Drugmaker Medimmune is recalling nearly 5 million doses of swine flu vaccine because the nasal spray appears to lose strength over time.

The Food and Drug Administration said Tuesday that the vaccine is safe and patients who received the product should be protected from the H1N1 virus.

The FDA says the vast majority of the doses have already been distributed and used. Agency officials say the vaccine met potency specifications when it was distributed in October and November.

Last week, another vaccine maker recalled swine flu shots for children because tests indicated those doses lost some strength.