The U.S. hasn’t seen a new confirmed human bird flu case in nearly 4 months — why?

Soaring egg prices and concerns about bird flu spreading from wild birds and cattle to humans were top of mind for many Americans at the beginning of 2025. The topic recently made its way back into headlines when Moderna announced that the Trump administration was canceling a $766 million contract to develop a bird flu vaccine for humans.

Over the past 15 months, the U.S. Centers for Disease Control and Prevention has confirmed 70 human cases of bird flu across 13 states. The last known cases were reported nearly four months ago, in February, in Nevada, Ohio and Wyoming.

California was a hot spot for human bird flu cases, with 38 infections confirmed by January this year. No cases have been reported in the state since. The Associated Press has reported, citing state records, that testing efforts have “fallen off.” During 2024 at least 50 people were tested per month, compared with three people tested in March, one in April and none in May.

“In recent weeks, the overall number of new animal H5N1 virus infections reported by [the U.S. Department of Agriculture] has declined, reducing the number of people exposed to infected animals, and human cases have declined as well,” a CDC spokesperson told Yahoo News in an email.

While there’s the possibility that the H5N1 virus is waning in the U.S. for the time being, experts are worried that the efforts to monitor bird flu infections is the factor that’s actually waning. Jennifer Nuzzo, director of the Pandemic Center at Brown University, is worried because while these latest cases have been relatively mild, the virus has been “quite deadly,” historically speaking.

“Of the known human cases that have been identified, about half of them have died, Nuzzo told Yahoo News, “and that is a truly staggering percentage.”

Yahoo News spoke to Nuzzo and Dr. Robert Hopkins, medical director at the National Foundation for Infectious Diseases, in an effort to gain some insight as to why those CDC case numbers haven’t budged in humans.

Why haven’t we seen an increase in human bird flu infections since February?

“The short answer is, we don’t know,” Nuzzo said. But experts like her have offered some possibilities:

1. Migratory bird patterns have changed: Wild birds are primary spreaders of avian flu. The United States could be experiencing a natural and temporary decline in human cases, which tend to peak in the fall and early winter, possibly because migration patterns have seasonally changed as well, a CDC official noted during a May call with U.S. doctors.

“As we expect to see bird migration patterns rev back up here in the next few months, I suspect we’re going to see increased levels of H5N1 in migratory birds,” Hopkins said. “That’s probably going to result in more poultry flocks and more other animals infected. I think that again raises our risk.”

2. It’s possible that less of the virus is circulating: Humans can become infected with bird flu through exposure to sick wild birds, poultry, backyard flocks or cattle. Avian flu infections are still being detected in birds and cattle but not as frequently as they were several months ago. “Since February 2025, there have been fewer affected dairy herds and commercial poultry flocks in the United States, and we believe, fewer workers are being exposed to sick animals,” a CDC spokesperson told Yahoo News in an email.

3. Testing at the state and local levels for H5N1 has decreased: Nuzzo suggested that perhaps there has been an overall reduction in human testing for H5N1, rather than a decrease in viral spread. “One of the concerns that I have is that states have appeared to be conducting just less general testing for H5N1,” Nuzzo said. “Those things coupled with a general lack of any communication from federal agencies about H5N1 leaves us very concerned that what we might be seeing is just a contraction of surveillance and not necessarily an abatement of the virus.”

A CDC spokesperson told Yahoo News that the agency’s “guidance and surveillance efforts for human cases of H5N1 virus infections have not changed. State and local health departments continue to monitor for illness in persons exposed to sick animals. However, no new human cases of H5N1 virus infections have been identified and reported to CDC in recent months.”

4. Less prevalence in wastewater surveillance: People who contract bird flu can shed pieces of the virus through wastewater, like from handwashing, bathing or laundering clothes.

But as the CDC website notes, “Wastewater data cannot determine the source of influenza A viruses.” Nuzzo agrees that wastewater surveillance alone isn’t the best method in determining whether human bird flu infections are waning but “probably the best thing we have in terms of figuring out how much virologic activity is occurring.”

