One of the largest measles outbreaks in the United States is centered in southwestern Utah and neighboring Mohave County, Arizona. The Southwest Utah Public Health Department has reported 27 cases, nearly all among unvaccinated school-age children. In Mohave County, health officials have confirmed 42 cases, including one hospitalization. Both outbreaks are believed to be connected.
Vaccination rates well below herd immunity
Public health experts point to low vaccination coverage as the main driver of the spread. In Washington County, Utah, only 79% of kindergartners are vaccinated against measles — far below the 95% threshold needed for herd immunity. Some schools in Arizona report vaccination rates below 10% for kindergarteners.
“We are just at that low rate of uptake that does leave us open for this kind of an outbreak,” said David Heaton, spokesperson for the Southwest Utah Public Health Department. He noted that vaccine hesitancy was already a challenge before the COVID-19 pandemic, but mandates during that time increased resistance for some families.
Arizona cases and community history
The Arizona outbreak is concentrated in Colorado City, a geographically isolated community with a history of limited outside interaction. State health officials report that this is the highest number of measles cases since the 1990s.
At Cottonwood Elementary School in Colorado City, just 7.7% of kindergarteners received the measles-mumps-rubella (MMR) vaccine for the 2024–25 school year, while exemption rates exceeded 80%.
National vaccination decline
Falling immunization rates are not limited to Arizona and Utah. Since 2019, 77% of U.S. counties have reported declines in childhood vaccination. As of September 23, the Centers for Disease Control and Prevention (CDC) confirmed 1,514 measles cases across 40 outbreaks nationwide — the highest in more than 30 years.
“This is exactly what you expect to see when a highly infectious, vaccine-preventable disease enters a community with low vaccination rates,” said Dr. Bob England, a former Arizona public health official.
Officials urge vaccination
The CDC and state health departments in Utah and Arizona are working closely to contain the outbreak. “Vaccines not only protect individual children from measles but also contribute to community immunity,” said Jim O’Neill, acting CDC director. Officials are urging families to check vaccination records and get up to date.
Experts warn that low vaccination “pockets” leave entire regions at risk. Counties in Utah, Arizona, and Nevada surrounding the outbreak area all report MMR rates below herd immunity levels. “That really adds up over time,” said Dr. England.
Why local data matters
Dr. Nathan Lo, an infectious diseases physician at Stanford University, emphasized the importance of looking at county-level vaccination data. “If you only have state-level data, vaccine coverage may seem relatively OK. But outbreaks happen in localized areas where protection is far below herd immunity.”
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