Those warnings have proved prophetic. And the federal Centers for Disease Prevention and Control (CDC) now finds itself not only trying to blunt the spread of the virus, but also playing catch-up with testing methods that have been largely resisted among America’s farmers.

The question now is whether new interventions can ward off mass human-to-human transmission of this strain—because to some experts, it’s only a matter of time.

“We will have a bird flu pandemic,” Robert Redfield, former director of the CDC, bluntly predicted in a television interview in June. “It’s not a question of if; it’s more a question of when … Once the virus gains the ability to attach to the human receptor and then go human to human, that’s when you’re going to have the pandemic.”

A CDC study released Thursday did little to alleviate that concern. The report found that a significant percentage of H5N1 infections went undetected in dairy workers who worked on farms with cows that were confirmed positive for the virus last summer. Among 115 farm workers who underwent blood tests in Michigan and Colorado, eight had evidence of recent infection in the form of antibodies—but only half of them could recall having symptoms. “All eight had either been milking cows or cleaning the milking mechanisms, officials said. 

Among other things, that result suggests that many more American farm workers could become or already have been infected with the virus without knowing it—all the more reason, the experts say, for federal and state health agencies to aggressively offer testing and enhanced personal protective equipment(PPE) to those with boots on the ground at U.S. dairy and poultry farms.

“This generally confirms what we knew: There are more people that are getting infected on farms than the official tally. The serology bears that out,” says Amesh Adalja, senior scholar at the Johns Hopkins University Center for Health Security. “That’s the reason why so many of us have been wanting more aggressive serological testing on farms, in order to understand the extent of infection and better understand the risks that the virus poses (there).”

During a press briefing on Thursday, CDC principal deputy director Nirav Shah said that nothing in the new data “gives rise to a concern about person-to-person transmission,” adding that the agency believes the virus still poses only a low risk to the general public.

In the United Kingdom, though, government officials have already raised the risk level of the bird flu virus from medium to high. And in the U.S., human H5N1 cases in California and Washington are on the rise. Nationally, 46 human cases have been documented and confirmed during the 2024 outbreak, including a person in Missouri with no known exposure to either cattle or poultry, the two primary sources of exposure so far. All of the individuals have experienced only mild symptoms, such as conjunctivitis or cough, and none have been hospitalized.

From the outset of this year’s H5N1 spread, federal and state agencies have struggled to conduct adequate testing on farms and among farm workers. The reasons are legion, but they include the fact that farmers fear a curtailing of their operations if positive cases are identified, and that many immigrant workers don’t want to interact with any government officials, let alone submit to blood testing or risk having to stay home from work and not get paid if they do test positive.

The CDC lacks the authority to force testing, but the discovery of asymptomatic cases or cases so mild as to be unnoticeable may provoke more forceful suggestions from it and local health agencies, even if those who’ve long studied bird flu and other similar viruses aren’t shocked to learn that some people didn’t know they’d been infected.

“I don’t think any of it is particularly surprising,” says Richard Webby, an infectious disease researcher at St. Jude Children’s Research Hospital in Memphis and director of the World Health Organization Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds.

Webby noted that with almost any infectious disease, but especially one that is respiratory in nature, asymptomatic cases of infection are “absolutely anticipated.” But the ability of H5N1 to spread silently from individuals who don’t realize they’re infected could pose its own problems. And while there is no record yet of human-to-human transmission, experts fear the worst if and when that occurs.

“I don’t think it’s a foregone conclusion that H5N1 is going to be the cause of the next pandemic, but certainly an avian influenza virus in the future will be,” says Adalja. “And I think we have to get it right with this one. Even if it can’t cause a pandemic, we want to think of this as a trial run—and we’re not doing things that are really great, from a trial run perspective, with this virus.”

The CDC recommended Thursday that farm workers with a significant bird flu exposure be tested for H5N1 whether or not they experience symptoms, a measure that could be important as the traditional flu season begins and sorting out one virus from another becomes more challenging. “The best way to limit the virus’s room to run is to test, identify, treat and isolate as many cases as possible in humans and as quickly as possible,” said Shah.

The agency also wants the antiviral Tamiflu offered to workers who had a high-risk exposure to H5N1-infected animals, especially those who didn’t wear adequate PPE. Finally, the agency updated its recommendations to prioritize more PPE for those in higher risk activities, such as those working in milking parlors or in poultry culling operations.

The California Department of Public Health has already distributed PPE to dairy farms and their workers, as have some other states. But like other health agencies, it can’t force the workers to wear them. Anecdotally,  that has been a problem across the country, as workers often shun the gear in hot, stifling conditions.

In a separate report Thursday, dairy farm workers in Colorado confirmed that the hot, humid environments found in milking parlors can make wearing respirators and masks uncomfortable. Especially during summer months, that could lead to fewer protected workers. And even among those who wear masks, degradation of PPE is not uncommon when workers are in close proximity to contaminated milk or animal manure.

It makes for a dangerous combination of circumstances: often unprotected workers laboring in close proximity to infected dairy cows, where the path to infection can be as simple as being splashed with contaminated milk during the milking process. Researchers say that’s exactly why the health agencies need to double down on PPE education and provision.

“It can be probably hard to use a lot of this PPE,” Webby says. “But I think messaging it (is important), so at least the individuals who are at risk understand that there is a risk and understand that their PPE probably does help protect them. Even if it’s something they can’t wear all the time, anything is better than nothing. And messaging is the key.”

Such messaging was virtually absent in the early months of H5N1’s march through America’s dairies and poultry farms earlier this year. At one point in June, Agriculture secretary Thomas Vilsack was reported to have told scientific experts of the bird flu, “It’s just going to burn itself out,” according to Vanity Fair.

Now, those health officials and the agencies they direct are scrambling to find ways to safeguard both people and livestock against a virus that has already infected 446 dairy herds in 15 states and more than 100 million birds, mostly commercial poultry, in addition to the documented human cases. Bird flu’s reported 52% mortality rate in Europe since 2002 is a grim reminder of the stakes—and only massively enhanced prevention, testing and treatment efforts will do, along with complete public transparency about the results of those efforts.

Adalja and some other experts do not believe that this subtype of H5N1 circulating in dairy cows in the U.S. is as severe in humans as other clades have been. But, “what is concerning is the fact that this has been something that’s been allowed to get this way,” Adalja says. “This is something that many of us in the field have been talking about being more aggressive about for months.”

The clock is ticking louder.