Hopkins says the presence of H5N1 showing up in wastewater surveillance has slowed. “So less virus means less transmission,” Hopkins said. “They are positive factors for right now, and they don’t mean that we won’t have greater problems going forward,” he cautioned.

For the week ending May 24, out of 340 sites nationwide as part of the CDC’s National Wastewater Surveillance System, there were 17 sites, in New Jersey and Oregon, that reported any detection of H5N1, according to the CDC.

5. The current virus strain isn’t good at infecting humans: The CDC has confirmed instances of bird flu that have passed from birds to animals, from birds to humans and from animals to humans. What about human to human?

“It appears that this H5 virus has not developed the mutations necessary to easily transmit to humans and to easily transmit from person to person because we don’t have any confirmed person-to-person spread either,” Hopkins said.

Experts worry federal government cuts could decrease bird flu monitoring in humans

“That’s my deep fear,” Nuzzo said. “I do know there have been concerns about attention on [bird flu] from the agricultural sector, in part due to USDA cuts and letting go of the staff who worked on H5N1.”

With the Trump administration’s efforts to reduce the size of the federal workforce, more than 15,000 USDA employees have taken one of two financial incentives to leave the agency. Three of 14 experts responding to disease outbreaks at the National Animal Health Laboratory Network are among the staff that have accepted the offers.

Nuzzo said even before the cuts, experts like herself were concerned that states weren’t being aggressive enough in responding to H5N1 either because they didn’t think it was a problem or because it was on the back burner.

“In 2024, federal agencies regularly held briefings on the topic with both outside experts like myself, but also state and local health officials,” Nuzzo said. “I think absent that, it’s just hard to know what is going on, and it’s hard to not assume that things aren’t going on. It also makes it hard to say, one way or another, how much of an issue [bird flu] is.”

Should the general public be concerned about bird flu right now?

The CDC says the current risk to the general public is low, although it is higher for people who work with cattle and poultry or who are in contact with wild birds.

Right at this moment, Nuzzo said, the general public shouldn’t be worried about bird flu.

“The people that I’m worried about are people who are being exposed to infected animals,” Nuzzo said, “and that’s largely people in the agricultural sector, the farm workers who have disincentives for coming forward if they become infected.”

Immigration labor accounts for 51% of all dairy labor, according to the National Milk Producers Federation. Experts are concerned that undocumented workers are avoiding bird flu testing out of fear of coming forward and being deported under President Trump’s immigration crackdown.

“The fear among undocumented workers — especially in the current climate — is real and can significantly impact public health efforts,” Dr. Krutika Kuppalli, associate professor in the division of infectious diseases at the University of Texas Southwestern, told the Telegraph.

“If people don’t feel safe coming forward for testing or care, we risk missing early signs of spread, especially in high-risk populations like dairy farm workers,” she added.

A CDC spokesperson told Yahoo News: “Farmworkers are typically a hard population to reach, and CDC continues to do outreach through trusted partners.”

Will bird flu be the next pandemic?

“I do think it’s important for people to realize that even before COVID, pandemics happened regularly,” Nuzzo said, adding that there were three influenza pandemics in the 20th century: 1918, 1957 and 1968.

There was also an influenza pandemic in 2009. “The fact that most people don’t even remember that we had a pandemic in 2009 is because of investments that were made by [then-President] George Bush in 2004, 2005,” Nuzzo said, referring to investments in vaccine development, virus surveillance and people on the ground to respond to a pandemic.

Nuzzo said she isn’t certain if H5N1 will trigger the next flu pandemic. However, she is certain there will be another pandemic, and it will likely be an influenza pandemic.

“The good news is that we can make flu vaccines; we do have tools to address it, but they’re only going to be helpful if we have them when we need them,” Nuzzo said.

When asked about the Trump administration’s decision to cancel the Moderna bird flu vaccine contract, she said: “Shutting down the investments today is a very short-sighted move that we very likely will regret in the future.”