Minnesota turkey farmers are clamoring to vaccinate their flocks this fall against avian influenza. But while initial tests in chickens are encouraging, experts say the vaccine has plenty of downside.

Minnesota has lost more than 9 million turkeys and chickens to avian flu and the requirement to kill surviving birds to stop the disease from spreading. By the time the virus retreated in early June, the flocks of 108 Minnesota poultry farms had been wiped out. So there's not much debate among the state's turkey farmers about whether a vaccine should be used if the deadly virus returns.

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"In Minnesota we're pretty much unanimous that this is something we would use, if we have it," said Steve Olson, executive director of the Minnesota Turkey Growers Association.

But even if vaccinating birds proves effective against avian flu, it is not without consequences. Many foreign countries ban imports of vaccinated poultry products because the vaccine causes meat to test positive for flu antibodies. That makes it difficult to distinguish between an infected bird and a healthy one that was vaccinated.

If trading partners don't want meat from vaccinated birds, there are ways to achieve that without shutting down all U.S. poultry exports, Olson said. A regional ban, limited to just turkeys, is one possibility, he added.

"We'd seek approval for use (of the vaccine) in turkeys in Minnesota only," he said. "If other states wanted to do that, they would seek approval from USDA for the same type of thing."

The National Turkey Federation also supports the use of a vaccine.

Ninety percent of Minnesota turkey products wind up out-of-state, though only about 12 percent of that is sold globally. Olson said that segment could shift to domestic buyers.

But other poultry producers are more dependent on foreign markets. Vaccination could result in lost business for farmers who raise broiler chickens for meat or breeders who sell pedigree animals.

The vaccine discussion is way ahead of the science, said Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota.

Talk of using a vaccine erupted after last month's news that a prototype vaccine is 100 percent effective in chickens. But Osterholm says the results got far more attention than they deserved because they were "based on just several birds. It was a very preliminary study."

Osterholm says there are many more steps to complete before an avian flu vaccine could be approved, manufactured, stockpiled and released for sale by the U.S. government.

Avian influenza testing samples were refrigerated at the University of Minnesota's Veterinary Diagnostics Laboratory, April 8, 2015. Minnesota turkey farmers are clamoring to vaccinate their flocks this fall against avian flu. Riham Feshir | MPR News

It's not clear if the United States Department of Agriculture would even allow the vaccine to be used.

The World Organisation for Animal Health says vaccination is justified only when a government cannot manage an outbreak through animal culling and when the risk to human health is great.

The situation in the U.S. hasn't met that threshold yet, said Director General Dr. Bernard Vallat.

"This strain (of avian flu) was a strain that was able to infect a farm without sufficient biosecurity. But it was not really the worst strain circulating in the world," Vallat said.

Countries that use vaccine to combat avian flu are supposed to have an exit strategy. Long-term use risks establishing a constant presence of the virus in a region or leads to harmful mutations of the bug.

Vaccination has become a trap in developing countries lacking strong eradication practices, said University of Minnesota avian flu researcher Dr. Carol Cardona.

"In many places they started with the idea that it would be one and done, and it hasn't turned out that way," Cardona said. "There's been places where conflict has intervened in Egypt and vaccine just continued."

Cardona, though, says the success of current U.S. eradication efforts means there's a better chance vaccination wouldn't become a trap here. She thinks most poultry farmers would only use the vaccine until they could find the money to upgrade their biosecurity.

For those farmers, she said, a vaccine would be cheaper and it would buy them time.