The U.S. Department of Agriculture is taking bids from animal health companies for a bird flu vaccine.

The department's Animal and Plant Health Inspection Service says it's preparing for the possible return this fall of the H5N2 virus that resulted in the death of 48 million birds.

Companies may use a government-prototype seed strain or create their own that could be mass produced for a vaccine if needed.

The USDA won't approve a vaccine for use unless it's deemed necessary to control another outbreak.

The agency said Tuesday it wants to acquire an emergency stockpile of vaccines available for delivery anywhere in the United States within 24 hours.

The USDA seeks companies capable of producing at least 100 million doses and up to 500 million doses.