A major measles outbreak in South Carolina has ended and the most active outbreak in the U.S. is now in Utah. The virus is continuing to spread in Arizona, too.
Utah’s outbreak has led to more than 600 measles cases since last summer. Arizona has had 308 cases in that time, most of which have been near the Utah border in Mohave County.
So far just 10 of Arizona’s cases have been in the Valley.
“I think what has helped us here in Maricopa County is, in general, in a lot of areas, we are reaching those higher levels of immunization,” said Dr. Nick Staab, chief medical officer for the Maricopa County Department of Public Health.
Measles is so contagious that at least 95% of the population needs to be immune in order to contain its spread, Staab said.
Statewide, vaccination rates are below the 95% herd immunity threshold — as of 2025, about 89% percent of the state’s kindergarteners were fully vaccinated for measles, according to the Arizona Department of Health Services. But rates vary by county. In Maricopa County, about 88% of kindergarteners had been fully vaccinated in 2025, compared to about 78% in Mohave County.
“The potential for a larger measles outbreak is anywhere where we have pockets of under-immunized people,” Staab said.
About 96% of Arizona’s recent measles cases have been among unvaccinated people, according to ADHS.
While caseloads in Maricopa County have remained low so far, Staab said some recent measles cases in the Valley have had no known source of exposure, indicating the virus is spreading in the community beyond just the cases that have been confirmed.
“It is likely that there are other cases that are not detected,” Staab said.
Staab urges Maricopa County residents to take precautions as the virus continues to spread. Anyone who is not fully immunized for measles should get vaccinated, he said.
And parents of babies who are too young for the vaccine should be vigilant, Staab said.
“I would recommend as a pediatrician that all parents with young infants are monitoring who their infants come in contact with,” Staab said. “With regard to measles, it’s not unreasonable I think to ask folks who you’re hanging out with what their exposures are, whether they’re immunized.”
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