone press conference on Tuesday, Dr Anne Schuchat of the Centers for Disease Control and Prevention said 60 million Americans had received the swine flu vaccine, and intensive monitoring for side effects had raised no safety concerns. "We're not seeing any worrying signals," she said.
Vaccine supplies have grown a lot in recent weeks, and she urged parents of children aged 10 or under to get the second dose, because studies show this age group needs both doses for the best protection.
"Flu activity has slowed, but it isn't over," Schuchat said. "None of us knows what flu activity will look like in the coming weeks and months, and it's very important not to become complacent."
The first wave of the swine flu pandemic began last April, when the strain was discovered. A bigger wave began in late summer and is now declining: infection is now widespread in 11 states, down from 48 in late October.
A new Harvard poll published on Tuesday found that concern about swine flu has fallen along with the number of new cases. Only 40% of people now say they are worried that they or a family member will get swine flu in the next year, down from 52% in September. Most parents who sought the vaccine for their children were able to get it, but more than a third of those polled said they did not plan to vaccinate their children. The Harvard School of Public Health polled more than 1,600 adults on 16 and 17 December.
Also on Tuesday, the first known case of swine flu in a dog was confirmed: a 13-year-old dog in New York, infected by its owner. Animals can carry and spread flu viruses, Schuchat said, but such cases are rare and people shouldn't be afraid to enjoy their pets. A few cats and ferrets have also been diagnosed with swine flu.
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