Measles is once again endemic in the U.S.

Measles is once again endemic in the United States, 26 years after it was declared eliminated in the country, an analysis of genetic data from outbreaks of the disease has found.

Data from the outbreaks will be the focus of deliberations by an international panel of scientific experts convening today to decide whether the United States should lose its official designation as a nation that has eliminated measles. An announcement of the decision is expected Nov. 17.

The New York Times asked several scientists to analyze the publicly available genetic data before the meeting.

Achieving elimination status in 2000 was a hard-fought public health victory for the United States. But with skepticism about the safety of vaccines growing in recent years, outbreaks have become more frequent. Over the past two years, cases have soared, with nearly 5,200 reported so far this year. Last week alone, Pennsylvania, New York and Wisconsin each reported a death associated with measles, bringing this year’s total to eight, more than the combined deaths from 2000 through 2024.

To officially lose elimination status, a committee of the Pan American Health Organization, a branch of the World Health Organization, must determine that the country has had uninterrupted transmission of measles for more than 12 months.

Publicly available data shows that several chains of transmission have continued for more than a year in the United States, according to scientists who analyzed the data at the request of the Times.

Three chains that were continuing in July 2026 — two in Utah and one in Arizona — can all be traced to an outbreak in Texas more than a year earlier. One of those, which was still continuing in Utah in September, had been circulating for more than a year in that state, occasionally spilling over into Oregon and Washington.

“This alone is strong evidence that there has been ongoing transmission for over a year,” said Bill Hanage, a co-director of the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health.

Another transmission chain detected in Virginia at the start of the year was still spreading in Iowa, Indiana and Pennsylvania as of mid-October. (There is often a delay of several weeks before genetic data is available online.)

“There’s a general pattern of outbreaks seeding other outbreaks,” said J.T. McCrone, an evolutionary biologist at the Fred Hutch Cancer Center who helps oversee Nextstrain, the publicly available database that contains genetic information for measles and many other pathogens.

“Measles is very contagious; it spreads very easily,” McCrone said. “We have a lot of connected communities in the U.S., and so you can hop from population to population and maintain a transmission chain.”

Some critics consider the potential loss of elimination status a referendum on Health Secretary Robert F. Kennedy Jr., who has for decades questioned the safety of vaccines in general and the measles vaccine in particular. He has falsely said that the measles vaccine can be fatal, that it contains fetal debris and that its effectiveness plummets over time. None of the eight Americans who died from measles this year had been vaccinated against the disease. (Four were infants who were too young for the shot.)

Kennedy has repeatedly noted that measles is on the rise in many countries beyond the United States. Due in large part to a rise in vaccine hesitancy and misinformation since the pandemic, seven countries, including Canada, Spain and Britain, have lost their elimination status in the past two years. Mexico is expected to lose it this month along with the United States.

But the explosion of cases in the United States has coincided precisely with his time in office. Last year, the Centers for Disease Control and Prevention recorded nearly 2,300 cases and three deaths.

Most cases of measles in the years since elimination were linked to travel abroad, particularly in countries where the disease is still endemic. But those importations did not spread widely and died out quickly because more than 95% of children in the United States had been immunized against the disease. That percentage dropped to 92.4% in the school year beginning in 2025, leaving nearly 300,000 kindergartners at risk.

Rising vaccine hesitancy has led to sharp increases in parents seeking exemptions for their children from school vaccination requirements. The COVID-19 pandemic exacerbated the problem, first by derailing routine immunizations and then by intensifying mistrust of vaccines. Kennedy’s statements from his pulpit as the country’s top health official also fueled misplaced fears that childhood vaccines might be more dangerous than the diseases they are intended to prevent.

Regaining elimination status will require more than ending the current transmission chains, experts said. Public health officials will need to increase immunization rates across the country and cut down on the number of pockets into which the virus can jump.

The measles virus evolves slowly, allowing the vaccine to be effective even without frequent updates. But its slow evolution also means that pinpointing how the virus is changing or moving requires deeper analyses.

Scientists have typically relied on snippets of the measles virus, which was enough to identify the sporadic cases in the country. But they now have the technology to decode the full genetic sequences of the samples, giving them clearer insight into how the virus is changing and moving and enabling them to distinguish endemic circulation from travel-associated cases.

“If this outbreak had occurred in 2006, we wouldn’t have the technology to be able to do this,” said Robert Potter, a clinical microbiologist at the Children’s Hospital of Philadelphia.

“We never really had to study measles from a genomic perspective before because there was no measles to sequence,” he added. “It’s giving us a lot of new information, but it’s information that we didn’t necessarily need to have.”

This article originally appeared in The New York Times.

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