Measles outbreak in US: Utah and Arizona hardest hit with over 100 known cases

A new measles outbreak has emerged along the Utah-Arizona border, affecting more than 100 people since August and marking the second-largest cluster of cases in the United States this year. Most of those affected are unvaccinated, highlighting ongoing concerns about declining immunisation rates in parts of the country.

The outbreak comes as the US faces one of its most challenging years for measles in over three decades. Earlier this year, the “Southwest outbreak” swept across Texas, New Mexico, and Oklahoma, infecting more than 880 people.

Epidemiologists say the scale of current cases mirrors the large outbreaks of the early 1990s, before widespread vaccination and school mandates led the United States to declare measles eliminated.

“We certainly have not had anything like this in many, many, many years,” Dr Walter Ornstein, emeritus professor at Emory University and former director of the US Immunisation Program at the Centers for Disease Control and Prevention (CDC) was quoted by The New York Times as saying.

Patterns and parallels

The outbreak in Utah and Arizona shares similarities with the Southwest cluster that began in western Texas earlier this year. Both started in rural towns with sizable populations of unvaccinated children and quickly spread to neighbouring states. Pediatric infectious disease specialist Dr. Adam Ratner told The News York Times, “It’s a very similar situation,” stressing the vulnerability of under-immunised communities.

However, experts note a key difference. Many large outbreaks over the past two decades were confined to close-knit religious communities, such as the ultra-Orthodox Jewish communities in New York in 2019 or the Amish in Ohio in 2014. The current Utah-Arizona outbreak is centered in Colorado City, Ariz. and Hildale, Utah, cities historically linked to the Fundamentalist Church of Jesus Christ of Latter-day Saints.

Local health officials say the virus has spread beyond this group into the broader community, where vaccination rates have dropped sharply since the COVID-19 pandemic.

In Mohave County, Arizona, full measles vaccination rates among kindergartners fell from 90% in 2019-20 to 78% in 2024-25. Southwest Utah saw a similar decline to roughly 78%, well below the 95% coverage needed to prevent measles outbreaks. Both states allow personal, religious, or medical exemptions from school vaccination requirements, a factor public health experts say has contributed to the virus gaining a foothold.

Impact of declining vaccinations

Dr. William Moss, an epidemiologist at Johns Hopkins Bloomberg School of Public Health was quoted by The New York Times as saying the outbreak was unsurprising given the decline in vaccination rates and the rise in exemptions. Jessica Payne, head of Utah’s immunisation program, attributes the drop to the politicisation of vaccines during the pandemic, noting that anti-vaccine sentiment has become “pervasive in the culture” of Southwest Utah.

Nationally, the trend mirrors a worrying rise in under-vaccinated communities. This year, there have been 44 outbreaks of at least three cases, compared to just 16 in 2024. Severe measles infections can lead to pneumonia, brain swelling, and in rare cases, death. In 2025, one in eight diagnosed patients required hospitalisation, and two children died, the first measles-related deaths in the country in a decade.

Public health response

The CDC is coordinating with Utah and Arizona health departments while also assisting smaller outbreaks in Minnesota and South Carolina. Officials continue to investigate the origin of the Utah-Arizona outbreak, including whether it links to the Southwest outbreak.

Genetic sequencing and contact tracing will determine if the US risks losing its measles elimination status, a designation for countries without continuous measles transmission for more than a year.

Dr. Ornstein warned that the situation extends beyond the US, with Canada and Mexico also experiencing large outbreaks. “Our whole continent may lose elimination status,” he said.