Just as one large measles outbreak peters out in the United States, another outbreak of the virus has taken off along the border of Utah and Arizona.
The new outbreak began in August and has sickened more than 100 people, making it the second-largest cluster of cases in the country this year. A majority of the cases are in unvaccinated people.
It comes during an already bleak year for the nation’s public health: The number of measles cases hit a 34-year high this summer, largely driven by the so-called “Southwest outbreak,” which grew to more than 880 cases across Texas, New Mexico and Oklahoma. Several epidemiologists agreed that the current scale and spread of cases most closely resembles the large outbreaks of the early 1990s — before nationwide immunization campaigns and school vaccine mandates helped the United States declare the virus eliminated.
“We certainly have not had anything like this in many, many, many years,” said Walter Ornstein, an emeritus professor at Emory University and former director of the United States Immunization Program at the Centers for Disease Control and Prevention.
There are several parallels between the current situation at the Utah-Arizona border and the outbreak that exploded from the Western edge of Texas in January: Both started in rural towns with a sizable population of children who had not been immunized against measles, mumps and rubella. And in both outbreaks, the virus traveled to a neighboring state and took root in similarly vulnerable pockets.
“I’m worried about it,” said Dr. Adam Ratner, a pediatric infectious disease expert who recently published a book about the resurgence of measles. “I think it’s a very similar situation.”
But experts have also noticed a key difference.
For the last two decades, most large measles outbreaks have had ties to close-knit communities that have long had low vaccine uptake, said Dr. William Moss, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health who has studied measles for more than 25 years.
The 2019 measles outbreak in New York, for example, almost exclusively spread through communities of ultra-Orthodox Jews. The largest outbreak before that, in 2014, was overwhelmingly confined to an Amish community in Ohio. And the outbreak in West Texas earlier this year spread mainly through a large Mennonite community.
In the current outbreak, cases have been clustered in Colorado City, Ariz., and Hildale, Utah — adjoining cities with historical ties to the Fundamentalist Church of Jesus Christ of Latter-day Saints, a polygamist offshoot of the Mormon Church. However, local public health officials said the virus had spread beyond members of that religious group into the broader community, where vaccination rates have dropped steeply since the pandemic.
In Mohave County, Ariz. — which now has the second-highest case count of 2025, only after the Texas county at the center of the Southwest outbreak — roughly 90 percent of kindergartners were fully vaccinated against measles in the 2019-20 school year.
But by the 2024-25 school year, the vaccination rate had dropped to 78 percent. (About 95 percent of a community needs to be vaccinated to stem the spread of measles, which is one of the most contagious known viruses.)
Data from Southwest Utah tell a similar story: Vaccination rates dropped nearly eight percentage points over the course of the pandemic to about 78 percent.
Dr. Moss said it comes as no surprise that this outbreak has taken root in states with relaxed laws surrounding school vaccine mandates. Both Utah and Arizona allow parents to opt their children out of those requirements for personal, religious or medical reasons.
In Mohave County, the percentage of kindergartners with personal exemptions nearly doubled from the 2019-20 school year to the 2024-25 school year. Exemption rates in Southwest Utah, the vast majority attributed to personal beliefs, also nearly doubled during that time period.
“We’ve just been holding our breath and waiting for this to happen,” said Jessica Payne, an epidemiologist who leads the immunization program at the Utah Department of Health and Human Services. “Now it has.”
Ms. Payne attributes the decline in routine shots to the politicization of vaccines that developed during the pandemic, which she believes has made anti-vaccine sentiment “pervasive in the culture” of Southwest Utah. In the 2024 presidential election, Donald J. Trump handily won in both Colorado City and Hildale.
The trend mirrors the national decline in vaccination rates and rise in exemptions, which experts fear has caused pockets of under-vaccinated communities to multiply around the country. There have been 44 reported outbreaks of at least three cases this year, compared to just 16 outbreaks in 2024.
Measles symptoms typically resolve in a few weeks. But in severe cases, the virus can cause pneumonia, making it difficult for patients, especially children, to get oxygen into their lungs. It may also lead to brain swelling, which can cause lasting damage, including blindness, deafness and intellectual disabilities. This year, one in every eight people diagnosed with measles was hospitalized and two children died, the first such deaths in the country in a decade.
In addition to helping coordinate the public health response in Utah and Arizona, the C.D.C. is also lending experts to health departments in Minnesota and South Carolina, which have smaller measles outbreaks, said Andrew Nixon, a spokesman for the Department of Health and Human Services.
Local health officials are still investigating how exactly the outbreak at the Utah-Arizona border began. While they believe the first case was in a resident exposed to the virus while visiting another state, it’s not yet clear whether there is a link to cases from the Southwest outbreak.
This detail may become important when a panel of independent reviewers from the World Health Organization evaluates whether the United States will maintain its elimination status — a designation given to countries that do not have continuous spread of measles for more than a year, said Dr. Daniel Salas, a public health expert at the Pan American Health Organization.
If contact tracing and genetic sequencing proves the two outbreaks are connected, the cases reported in Utah and Arizona will be included in the 12-month countdown that will run out this January.
While losing elimination status doesn’t necessarily lead to tangible changes, like travel restrictions, experts have called the possibility “deeply embarrassing” for the country, which should have the resources to control the virus.
The United States isn’t the only country struggling to contain the virus, Dr. Ornstein said. Large outbreaks have spread through Mexico and parts of Canada, which has reported even more cases than the United States this year and is expected to lose its elimination status later this month.
“Our whole continent may lose elimination status,” he said.
Jonathan Corum contributed reporting.
Teddy Rosenbluth is a Times reporter covering health news, with a special focus on medical misinformation.
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