In March, Christian Drouin, a French farmer in the Vendée, discovered that his chickens were dying of avian influenza. He had to take drastic action to cull his flock and prevent the infection from spreading. Normally, veterinarians would arrive to gas the birds with carbon dioxide. But the veterinary teams were overwhelmed with calls to cull flocks infected with the virus, now apparently endemic in Europe.
So Drouin was advised to switch off the ventilation fans in his poultry buildings. As the temperature rose, most of his 18,000 birds died of heat stroke over several hours. The next day, his neighbors helped him bury the carcasses. “After that, I lay down in the dark, stunned by what I had done,” he told Agence France-Presse.
Seeking to stamp out the highly pathogenic H5N1 strain of avian influenza, France and other countries have been culling record numbers of poultry—more than 16 million birds since December 2021 in France alone. Last year, the cost there exceeded €150 million. Now, faced with the desperation of farmers like Drouin, France, the Netherlands, and other hard-hit countries have restarted research into a solution long considered taboo: vaccinating flocks.
Ministers in France and other EU countries are discussing the idea, and Dutch scientists have already begun trials of chicken vaccines. In southwestern France this week, researchers will begin to immunize ducks with a newly developed vaccine. And in October, stakeholders will gather at the World Organisation for Animal Health to discuss how to lower international barriers to shipping vaccinated poultry.
For now, many countries refuse such shipments because they aren’t confident countries with vaccinated birds have controlled the virus. Flu experts worry vaccination may not completely halt outbreaks, perhaps raising the long-term risk that the avian virus leaps to humans. And developing and administering vaccines will be costly.
For all these reasons, vaccination is the last resort, says avian pathologist Jean-Luc Guérin of the National Veterinary School of Toulouse. “We use this tool only if we admit that we cannot control the infection by classical ways.” The United States has not authorized the use of avian influenza vaccines because of the trade implications, hoping culling will stop its current outbreak. The U.S. poultry industry is taking a wait-and-see approach. But in Europe the devastation wrought by the virus, and the cost and logistics of culling millions of birds, may be changing the calculus.
In places where the extremely infectious new strain of avian influenza has taken hold, vaccination “really has the capacity to make a huge difference,” says Richard Webby, a virologist at St. Jude Children’s Research Hospital who studies influenza in birds and other animals. In the long term, researchers say, living with H5N1 may require not just vaccines, but a restructuring of dense European poultry operations.
For 3 decades, ever more strains of avian influenza have been emerging in Asia. The current H5N1 strain arrived in Europe in 2021. It was first detected in the United States in January and continues to spread.
Some researchers are concerned that vaccinating, if not done carefully, will allow H5N1 to persist and continue to mix with strains in wild birds, with the risk that it might evolve to spread among people. The risk for the European Union and United States, although low, is probably the highest since H5N1 emerged 25 years ago, Webby says. “We really don’t want this virus lurking around in poultry farms,” adds Adel Talaat, an infectious diseases expert at the University of Wisconsin, Madison.
In an encouraging sign, vaccines have lessened the impact of recent outbreaks, at least in China. In 2017 the country began mandatory vaccination of poultry against an H7N9 strain that was able to spread to people. Vaccination slashed the prevalence of the virus in poultry and the number of human infections dropped to zero. That accomplishment “could be replicated everywhere,” says virologist Hualan Chen of the Harbin Veterinary Research Institute, who developed the vaccines.
The campaign also paid off for Chinese farmers, who resumed producing broiler chickens, according to a cost-benefit analysis published in March in Preventive Veterinary Medicine. And the United States continued to accept Chinese poultry products, showing trade barriers are not insurmountable. Vaccines also benefit animal welfare by reducing the need to cull flocks, adds Chen, who “strongly” recommends vaccinating poultry against H5 strains.
Still, the virus strain now in Europe, H5N1, can be difficult to control with vaccines because it infects many species, including ducks, whereas H7N9 is chiefly a problem in chickens, Webby says. And economics may favor culling for sporadic outbreaks.
But in hard-hit France, vaccine trials are starting. Two vaccines will be tested on ducks raised for foie gras. Ducks carrying bird flu are the “ultimate reservoir,” Guérin says, because they can spread the virus for up to 15 days before showing symptoms. In the trials, birds will be vaccinated on farms, then exposed to the virus in a lab. The goal is to reduce the amount of virus circulating and so protect other poultry species.
One vaccine, Volvac Best, is made by Boehringer Ingelheim and already used in countries outside Europe, including Mexico and Egypt, to immunize chickens against Newcastle disease and H5N1. Ceva created the other vaccine, the first RNA vaccine to be tested in poultry, specifically for ducks. Results should be available by the end of the year, says Gilles Salvat, a veterinary health expert at the French Agency for Food, Environmental and Occupational Health & Safety. If the vaccines prove effective at lowering viral levels, Salvat hopes they might be ready for market by the end of 2023.
After farmers vaccinate flocks, they’ll need to make sure the virus isn’t circulating silently in any birds that were missed or didn’t respond fully to a vaccine. They will need to swab birds and test for the virus, which can spread on boots, clothing, tires, and even the wind. Such measures will also reduce the risk of spread to humans or wild species, says Carol Cardona, a veterinarian and avian influenza specialist at the University of Minnesota, Twin Cities.
“Vaccines can help, but are not the golden bullet,” says Ron Fouchier, a virologist at Erasmus University Medical Center. The approach may still require some culling, he says, because viruses will continue to evolve and may occasionally escape vaccines.
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