A bird flu vaccine doesn't work well enough to approve it for emergency use against the current outbreak that's shaken the Midwest poultry industry, the U.S. Department of Agriculture said Wednesday.

The USDA's Animal and Plant Health Inspection Service said in a statement Wednesday that the current vaccine is not well matched against the highly pathogenic H5N2 virus and doesn't provide enough protection.

"The vaccine currently available offers just 60 percent effectiveness in chickens, leaving 4 in 10 birds unprotected. The vaccine's effectiveness in turkeys is still being studied," it said.

By the USDA's count, bird flu has cost chicken and turkey producers more than 45 million birds since early March, mostly in Iowa and Minnesota.

The USDA said it will continue to support efforts to develop more effective vaccines, and will re-evaluate its decision as those become ready for use. The agency said it will carefully consider both the efficacy of any new vaccine and the potential foreign trade losses.

A major concern is that several significant U.S. trade partners have told the USDA they might ban all imports of U.S. poultry and eggs, which could cost producers billions of dollars in lost exports. The reason other countries might balk is that tests for the disease in poultry products look for the same antibodies that vaccines trigger an animal to produce.

If a vaccine is ultimately approved, the USDA said it would be targeted to the states and poultry sectors where it could be most effective — where quarantines, culling infected flocks and enhanced biosecurity can't stop the spread.