The H5N1 virus, also known as bird flu, typically does not infect humans, but recent reports of mammal-to-mammal transmission in minks has health experts concerned that the virus may eventually have the potential to spread to humans and trigger another pandemic.

Experts concerned over mammal-to-mammal transmission

According to MedPage Today, the current H5N1 outbreak "has become one of the largest and longest avian flu outbreaks in history." So far, almost 50 million wild and domestic birds in the United States have been culled or killed, and another 50 million have been culled or killed in Europe due to the virus.

Aside from birds, the virus has also infected other animals, such as bears, foxes, skunks, raccoons, and more. Transmission among humans has been largely limited during the current outbreak. Only one person in the United States has been infected so far.

However, a study published last month in Eurosurveillance has raised concerns among epidemiologists about the potential for mammal-to-mammal transmission, which could then increase the risk of H5N1 to human health.

In the study, researchers detailed an H5N1 outbreak among farmed minks in Spain that occurred in October 2022. According to the researchers, it was suspected that the virus had been transmitted among the animals based on "the increasing number of infected animals identified after the confirmation of the disease, and the progression of the infection from the initially affected area to the entire holding."

The researchers also noted that the virus had a novel mutation in the PB2 gene that could impact public health. This mutation can lead to the acquisition of another mutation that allows the virus to recognize human receptors.

In addition, U.K. Health Security Agency released a report in December saying that avian influenza currently represents a Level 3 out of 5 risk to human health: "Evidence of viral genomic changes that provide an advantage for mammalian infection."

How much of a risk does bird flu pose to humans?

Since H5N1 first emerged in Hong Kong in 1997, it has caused periodic outbreaks among birds and other animals. Since then, there have been around 870 H5N1 infections among humans, and around half of these patients died. Although the virus has yet to completely spill over among humans, experts say it does have the potential to become a pandemic.

According to William Schaffner, an infectious disease expert from Vanderbilt University Medical Center and a spokesperson for the Infectious Diseases Society of America, the study "sent up a yellow cautious light" about H5N1 in the infectious disease public health community.

"There was no evidence of infection among any of the mink caretakers, who had very sustained, close contact with those animals, so everybody took a deep breath," Schaffner said. "But nonetheless, everybody [in the infectious disease public health community] is a little bit anxious, and they are watching this."

Other experts say that while the risk of H5N1 causing a pandemic has not necessarily changed, it is still too soon to tell whether the current outbreak and conditions will eventually begin infecting humans more readily.

"In '97 there was a pandemic threat. And that pandemic threat has continued to exist, and we cannot really quantify the threat because we still don't really understand what it takes," said Ron Fouchier, a virologist at Eramus Medical Center in Rotterdam.

"Trying to predict what H5N1 will do in the human population absolutely requires a great deal of scientific humility," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "I will never, ever, take H5N1 for granted. I just don't know what it's going to do."

Preparing for the next pandemic

Although the current risk of H5N1 to humans is low, experts say that it is important to be prepared for future pandemics in general, regardless of the virus that ultimately causes it.

H5N1 "is not the only virus that we should be concerned about for pandemic preparedness," said Tim Uyeki, CMO of CDC's flu division.  "There are many other avian influenza A viruses circulating in birds and poultry that have sporadically transmitted to people and have caused a wide range of illness, including severe lower respiratory tract infection and fatal outcomes."

"There will be a new pandemic for sure. Whether it is this one, I don't know," Fouchier said. "But I think that the current situation is enough that countries actually [should] review their pandemic preparedness plans again."

Notably, experts have also said that they were concerned about how people would react to the public health measures needed to curtail a potential future pandemic, particularly after COVID-19.

"If we did have another pandemic right now I think it would be very difficult to get the public to do anything to try to limit or control transmission. That is to me a big setback," Osterholm said.

"What has become clear to me over time is that the big challenge is not the viruses," said Keiji Fukuda, a former CDC scientist and former assistant director-general of the World Health Organization. "… The real challenge is whether people, whether governments, whether policymakers have the ability to actually address the challenge in the way that needs to be done. And I don't see so much which encourages me, to be blunt. That's what gives me a pit in my stomach." (Fiore, MedPage Today, 2/7; Branswell, STAT, 2/8)