For nearly two years, the U.S. has had to swallow an uncomfortable reality: A highly infectious disease it once eliminated is spreading again.

More than 4,400 measles cases and at least three deaths have been reported nationwide this year, marking the country's worst outbreak in more than three decades. The spike followed nearly 2,300 cases in 2025. But many public health experts argue that the return of measles isn't the only concerning development — it's also what the resurgence signals about America that's causing alarm.

Measles is often described as one of the most contagious viruses, and it is uniquely unforgiving of weaknesses in public health defenses. A safe and highly effective vaccine has existed for decades.

But the virus's spread exposes declining vaccination rates, eroding trust in health institutions and growing challenges in persuading people to protect themselves, their children and their communities, experts said.

Together, those vulnerabilities have made measles a revealing test of the country's public health defenses. For some experts, the resurgence of a disease the U.S. eliminated more than two decades ago is a warning sign of its ability to contain future threats, from other vaccine-preventable diseases to emerging pathogens.

"Measles is the canary in the coal mine for so many different reasons," said Jennifer Nuzzo, an epidemiology professor and director of the Pandemic Center at Brown University School of Public Health. "It's the canary in the coal mine for other vaccine-preventable illnesses, where we could see a return of things that used to harm kids, but also for very acute, no-notice dangerous infectious disease emergencies." The U.S. is on track to lose its official measles elimination status during an upcoming review by the Pan American Health Organization, which will meet in Washington, D.C., in mid-November.

"It says a lot about a country if it is unable to control measles," said Nuzzo, who is one of 10 experts CNBC spoke with about the ongoing outbreak and its implications.

The U.S. has the tools to prevent measles from spreading. But pockets of low vaccination coverage are leaving the country vulnerable.

The measles, mumps and rubella, or MMR, vaccine is about 97% effective against measles after two doses, while a single dose is 93% effective, according to the Centers for Disease Control and Prevention. Yet data aggregated by the CDC shows that vaccination coverage for that shot among U.S. kindergarteners fell to 92.4% in the 2025-2026 school year, which is below the 95% threshold public health officials generally recommend to prevent sustained transmission.

That translates to 280,000 kindergarteners who did not complete their MMR vaccination series, according to the CDC. Childhood vaccination exemptions in the U.S. also reached a record high 4.2% for incoming kindergartners during the 2025-2026 school year, up from 3.6% the previous year, the agency says.

"The bottom line is that not enough people are vaccinating their kids," said Brown's Nuzzo.

While some cases stem from international travel, much of the recent sp Among states across the U.S. collecting MMR vaccine data, 67% of counties and jurisdictions now have immunization rates below 95%, according to a September investigation by NBC News. Recent U.S. outbreaks have often occurred in "closely knit communities" where people have frequent contact and low vaccination coverage, said Dr. William Moss, an epidemiology professor at Johns Hopkins Bloomberg School of Public Health.

But declining vaccination rates do not mean most Americans have turned against immunization. Polling suggests broad support for childhood vaccines. A July KFF survey found that many Americans sit in a more uncertain middle ground about the shots, leaving room for conflicting information to influence their decisions.

"This is not an edge-case issue," Nuzzo said, adding that the vast majority of parents still vaccinate their children.

Even if outbreaks are concentrated in a minority of communities, it can create a "dangerous situation" far from those areas, she said. Infants too young to be vaccinated and people who cannot receive immunizations for medical reasons can be particularly vulnerable when the virus spreads into communities with high vaccination rates. Those risks come as the Trump administration reshapes federal vaccine policy.

President Donald Trump signed an executive order in August suggesting that the combined MMR vaccine be split into separate shots, while repeating false claims about its safety. Health and Human Services Secretary Robert F. Kennedy Jr. has also overhauled a key federal advisory committee responsible for recommending childhood immunizations, though the changes are frozen amid ongoing litigation.

Several experts told CNBC that such moves risk further eroding confidence in vaccines and weakening efforts to protect children.

Public health infrastructure was also "really decimated by Covid," and the CDC in particular lost many experienced scientists and physicians "who had spent their lives working on vaccine-preventable diseases," said Scott Weaver, a virologist who is the research director of Galveston National Laboratory.

He added that the CDC, as of September, is also no longer immediately incorporating state-reported "measles-associated" deaths into its national published tally, making it harder to track outbreaks in real time. The agency lists three measles-related deaths in 2026, but Pennsylvania has reported at least five deaths.

"We have folks in federal health leadership right now that are throwing gasoline on the fire, and that is not helping the situation," said Jess Steier, a public health scientist and founder and CEO of Unbiased Science, an organization that aims to make complex scientific concepts understandable.

