The first bird flu vaccine won federal approval Tuesday as a stopgap against a potential pandemic until more effective vaccines can be developed. The vaccine is the first to win US Food and Drug Administration approval for use in protecting humans against the H5N1 influenza virus. It would be used if the strain should mutate into a form that could spread easily from person to person to spark a pandemic. The Sanofi Aventis SA vaccine already is being stockpiled for use in an outbreak of bird flu. It will not be commercially available. Approval came on the recommendation of FDA advisers, who in February said the vaccine would be better than nothing.