In June, measles started spreading in Utah and Arizona, as local health departments began receiving reports from families with children experiencing full-body rashes. Parents mentioned that their whole families had just recovered from measles.
However, testing was often declined by patients, which meant there were no official cases recorded until August 8. That’s when the Mojave County, Arizona, Department of Public Health was alerted to a 10-year-old boy from a close-knit community near the border of northern Arizona and southern Utah.
This outbreak has escalated to over 600 reported cases, marking it as the most active measles outbreak in the United States. The situation is pivotal in determining whether the U.S. has lost its measles elimination status, a designation that indicates a halt in routine transmission of the virus within the country for more than a year. A meeting to discuss this matter is on the calendar for November.
The U.S. Centers for Disease Control and Prevention (CDC) engaged one of its disease investigators to analyze molecular evidence – essentially, genetic clues found within the measles viruses infecting patients – to ascertain more about when the outbreak began and how extensive it actually is.
Last year witnessed a significant rise in reported measles cases, totaling 2,267—the highest in over 30 years. This trend shows little sign of slowing down, with over 1,700 cases reported within the first four months of this year, particularly in communities that have rejected vaccination.
Almost immediately after the outbreak was confirmed in Utah, health officials suspected that the actual number of cases was likely much larger than reported.
Undercounting is common in disease outbreaks, often due to physicians overlooking unfamiliar conditions or patients not seeking medical help for various reasons. It appears both of these factors contributed to the ongoing outbreak.
Given that measles has not been widespread in the U.S. for 30 years, many healthcare providers have never encountered a case. Moreover, the area experiencing the outbreak has connections to a sect of the Fundamentalist Church of Jesus Christ of Latter-Day Saints, whose leader, Warren Jeffs, fostered distrust toward vaccines.
The measles vaccine is highly effective, preventing the disease around 97% of the time with two doses. Because the virus is exceptionally contagious—able to linger in the air for up to two hours—it requires high vaccination rates, ideally over 95%, to prevent transmission. Unfortunately, misinformation and a lack of trust in public health have greatly diminished community protection.
To get a better understanding of the outbreak, public health officials wondered if examining the genetic evolution of the virus could provide insights into its timeline and size, as explained by Dr. Annie Wang, an Epidemic Intelligence Service officer who worked with Arizona’s Pima County Health Department during the outbreak.
EIS officers are specialized public health experts often assigned to assist with outbreak investigations at state and local levels. Wang shared the findings of their investigation during the CDC’s annual EIS conference.
This method of tracing an outbreak’s origins relies on tracking how fast viruses mutate as they transfer between individuals. Measles is relatively stable, undergoing few changes during transmission, though it does accumulate minor variations over time.
Wang and her team decided to investigate the entire genome of the virus rather than just the usual small segment used to identify strains. They sequenced this for samples from 165 patients.
Their analysis allowed them to estimate the origin of the virus that initiated the Arizona and Utah outbreak.
According to Wang, those early cases likely began six weeks to possibly even 14 months prior to the diagnosis of the 10-year-old boy. However, the exact location of the first infection remains undetermined since they only examined viruses from the outbreak in Utah and Arizona.
If the initial infection did occur in the U.S., this research could suggest that measles has been spreading domestically for longer than a year—important for understanding its elimination status.
Wang and her team also concluded that the ongoing outbreak could be at least 6.5 times larger than what has been officially documented.
There are additional indications that the extent of measles transmission in the U.S. is not being accurately reported.
An EIS investigation tracking measles in wastewater from Oregon revealed the virus was detected 100 times across 23 of 24 monitored counties between October and February. Despite this, only six measles cases were reported in Oregon during that period, all outside the monitored areas.
“The frequency and spread of detections suggest measles is indeed more common than the six reported cases,” stated Dr. Jeffrey Tamucci, an EIS officer with the Oregon Public Health Authority.
He posed the question: “What’s causing these missing cases? Are patients not seeking care? Are they misdiagnosed? Are providers failing to test for compatible symptoms? We don’t have the answers yet; we’re still working on it.”
The CDC is also undertaking a national analysis akin to the one in Arizona and Utah, focusing on genomic data, noted Dr. Noel Brewer, a public health expert at the University of North Carolina.
“State data is helpful, but it has limitations, particularly in connecting outbreaks across states,” Brewer said, mentioning that he was aware of the Arizona and Utah situation. Other states are also conducting genomic analyses.
Brewer leads the committee that will evaluate the CDC’s report on measles elimination status for the Pan American Health Organization. This committee is facilitated by the nonprofit Task Force for Global Health and funded by the CDC.
Brewer anticipates having the CDC’s report by the end of June and feels it will be decisive and compelling.
“Regardless of whether this in-depth analysis uncovers a clear link among the cases, it’s evident that measles is widely present throughout the U.S. and does not show signs of stopping,” he added.
In early April, CDC researchers released a substantial amount of data from advanced genetic sequencing of about 1,000 measles viruses circulating last year in the U.S. This information will help determine if the outbreaks in Texas, Arizona, and Utah are connected or if they stem from separate introductions from abroad.
Dr. Kristian Andersen, an evolutionary biologist at the Scripps Research Institute, mentioned that analyzing broader datasets from the U.S., Canada, and beyond shows strong connections between what’s happening in Utah and Arizona to past outbreaks in the U.S., especially linking back to states like Texas and New Mexico.
“The larger outbreak involving South Carolina and Washington appears to arise from endemic transmission inside the U.S. and Canada, rather than from ‘immigration’ or travelers entering the country,” Andersen noted.
