US will vaccinate birds against avian flu for first time — what researchers think

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US authorities have authorized the vaccination of California condors in a bid to protect them from a highly pathogenic strain of bird flu.Credit: Beth Morris/Getty

US officials have authorized the vaccination of the critically endangered California condor (Gymnogyps californianus) against a type of avian flu spreading globally. It is the first time that the United States has approved inoculation of any bird against highly pathogenic avian influenza (HPAI).

The authorization comes as an H5N1 strain of avian influenza, a type of HPAI, has spread to an unprecedented number of countries, lasted longer than a typical outbreak and killed hundreds of millions of birds, as well as many mammals, worldwide. Some countries already vaccinate birds, including commercial flocks, against avian flu. The severity of the outbreak is driving some nations that have been hesitant, including the United States, to follow suit.

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The decision to vaccinate California condors, a closely monitored species, does not mean that authorities are planning to vaccinate the country's poultry stock. Still, researchers will be watching how the inoculation campaign goes, with an eye towards administering the vaccines more widely.

"This authorization opens the opportunity to add another tool for how we address this threat," says Ashleigh Blackford, the California-condor coordinator at the US Fish and Wildlife Service (FWS), headquartered in Washington DC.

The strain of HPAI that is currently spreading first infected birds in China in 1996 and has since caused sporadic outbreaks that usually flare up in the autumn and fizzle out by the following spring. But since late 2021, outbreaks have persisted through the summer in a record number of bird species, along with mammals such as raccoons, foxes and seals.

H5N1 has killed birds before, but "never to the extent we’re seeing in North America and Europe now", says Samantha Gibbs, a wildlife veterinarian at the FWS, who specializes in avian influenza.

Since October 2021, more than 70 countries on five continents have reported outbreaks. And on 22 May, Brazil, which is the world's largest poultry exporter, became the eighth South American country to declare an emergency, in response to the country's first detection of HPAI in wild birds.

H5N1 can infect people, but cases are rare and typically involve close contact with infected poultry. No documented cases of human-to-human transmission of this strain of the virus have occurred thus far, but researchers worry that, with more opportunities to infect mammals, the virus could eventually evolve the ability to spread among people.

Several vaccines against H5N1 are available, but administering the jab to birds has long been controversial among researchers and farmers. Poultry producers worry about the cost and difficulty of vaccinating millions of birds, as well as trade restrictions: many countries forbid the import of vaccinated birds owing to concerns that they could make it difficult to track the virus's spread.

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Until this outbreak, US authorities had claimed that strict biosafety protocols, such as enhanced disinfection procedures for farm workers or wildlife managers, and culling infected birds were enough to mitigate the worst effects of HPAI. The high mortality rate of the current strain of H5N1 in the California condor, which is the largest bird in North America, has forced health officials to re-evaluate this approach.

The species had a brush with extinction in 1987: only 27 wild condors remained at that time, so US officials captured them, bred them in captivity and released them back into the wild. Today, the birds remain one of the world's rarest avian species: as of last year, there were 537 condors, and 63% of them lived in the wild.

So far this year, officials have found 21 dead condors, 15 of which tested positive for HPAI. (The bodies of four birds couldn't be recovered, so their HPAI status is unknown.) It took more than 20 years to rebuild the population to its current level, Blackford says. "This is not the direction we want to be going in."

A poultry-vaccination campaign in France — which will include ducks that are farmed for foie gras — is due to begin in September.Credit: Stephane Mahe/Reuters

Inoculating these birds is a "perfect" use of the vaccine, says Karen Grogan, a clinical poultry veterinarian at the University of Georgia in Athens. But it's unclear whether it will be effective in the condors, because it was licensed for use in chickens. "Even between chickens and turkeys," which are a part of the same family of birds, "a vaccination might be efficacious in one but not the other", she says.

Last week, scientists gave the jab to 20 black vultures (Coragyps atratus), which are part of the same family as the condors but are not endangered, to test the vaccine's safety and efficacy. They will also vaccinate captive California condors before inoculating the wild population during an annual routine health check, Blackford says. The jabs use an inactivated form of an earlier H5N1 strain.

This limited vaccination campaign in the United States is just one part of a broader movement among researchers to study the safety and efficacy of vaccination against avian influenza in birds.

The objective for the condor programme is to conserve an endangered species, says Jean-Luc Guérin, an avian pathologist at the National Veterinary School of Toulouse, France. In other regions, there are aims to curb the spread of the virus altogether.

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For example, the French government has ordered 80 million HPAI jabs as part of a mass-vaccination campaign for the country's poultry stocks, starting with ducks. This campaign, the first of its kind in the European Union, is due to start in September. China, meanwhile, has been vaccinating its poultry stock for nearly 20 years and has had some success in reducing infections in poultry and people1.

Although the US Department of Agriculture is testing four vaccine candidates against H5N1 for use in poultry, it has not signalled that it plans to deploy a wider vaccination campaign for commercially farmed birds.

Gibbs says that, although vaccinations can be an important tool to keep outbreaks in check, the fundamentals of conservation work will be even more important to protect the health of wildlife, and ultimately of people. Efforts to preserve and restore bird habitats will ensure that animals "can fight off not just bird flu, but whatever is going to come next", Gibbs says. "If we set them up for success through conservation efforts, then we don't have to play this catch-up game with pharmaceuticals."

doi: https://doi.org/10.1038/d41586-023-01760-0

Wu, J. et al. Emerg. Infect. Dis. 25, 116–118 (2019).

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