LOS ANGELES (AP) – Antibodies found in early results of a historic new vaccine trial are expected to give endangered California condors at least partial protection from the deadliest strain of avian influenza in U.S. history.

The California condor is the only bird species in the U.S. that has been approved for the new emergency-use vaccine, which was administered this summer to condors bred in captivity during a trial at the Los Angeles Zoo, the San Diego Zoo Safari Park and the Oregon Zoo.

Authorities launched the study after the avian influenza deaths earlier this year of 21 free-flying condors in Arizona, part of a Southwest flock usually accounting for a third of the wild population.

Wildlife officials feared that the outbreak’s toll on the California condor population could erase any gains made to rebuild the wild population, spurring the efforts to fast-track the vaccine.

After 40 years of recovery efforts to prevent the extinction of the iconic vulture with a 10-foot (3-meter) wingspan, the wild population today has fewer than 350 condors in flocks spanning from the Pacific Northwest to Baja California, Mexico.

“Losing 20 birds is effectively akin to setting the recovery program back by 10 years,” said Dr. Hendrik Nollens, vice president of wildlife health for the San Diego Zoo Wildlife Alliance.

The bird flu reached the U.S. in February 2022 after wreaking havoc across Europe. U.S. agriculture officials consider this year’s cases to be part of last year’s outbreak, which was recorded as the country’s deadliest ever.

Authorities confirmed the flu’s presence earlier this month in commercial poultry flocks in South Dakota and Utah, heightening concerns ahead of the spring migratory season. The outbreak cost poultry producers nearly 59 million birds across 47 states, including egg-laying chickens and turkeys and chickens raised for meat. The flu also caused spikes in egg and turkey prices for consumers and cost the federal government more than $660 million.

Early results indicate that when 10 condors were vaccinated with half a milliliter (0.016 fluid ounces) on two occasions – an initial injection and a booster administered 21 days later – 60 percent of the birds showed measurable antibodies expected to protect them from avian flu after exposure.

“We’re thankful that we’re getting any immune response,” said Ashleigh Blackford, the California condor coordinator for the U.S. Fish and Wildlife Service.

The bird flu trial`s progress will allow wildlife officials to move forward and release roughly two dozen vaccinated condors into the wild in California and Arizona by the end of the year. The government is awaiting additional results before deciding whether free-flying condors should be captured and inoculated. Officials already vaccinate condors in captivity and in the wild for West Nile virus.

Dr. Carlos Sanchez, the Oregon Zoo’s director of animal health, said wildlife officials faced questions about undertaking the bird flu vaccine study.

“Human intervention, veterinary intervention, is not something we do all the time or take lightly,” he said. “It wasn’t an easy decision.”

The shots initially were tested on black vultures to make sure they could be safely injected into condors in managed care beginning in July. The post- inoculation monitoring and testing lasted 42 days and officials said no adverse reactions occurred.