The United States is testing four bird flu vaccines for poultry, 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 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, 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, one from Merck Animal Health, and two developed by the department’s Agricultural Research Service.