If the U.S. fails to materially improve vaccination rates, the measles resurgence could extend beyond ongoing outbreaks.

At "current" state-level vaccination rates, measles could become endemic again in the U.S., resulting in an estimated 851,300 cases over 25 years, according to a large-scale epidemiological model published in JAMA in April 2025 by researchers at Stanford and other institutions. Under that scenario, the model predicts the measles cases could lead to 170,200 hospitalizations and 2,550 deaths over that period.

The model, which simulated infectious disease spread in the U.S. at various childhood vaccination levels, relies on immunization data from 2004 to 2023 for that baseline scenario.

Michael Osterholm, an epidemiologist and the director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said he views the measles situation in Canada as a direct warning for the U.S.

Canada lost its official measles elimination status in November after a multi-province outbreak that generated more than 5,400 cases across 10 jurisdictions in 2025. Canada has a population of more than 40 million people.

"We have a population of 350 million. If you extrapolate, if we had the very same experience that Canada had over the next year or two, that would be up to 50,000 cases of measles in the U.S.," Osterholm said. "I think that's a potential future we have to be mindful of." An even larger measles outbreak isn't the only potential problem.

Vaccination coverage for other childhood diseases has similarly declined. The share of U.S. kindergartners vaccinated against polio fell to 92.4% in the 2025-2026 school year, while coverage for diphtheria, tetanus and acellular pertussis, or DTaP, dropped to 92%, according to CDC data.

The U.S. has already seen a sharp rise in pertussis, or whooping cough. U.S. reported cases reached roughly 43,300 in 2024 and nearly 28,000 in 2025, up substantially from pandemic-era levels, according to the CDC. Brown's Nuzzo said the resurgence reflects, in part, fewer people getting vaccinated and the fact that protection from the pertussis vaccine wanes over time.

The simulation model published in JAMA also examined how falling childhood immunization rates could affect the future spread of several diseases previously eliminated from the U.S. through vaccines. Under current vaccination levels, the researchers found that diseases including rubella and polio were unlikely to become endemic in the U.S.

But in a hypothetical scenario in which childhood vaccination rates fell by 50%, the model projected 9.9 million cases of rubella, 4.3 million cases of polio and 197 cases of diphtheria over 25 years.

Rubella can cause devastating complications when an infection during pregnancy passes to a developing fetus, while polio can cause paralysis and death, said Dr. Nathan Lo, assistant professor of infectious diseases at Stanford Medicine.

The projections are not predictions that these outbreaks will occur, but they illustrate how much risks could grow if vaccination coverage deteriorates.

"What vaccines often buy you is not just the protection, but also less uncertainty," said Mathew Kiang, an assistant professor of epidemiology and population health at Stanford Medicine who also worked on the simulation model. "I think even more frightening is just the really wide uncertainty that we just wouldn't be sure of what could happen." The measles resurgence is also raising questions about how effective the U.S. can be at detecting and responding to emerging public health threats before they spread.

"Public health is all about prevention," said Dr. Manisha Juthani, president of the Association of State and Territorial Health Officials and commissioner of the Connecticut Department of Public Health. She said the country must maintain its ability to detect threats, invest in surveillance, and recognize that infectious diseases do not respect national borders.

Those capabilities are crucial for curbing emerging pathogens, especially when scientists initially know little about how a new disease spreads, how severe it is or how to treat it. A pathogen could spread before vaccines or treatments are available, making early detection, clear public communication and coordination especially important.

Other experts argue that weaknesses in those capabilities could also have implications for the country's defenses against deliberate biological attacks.

In an April 2025 op-ed, Steph Batalis, a research fellow at Georgetown University's Center for Security and Emerging Technology, argued that the measles outbreak exposed a broader vulnerability in the U.S.' biodefense strategy and, specifically, a weakened ability to deter potential biological attacks.

She argued that the country's struggle to contain a known, vaccine-preventable disease "might only embolden those who see our public health vulnerabilities as an opportunity." But Osterholm, who works extensively on bioterrorism, said low MMR immunization rates and rising measles cases do not necessarily indicate that Americans would refuse a vaccine or antibiotic during a biological emergency. The shots and drugs used to counter biological threats can differ from those routinely administered to the public, he said, and the prospect of a severe outbreak could prompt urgency among the public to seek protection.

That point highlights an important limitation: The measles resurgence by itself does not establish whether the U.S. could withstand entirely different or novel public health threats.

But as a disease the U.S. knows how to prevent spreads, experts say the stakes of addressing those vulnerabilities extend to whatever might come next.