MINNEAPOLIS — The U.S. Department of Agriculture is working on a vaccine to counter a deadly strain of bird flu, as losses to poultry producers mount.

A pure “seed strain” would target the H5N2 virus — which has already cost Midwest turkey and chicken producers more than 6.7 million birds since early March — as well as some other highly pathogenic viruses in the H5 family that have been detected in other parts of North America. If the USDA decides the vaccine is necessary to stop avian influenza, it will provide that seed strain to drug manufacturers.

The process, though, is fraught with questions about which birds would get the vaccine, how it might affect exports and whether it would be effective against the rapidly spreading strain.