US government is testing avian flu vaccines for birds, but ending historic outbreak isn't that simple
(CNN) -- The United States is facing what some experts are calling "a new era for bird flu."
Since January 2022, the country has been battling the biggest outbreak yet of highly pathogenic avian influenza in wildlife. The virus is a major threat to commercial and backyard flocks, and it has started to show up in hundreds of mammals, including a handful of pet cats.
The risk to humans is low; there has been only one human case of this virus in the US since the outbreak began, according to the US Centers for Disease Control and Prevention, and nine cases globally, mostly among people who work with birds. The CDC says there are trials underway of vaccines that could be used to protect humans in case the virus changes and becomes more of a threat.
Separately, the US Department of Agriculture, the US National Poultry Research Center, and labs at a handful of American universities have been experimenting with vaccine candidates to be used in birds.
The USDA's Agriculture Research Service started trials of four vaccine candidates for animals in April and expects to have initial data on a single-dose vaccine available this month. A two-dose vaccine challenge study -- in which animals are exposed to the virus to see how well the vaccine works -- should produce results in June.
If the animal vaccines look to be protective, the USDA's next step would be to work with manufacturers on whether it would be feasible to use them.
One manufacturer, Zoetis, announced April 5 the development of a vaccine geared toward currently circulating virus strains. The company says it would take about a year to get to the distribution stage in the US.
Vaccines are already available in other countries, including China, Egypt, Indonesia, Italy, Mexico, and Vietnam, and some nations are vaccinating their commercial flocks.
However, in the United States, not all poultry experts are ready to use a vaccine, even if one becomes available -- at least, not yet. Instead, their focus remains on eradicating the virus.
Eradication and vaccination
As of April 26, the CDC says, nearly 58.8 million poultry have been affected by avian flu since January 2022. The virus has been detected in at least 6,737 wild birds, and the number is likely to be much higher. There have been poultry outbreaks in 47 states.
Although this is the worst outbreak in history, improved biosecurity measures have vastly reduced the number of cases in the commercial sector, according to the USDA. When the outbreak began in early 2022, there were 51 detections among commercial poultry. In March 2023, there were only seven.
The USDA says close surveillance work among its Animal and Plant Health Inspection Service (APHIS) and state and industry partners led to the reduction in cases.
Generally, there are two ways of confronting this kind of highly infectious disease in poultry, according to Rodrigo Gallardo, a professor in poultry medicine and a specialist in avian virology at the University of California, Davis.
"One of them is through vaccination action. And then the other one is through eradication," he said.
In the United States, the latter is the approach for now, Gallardo said.
If farmers detect even a single case in a flock, they will put down the birds right away.
"The virus keeps replicating and amplifying if the birds are alive, so the only way of stopping the replication and limiting the dissemination is by depopulation," Gallardo said.
Tom Super, the senior vice president for communications for the National Chicken Council, the national trade association for the US broiler chicken industry, said in an email to CNN that although it supports the ongoing discussions about a vaccination program, "currently we support the eradication policy of APHIS and believe that right now this is the best approach at eliminating [bird flu] in the U.S."
The US Poultry and Egg Association said it's "certainly a topic of discussion," but the organization doesn't have a position on implementing a vaccination program.
Complications of vaccinations
A vaccination program comes with several complications, Gallardo said. Vaccinated birds would be protected, but with this highly infectious disease, they still could shed some virus that could infect unprotected birds.
"So vaccination, in that case, creates amplification if it is not done right," Gallardo said.
Plus, it's difficult to detect the disease in vaccinated birds. Birds that are vaccinated don't always show signs if they're sick, so it would be hard to know which birds to keep separate from the others. Tests also have a hard time telling the difference between antibodies generated by vaccination and antibodies from an infection.
"If you're not able to diagnose it, it might spread more than what it would do if you are able to diagnose it and eradicate it," Gallardo said.
Countries that have chosen the vaccination route see more endemic strains develop, meaning the virus is never really totally wiped out.
"This is a very variable virus, and if you don't update the vaccine that you're applying to meet the change in the virus, then you won't be able to completely protect the birds. Partial protection means more birds will be spreading the virus," Gallardo said.
A vaccine has never been used against highly pathogenic avian influenza in the US, according to the USDA. The agency created a vaccine after an outbreak in 2014 and 2015, but that involved a different strain, so it wouldn't work on the latest version of the virus.
The logistics of a vaccine like this are difficult, said Dr. Yuko Sato, an associate professor in the College of Veterinary Medicine at Iowa State University.
"You have to make sure that the new vaccine will protect against this current virus and hope that it doesn't mutate or change so that the vaccine will continue to be protective," Sato said.
"The vaccine is not a silver bullet. This is not going to prevent infection of the birds, so in order to have an exit strategy as the country, you would have to make sure that if you vaccinate, if you still have positive birds, you have to be able to make sure that you could stamp out the virus. Otherwise, we'll never be looking at eradicating the virus from the United States."
Vaccines may threaten bird businesses
Another concern: Birds are a big business in the US.
The US has the largest poultry industry in the world, with 294,000 poultry farms. The market size for chicken and turkey meat production alone for 2023 is projected to generate $57.8 billion, according to market analysis firm IbisWorld.
Bird flu has hurt business in the US, but it could do so in a bigger way if the nation vaccinates poultry, according to the National Chicken Council.
"The National Chicken Council does not support the use of a vaccine for [bird flu] for a variety of reasons -- the primary one being trade. Most countries, including the US, do not recognize countries that vaccinate as free of [bird flu] due to concerns that vaccines can mask the presence of the disease. Therefore, they do not accept exports from countries that do vaccinate," Super wrote in his email.
The US broiler industry is the second largest exporter of chicken in the world. It exports about 18% of the chicken meat produced in the United States, valued at more than $5 billion annually.
"If we start vaccinating for [bird flu] in the U.S., the broiler industry will lose our ability to export which will have a significant impact on the industry -- while costing billions and billions of dollars to the U.S. economy every year," Super said.
With the way the disease is spreading, scientists would also probably have to vaccinate wildlife -- which is nearly impossible.
Of the birds affected in this outbreak, about 76% are commercial egg-laying hens, 17% are turkeys, and only 5% are broilers, the chickens used for meat, Super said. The rest of the cases have been among ducks, backyard chickens, and game birds.
"So the U.S. poultry sector that least needs a vaccine would have the most to risk from using one," he said.
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