CHICAGO - The U.S. government is testing four potential bird flu vaccines for poultry, officials said on Friday, after more than 58 million chickens, turkeys and other birds have died in the nation's worst outbreak ever.

The trials, conducted by the U.S. Department of Agriculture's (USDA) Agricultural Research Service, are the first step in a lengthy process toward the possible first use of vaccines to protect U.S. poultry from the lethal virus.

Bird flu, also known as highly pathogenic avian influenza (HPAI), has killed hundreds of millions of birds around the world, raising interest in vaccines. The virus is largely spread by wild birds that transmit it to poultry.

The USDA is testing one vaccine from Zoetis Inc, one from Merck Animal Health, and two developed by the department's Agricultural Research Service.

Zoetis said it previously supplied its vaccine to a USDA stockpile in 2016, following a massive 2015 outbreak, but it was never used.

Initial data from a study using a single dose of a vaccine are expected in May, while results from studies on two-dose vaccine regimens are expected in June, the USDA said.

If the trials are successful and USDA decides to continue development, it would take at least 18-to-24 months for a vaccine that matches the current virus to be commercially available, the agency said.

The government needs to ensure vaccinations would not disrupt trading with major buyers, said Greg Tyler, president of the industry group USA Poultry & Egg Export Council.

Governments have previously focused on culling infected flocks to control the virus due to concerns importers would block shipments of vaccinated poultry to avoid the risk of infections.

The USDA said on Friday its "current strategy of stamping out and eradicating HPAI... continues to be the most effective strategy because it works."

France said last week it was launching an order for 80 million doses of vaccines to use in ducks in the autumn if final trial results are positive, the first EU member to start such a plan.