Federal health researchers are working on human and avian vaccines against a bird flu outbreak that has killed several million turkeys and chickens across the country.

The highly pathogenic H5N2 bird flu has touched down in 16 states, particularly in Minnesota and Iowa where, respectively, more than 2.5 million turkeys and 3.8 million egg-laying hens have been lost to the virus. The U.S. Department of Agriculture late Wednesday reported 13 new outbreaks in Minnesota, bringing the total number of affected farms to 44.

Meanwhile, Wisconsin agriculture regulators Wednesday reported two new outbreaks in Wisconsin, one affecting an egg operation with 800,000 hens, the second-largest U.S. incident since the bird flu started spreading earlier this year.

Although no human illnesses have been reported in connection with the H5N2 outbreak and national and state public health officials believe the risk to be low, U.S. Centers for Disease Control scientists are trying to develop a human vaccine. "Preparedness" is the CDC's goal, Dr. Alicia Fry of the National Center for Immunization and Respiratory Diseases told reporters in a conference call Wednesday.

Separately, USDA scientists are working on an avian vaccine.

"We are really at the beginning of this," Fry said. "We are cautiously optimistic we will not see any human cases, but there is a possibility we may."

Minnesota turkey farms have been hit especially hard by bird flu this spring.

Public angst about human health risks from bird flu, in general, spiked in 2003 following the emergence of an H5N1 strain, primarily in Asia. Through March 3, 2015, the World Health Organization has linked 784 human infections and 429 deaths to that strain.

While the H5N2 strain circulating in Minnesota is a "cousin" of that Asian strain, it doesn't appear to present as much risk of severe human illness, said Joni Scheftel, state public health veterinarian.

Person-to-person spread of either strain would be extremely rare, she added. "This virus gives no indications that it would do that. Even the bad H5 viruses that have been identified around the world … do not have a propensity to be transmitted person to person."

The concern right now isn't that the virus could mutate into a more harmful or transmissible version, but that someone with a genetic profile of susceptibility to infection could come in contact with the current strain, Scheftel said.

In Minnesota, health officials have monitored 101 turkey farmworkers for infections. The state recommended preventive antiviral treatment for 67 of the 101 Minnesota workers whose lack of protective equipment at work might have put them at risk for exposure. Only 48 agreed to take the recommended Tamiflu, though, due to the low risks of transmission and illness from this strain.

"We've had zero human cases even among people who have had intense contact with sick birds," Scheftel said. "That's very different from the other viruses we've been alarmed about in the past. … Any H5 has the potential to infect people, so we're acting out of an abundance of caution."

The USDA's Poultry Research Laboratory in Georgia is working on the vaccine for poultry.

"We are making progress on that," David Swayne, the lab's director, said in the conference call Wednesday.

The USDA is working on a "seed strain," essentially a pure virus sample that could be the foundation for producing an effective vaccine. Animal testing will begin in early May, and if it proves successful, the USDA would look to the private sector for production.

A successful vaccine could be particularly important given that avian flu researchers expect that the H5N2 could last for a few years now that it's entrenched in North America. It's believed that the flu originated from migrating waterfowl that don't get sick from the virus but spread it through their feces.

When birds migrate south next fall, another wave of outbreaks could occur. Animal health researchers say the H5N2 virus thrives in colder weather but should wither in heat. "When warmer weather comes in consistently across the country, I think we will stop seeing new cases," said John Clifford, the USDA's chief veterinary officer.

In the meantime, state disease investigators might not be finding human infections, but they are finding heartbreak as they contact turkey farmers who've lost entire flocks.

"It's extremely shocking to see that number of birds die," Scheftel said. "These people are somewhat traumatized."

The Associated Press contributed to this report.

Mike Hughlett • 612-673-7003

Jeremy Olson • 612-673-7744