What bird flu experts are watching for in 2025

After a year of rising bird flu cases among people in the United States, researchers are keeping a watchful eye on the H5N1 virus to slow further spread.

Since early 2024, the country has recorded 66 cases of highly pathogenic avian influenza caused by H5N1. Farm workers who caught the virus from infected cattle or poultry make up most cases. The first human death related to the disease was announced January 6, but other recent illnesses have often been mild, with symptoms such as pink eye, fever and a cough.

Although the public health risk remains low, researchers worry that insufficient tracking of the virus, particularly in cattle, could increase H5N1’s human pandemic potential. Government action to handle bird flu has been lackluster, scientists say, and the incoming Trump administration’s views on vaccines and other policies may affect the virus’s containment and course.

“It could become a pandemic tomorrow, and it could never become a pandemic. We just don’t know,” says virologist Angela Rasmussen of the University of Saskatchewan in Saskatoon, Canada. But “at a pandemic scale, even a ‘mild’ pathogen can be incredibly destructive.”

Here’s what bird flu experts are watching for in 2025.

Containment of the cattle outbreak

About two-thirds of the recent bird flu cases in humans came from exposure to infected dairy cows. But the outbreak in cattle has yet to be contained.

The U.S. Department of Agriculture first announced finding H5N1 in dairy cows in March, although researchers suspect the outbreak began in December 2023. More than 900 herds of cattle across 16 states have now tested positive. The vast majority are in California, which declared a state of emergency last month.

That same month, the USDA began a national milk testing strategy, sampling milk from multiple cows in bulk tanks. Twenty-eight states are currently testing this way, and around two-thirds of those states have not shown any H5N1-infected cattle since March 2024. As the data come in, Rasmussen says, more states will probably have herds that test positive for the virus.

Additionally, an April order by the USDA requires dairy cattle to test negative for H5N1 prior to transportation to a new state. But plenty of within-state movement is still happening, says virologist Seema Lakdawala of Emory University in Atlanta. A temporary stay-at-farm order, at least until health officials know how widespread the outbreak is, would be ideal.

Considering an entire herd “infected” is also a flawed way of thinking, Lakdawala says. The more H5N1 jumps between cows in a herd — which can contain thousands of animals — the more opportunities it has to acquire genetic changes that make it more dangerous to humans. 

Lakdawala wants to see each dairy cow get tested prior to milking, which shouldn’t present a huge logistical challenge, she says. Squirting some milk directly from a cow’s udder onto a testing device — similar to a COVID-19 rapid test — could quickly uncover an infection. But the USDA either hasn’t come up with a way to implement this or it hasn’t gotten buy-in from farmers, Lakdawala says.

Partnering with private testing companies could help, but “there is no current strategy to isolate infected cows in a herd to reduce the transmission of this pathogen,” she says.

Genetic changes to H5N1

Despite mild symptoms in most of the human H5N1 cases in the U.S., the individual who died with it — a Louisiana man over the age of 65 who had underlying medical conditions — had the country’s first severe illness. Additionally, a 13-year-old girl in British Columbia, Canada needed her blood externally oxygenated, among other measures, in November after catching H5N1. She is no longer in critical condition but remains hospitalized.

These two cases hint that harsher illness may occur when the virus infects people outside the demographics of farm workers, who are often healthy young adults, Rasmussen says.

The serious illnesses also arose from the virus subtype D1.1, which is circulating in poultry and wild birds. A different subtype, called B3.13, is spreading among dairy cows. This could mean the D1.1 subtype possesses some genetic component that increases disease severity, Lakdawala notes.

Genetic analyses of the viruses from both serious cases showed mutations gained after infection that could allow H5N1 to more easily infect people. Those mutations were found in the genetic code for a protein called hemagglutinin, which helps the virus latch onto and invade cells, says virologist Jesse Bloom of the Fred Hutchinson Cancer Center in Seattle. H5N1’s hemagglutinin struggles to stick to cells in the human respiratory tract, which is currently a major barrier to the virus’s spread in people. Changes to the protein would probably be key to allowing person-to-person transmission, he notes.

Still, Rasmussen says, “we don’t actually know how significant some of these mutations are … Just because they do something in the lab doesn’t necessarily mean that they’re going to actually do that” in the real world.

Coinfections of both H5N1 and a seasonal flu virus are also worrisome. If the viruses combine their genetic pieces in a process called reassortment, that could produce a more transmissible virus, Rasmussen says. “Historically, numerous influenza pandemics, including 1918, and including the 2009 H1N1 pandemic, have been associated with a recent reassortment event.”

That possibility provides another reason to get an annual flu shot, she says.

Government action

Many researchers believe that insufficient federal actions have been taken to manage bird flu thus far. Since 2022, the USDA has spent nearly $2 billion on H5N1 measures, which includes reimbursing farms for losses. Preventatively, “it’s been a pretty lackluster response,” Rasmussen says.

From December 2023 to March 2024, when the USDA announced its finding of bird flu in dairy cows, the virus was spreading undetected among cattle and moving to new states, Rasmussen says. That lag time is a big reason why the outbreak is still growing, and why no one knows its extent.

Dairy farm workers need better protection, too.

“I’ve been in multiple [dairies]. What I see in terms of [personal protective equipment] are gloves, and that’s it — there’s no eye protection; there’s no mucosal protection,” such as face shields and masks, Lakdawala says. “We need to start requiring them … at these farms to reduce exposures.”

With such a high exposure risk to H5N1, dairy employees should also be offered H5 vaccines that the federal government has been stockpiling for emergencies, both Lakdawala and Rasmussen say. Around 10 million doses should be available by the end of the first quarter of 2025.

But the Biden administration currently has no plans to deploy them.

“I think that it’s past time that people who are at a high occupational risk of getting infected are offered this particular vaccine,” Rasmussen says.

Meanwhile, the U.S. Department of Health and Human Services announced on January 3 that it’s awarding more than $300 million for bird flu monitoring and preparedness.

Even though Rasmussen thinks the number is too small, it’s “maybe the last money that’s going to go towards this,” she says. She worries about what will happen when President-elect Trump takes office later this month.

His nominated HHS secretary, Robert F. Kennedy Jr., has proclaimed skepticism about vaccines, and his nominated U.S. Food and Drug Administration commissioner, Marty Makary, opposed COVID-19 vaccine mandates.

Their decisions could delay or prevent people from receiving flu shots — not just H5 versions — and halt development of future vaccines, Rasmussen says.

No one can predict whether an H5N1 pandemic will happen, but it’s crucial to be prepared. “We are not doing a good job,” Lakdawala says.