WASHINGTON - A bird flu vaccine won federal approval for the first time Tuesday as a stopgap measure against a potential pandemic until more effective vaccines can be developed.

The vaccine is the first to win Food and Drug Administration approval for use in protecting humans against the H5N1 influenza virus. It would be used if the strain mutated into a form that spread easily from person to person, sparking 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.