The makers of the nasal spray version of the flu vaccine say they now know why it didn't protect young children against swine flu — the doses got too warm.
The spray FluMist works well for most flu strains, but small studies found it didn't work very well against the swine flu bug that first emerged in 2009.
The maker AstraZeneca has been investigating since the problem came to light last year. At a medical meeting Thursday, officials said they concluded that the swine flu part of the vaccine is unusually sensitive to heat. The vaccine is refrigerated.
A company official said a more stable strain would be used in the future.
The swine flu strain hasn't been a big player this flu season.