The Pan American Health Organization, the regional arm of the World Health Organization for the Americas, made the determination after the U.S. recorded more than 5,400 measles cases in 2026 — more than double last year's total and the highest count in over three decades. PAHO is scheduled to hold a news conference on Tuesday.

Under WHO criteria, a country loses elimination status when locally transmitted measles infections or outbreaks persist for 12 months or longer. About a third of this year's cases are tied to outbreaks that began in 2025 and have continued to spread, according to the Centers for Disease Control and Prevention.

Pennsylvania has been the epicenter. State health officials have reported more than 1,500 cases and seven deaths associated with measles this year — the largest outbreak in the nation and the deadliest. New York and Wisconsin have each reported one death. Nationally, the CDC has tallied 5,412 cases, more than 590 hospitalizations and at least six measles-related deaths, with some states reporting additional fatalities. Cases have been confirmed in 45 states and Washington, D.C.

The 2026 total shatters last year's record of 2,242 cases, which was itself the highest in more than 30 years and was linked to 48 separate outbreaks. The CDC has reported 43 new outbreaks in 2026, defined as three or more related cases.

"It means that we're at risk of being exposed to measles and getting measles if we're not vaccinated," said Dr. Céline Gounder, a CBS News medical correspondent. "It means ERs and pediatricians need to consider measles in anyone with a fever and rash, whether they've traveled overseas or not. And we may now become an exporter of measles to other countries."

Dr. Omer Awan, an epidemiologist and senior public health contributor for Forbes, called the loss a setback to the nation's public health infrastructure.

"It's frustrating and heartbreaking ... because we have the tools to fight this," Awan said. "The tool is the MMR vaccine, which we know is 97% effective at preventing the onset of measles. It'd be one thing if we didn't have the tools to fight this, but the tools are at our disposal, we just aren't using the tools that can help us mitigate the spread and beat measles."

Nearly all of this year's cases have occurred in unvaccinated people. CDC data shows 96% of those infected in 2026 had not received the measles vaccine or were unsure of their vaccination status; only 2% of cases were in people who had been fully vaccinated.

Measles is among the most contagious infectious diseases. While many people recover, it can cause severe infections in the lungs and brain that lead to cognitive problems, deafness or death. Two doses of the measles, mumps and rubella vaccine are extremely safe and effective at preventing infection, according to medical research.

Some people — including infants, pregnant women and those with weakened immune systems — cannot be vaccinated and depend on population immunity to keep the disease at bay. Awan said those groups will bear "the brunt of the consequences" of increased spread and are most vulnerable to complications such as pneumonia or encephalitis.

To regain elimination status, a country must demonstrate no local spread or outbreaks lasting more than a year. PAHO recommends maintaining vaccination coverage of at least 95%, along with sensitive surveillance systems, laboratory capacity and a guaranteed rapid response to outbreaks, with data submitted to and verified by the organization.

Childhood vaccination rates have declined in many communities across the U.S. in recent years, often falling below the threshold needed to ward off outbreaks. Gounder noted that state lawmakers control school vaccine rules and that most legislatures reconvene in January.

"Nine states made opting out of vaccination easier in 2025. States ending nonmedical exemptions saw kindergarten vaccination rise 2 to 4 points," Gounder said. "Your state lawmakers will write next year's exemption rules."

The U.S. began collecting national measles data in 1912, when about 6,000 measles-related deaths were reported each year. In the decade before the vaccine became widely available in 1963, nearly all children caught measles before age 15, with an estimated 3 to 4 million infections and 400 to 500 deaths annually, mostly among children. Reported cases fell sharply after vaccination began, and the country was declared measles-free in 2000.

The U.S. held that status for over 25 years, weathering contained outbreaks — including a 2015 cluster traced to overseas visitors at Disneyland in Southern California and a 2018-19 outbreak in New York that was eventually curbed through public health measures, including limits on vaccine exemptions. Those spikes were a fraction of the spread seen this year.