Last year, starting in January, the United States experienced its largest documented measles outbreak in more than three decades, when an epidemic centered on West Texas sickened at least 762 people. Now a fast-moving outbreak in South Carolina seems poised to surpass it: Local officials have logged 700 infections, and the virus is still rapidly spreading.
As public-health officials scramble to contain the virus, they’re also trying to figure out whether these two outbreaks are connected—specifically, whether the version of the pathogen that kick-started the West Texas cases has been circulating within the nation’s borders ever since. If the answer is yes, it will mean that measles has once again become a permanent resident of this country, after 26 years of only limited outbreaks imported from abroad. Given that the U.S. clocked more than 2,200 measles cases in 2025—more than it has had in a single year since 1991—the experts I spoke with already consider this the reality that Americans are living in. One of the fastest-spreading viral diseases ever documented has once again become a routine threat.
At this point, researchers are working to find the connective tissue among some of the largest measles outbreaks in the U.S. within the past year, including the ones centered in West Texas, Utah, Arizona, and South Carolina. Technically, the epidemics still could have been caused by separate reintroductions of measles from at least one international source. But “that’s a hard stretch,” Robert Bednarczyk, a global-health researcher and epidemiologist at Emory’s Rollins School of Public Health, told me. The most likely and so far best-supported scenario, he said, is also the simplest one—that the virus spread so fiercely and quickly through these communities that it was able to hitch a ride elsewhere in the country when infected people traveled.
If further evidence proves that scenario true, the Pan American Health Organization could strip the U.S. of its official measles-elimination status—which the country has held since 2000—at a meeting scheduled for April. (A country achieves elimination status when it can show that the virus hasn’t been circulating for 12 consecutive months; it loses the status when researchers show that measles has been spreading for a year straight.) Already, PAHO has publicly confirmed that scientists have found the same strain of measles in Texas, New Mexico, Utah, Arizona, and South Carolina, as well as in Canada, Mexico, and multiple other North American countries. (In response to a request for comment, a PAHO spokesperson clarified that although these detections had been made, the committee was still seeking further evidence.)
Health officials also recently announced that South Carolina’s outbreak has seeded cases elsewhere, including Washington State. Still, the case for measles’ continuous transmission can’t yet be considered a slam dunk. To prove it definitively, researchers will need to show that geographically distant outbreaks in the U.S. are epidemiologically linked and that there is not sufficient evidence suggesting that the virus bounced back and forth between countries.
Two types of information are essential to these investigations. First, researchers look into the travel histories of infected people, who might have brought the virus from one state to another. Second, they compare genetic sequences pulled from the virus across locations. Measles mutates slowly enough that researchers can in many cases search for essentially the same strain when tracking its movements. But the virus does accumulate some changes in its genome, and the further apart two cases are in time, the more genetically distinct their genetic material should be. If measles was being continuously transmitted, scientists might expect to see slightly different iterations of the virus racking up mutations as it traveled, say, from Texas to South Carolina. If measles had been introduced separately to those locations, the sequences pulled from each state might more closely resemble genetic information from an international source, Pavitra Roychoudhury, a pathogen-genomics expert at the University of Washington, told me.
But some of the data that officials need may be lacking. For months, experts have been concerned that the U.S. has been severely undercounting its measles cases and that the virus had been circulating in some communities long before it was officially detected. Cases of the disease can be easy to miss, Helen Chu, an immunologist at the University of Washington, told me. The early days of measles are usually marked by common symptoms such as fever and cough; to the untrained eye, the virus’s rash can look like many of the reddish, patchy blemishes that many other pathogens cause. The overwhelming majority of measles cases in the U.S. have also concentrated in communities that have low vaccination rates, which often have less access to medical care and the sort of testing that would also collect viral samples. Many people who deliberately decline vaccination for their families are also skeptical of seeking medical care in general, or of public-health officials investigating outbreaks.
With inconsistent data, researchers may be left sorting through genetic sequences that neither point clearly to one another nor obviously implicate separate sources. “There’s a judgment call in that gray area,” William Moss, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, told me. “How different is different?” The last time measles spread endemically in the United States, this sort of genomic analysis was not commonplace.
Should PAHO find that measles is spreading concertedly in the U.S. again, the nation’s leaders may shrug off the change. At times, top officials at the Department of Health and Human Services appear to have dismissed the notion of continuous spread: In November, Jim O’Neill, Health Secretary Robert F. Kennedy Jr.’s deputy and the CDC’s acting director, posted on social media that “preliminary genomic analysis suggests the Utah and Arizona cases are not directly linked to Texas.” (Neither the Trump administration nor PAHO has released the details about the data it reviewed or how similar any identified strains are.) And last week, the CDC’s new Principal Deputy Director Ralph Abraham described the prospect of the U.S. losing its measles-elimination status as “just the cost of doing business with our borders being somewhat porous for global and international travel.” Certain communities, he added, “choose to be unvaccinated. That’s their personal freedom.”
In an email, Emily G. Hilliard, HHS’s press secretary, echoed Abraham’s comments, describing current outbreaks as “largely concentrated in close-knit, under-vaccinated communities with prevalent international travel that raises the risk of measles importation,” and noting that the United States still has a lower measles burden than Canada, Mexico, and much of Europe do.
In practice, the April decision will be a matter of semantics. Whatever the outcome, the U.S. has been weathering a worsening measles situation for years now, as vaccination rates have ticked down and outbreaks have grown larger and more common. In the past year, the Trump administration has made it substantially more difficult for local public-health-response teams to address and contain outbreaks too. HHS reportedly delayed communications from the CDC to officials in West Texas and held back federal funds to fight the outbreak for two months. More recently, HHS pledged to send $1.4 million to address South Carolina’s outbreak, though it began months ago. (One recent analysis suggests that measles outbreaks of this scale can cost upwards of $10 million.) The administration has repeatedly downplayed the benefits of immunization, while exaggerating the importance of nutritional supplementation for combatting measles. Kennedy has also spent decades repeating disproved claims that vaccines such as the measles-mumps-rubella immunization can cause autism. (Hilliard wrote that Kennedy has consistently said that vaccination is the most effective way to prevent measles, but she also emphasized in her email that people should consult with health-care providers about whether vaccination is best for their family.)
Should immunity erode further—as experts watching the Trump administration’s actions expect it to—measles will find it even easier to move across the country, until epidemics bleed so thoroughly together that their links become irrefutable. Already, the nation’s leaders have made clear where the U.S. stands on measles: It is an acceptable norm.
