Parts of southwestern Utah and northwestern Arizona are seeing one of the largest measles outbreaks in the United States.

Health officials have confirmed 23 cases in children in Utah, in which all but one were unvaccinated, and 42 cases in neighboring Mohave County, Arizona. Officials believe the cases in Arizona are tied to the outbreak in Utah.

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Health experts in both states are urging parents to vaccinate their children against the disease. 

“I’ve worked for this health department for about 18 years, and we’ve never seen a case of measles that I know of up until this point,” David Heaton, the public information officer for the Southwest Utah Public Health Department, told NBC News. “We are just at that low rate of [vaccine] uptake that does leave us open for this kind of an outbreak.”

“About a month ago, we started getting cases where we could see evidence of community spread, meaning that there was measles being passed on the ground in our five-county district,” Heaton said.

The outbreak in Arizona is concentrated primarily in Colorado City and the surrounding area. Colorado City is an intentionally isolated, rural community that has notoriety for being a colony of Mormon polygamists in tandem with neighboring Hilldale, Utah.

The trend follows a large outbreak in West Texas that spread across the country earlier this year. A similar outbreak was seen largely in the very religious and somewhat isolated Mennonite community.

The warning comes after a recent investigation found the U.S. lacks the vaccine coverage needed to prevent outbreaks of communicable diseases such as measles. The investigation found that as of the 2024-25 school year, no schools in Mohave County had kindergarten classes with herd immunity protection against measles.

A little under 93% of people in the U.S. are vaccinated against measles. According to the Centers for Disease Control and Prevention, a 95% vaccination threshold must be reached to achieve herd immunity.

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“It’s herd immunity that has protected us for decades from this disease, and it will continue to protect us if we don’t blow it,” Dr. Bob England, a former Arizona public health official affiliated with the Arizona Partnership for Immunization, said.

Measles is a highly contagious virus. Up to 90% of unvaccinated people will contract measles if exposed. Measles was declared eliminated in the U.S. in 2000, but it has resurged in recent months.