The nation's first vaccine against bird flu is even less effective than previously thought, according to Food and Drug Administration documents released Monday.
In clinical trials, the two-shot series appears to provide protection to just 45 percent of adults who received the highest dosage of the Sanofi Aventis SA vaccine.
An earlier, interim analysis of the same study of the vaccine suggested it sparked a protective immune response in 54 percent of patients, when measured 28 days after getting the second shot. The New England Journal of Medicine published those results in March 2006.
The FDA released the more recent results, contained in company and agency documents, ahead of a Tuesday meeting where it will ask a panel of outside experts to review the vaccine. The agency is not required to follow the advice of its advisory committees, but usually does. The vaccine is the first against the H5N1 influenza strain to seek FDA approval.