The AstraZeneca FluMist vaccine 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. Swine flu has returned each year since but wasn't a big player this flu season.
The problem first came to light last year, when swine flu was behind most illnesses. At a medical meeting Thursday, company officials said they investigated and concluded that the swine flu part of the vaccine is unusually sensitive to heat.
Flu vaccine is refrigerated, but it is allowed to be out at room temperature for up to two hours during distribution. The doses out on hot days were least effective, and company officials say it's because they degraded and lost potency.
The company plans to use a more stable strain in the future.
FluMist is made using live but weakened virus, and is only approved for ages 2 to 49. Flu shots, made from killed virus, do not appear to be as vulnerable to heat, experts say.
In June, a federal scientific panel — the Advisory Committee on Immunization Practices — took the unusual step of advising doctors to give FluMist to healthy young kids instead of a shot, if available. For years, studies have indicated the nasal spray is the better choice for young children because it prompts a stronger immune response in kids who have never been sick with the flu.
On Thursday, the panel rescinded its preference for FluMist.
Data presented to the panel on Thursday confirmed early indications that flu vaccines of all kinds, including FluMist, didn't work very well this winter. They were roughly 20 per cent effective. This year's vaccines didn't include the exact strain that ended up making most people sick.