Rep. Byron Donalds, a Republican from Florida who is running for governor, has blamed “rampant illegal immigration” during the Biden administration for measles outbreaks. But we found no link to illegal immigration in several recent large outbreaks, and infectious disease experts told us that the main driver of the cases is falling measles vaccination rates in the U.S.
Measles was eliminated in the U.S. in 2000, meaning there were no sustained outbreaks lasting more than a year. But since January 2025, cases have risen markedly and the country’s elimination status is now in question. Last year, there were 2,289 confirmed cases — the highest number since 1991. As of Aug. 30, there have been 2,903 confirmed cases in 2026, already breaking 2025’s tally.
Donalds weighed in on the cause of measles outbreaks during an Aug. 23 interview on CBS News’ “Face the Nation” in response to a question from host Margaret Brennan, who asked whether he would tell Florida parents to get their children vaccinated against the disease. “No, actually, my message to Floridians and, quite frankly, to Americans is the reason why we have seen a measles outbreak in certain parts of the country is because of rampant illegal immigration into the United States from the previous administration,” Donalds replied.
When Brennan asked him for evidence for this statement, Donalds referred to “the outbreaks in El Paso, Texas, in Southwest Florida, where I live, and many other parts where you know that you have a situation,” adding that people entering the country illegally “have not had that vaccination schedule.”
Donalds appeared to refer to one outbreak at an immigration detention center, but it’s unclear exactly which outbreaks he was linking to illegal immigration. We did not receive a reply to questions we sent to his office. There’s no evidence illegal immigration under President Joe Biden caused the recent uptick in measles cases in the U.S., and there is clear evidence of vaccination rates falling within U.S. children.
“The U.S. measles problem is due to gradually dropping rates of measles vaccination among children,” Noel T. Brewer, a professor of health behavior at the University of North Carolina at Chapel Hill, told us. “Most of the current measles cases are from domestic transmission. A sizable but much smaller portion of measles cases are from international travel. Immigrants play a very minor role in the US measles crisis.”
“There is no evidence that the 2025-2026 outbreaks of measles are due to immigration or immigrants,” Dr. Jonathan L. Temte, a professor of family medicine and community health at the University of Wisconsin School of Medicine and Public Health, told us, noting the U.S. has a long history of blaming immigrants for the spread of infectious disease. “The CDC does not track immigration status of individuals diagnosed with measles. Public health data shows that the vast majority of U.S. measles cases stem from unvaccinated domestic residents.”
As we will explain, the data indicate an increasing proportion of measles cases are acquired within the U.S., and not brought in from other countries. Among cases where people were infected abroad, most are related to U.S. residents traveling outside the country and then returning.
Nor could we find any evidence connecting measles in Southwest Florida to illegal immigration, despite Donalds’ implication of a link. In El Paso, Texas, and an adjacent county, there have been some measles cases in an Immigration and Customs Enforcement facility and a U.S. Marshals Service detention center. Little information has been confirmed about the cases among detainees, including the immigration status of those infected. But even if all of those individuals were undocumented, it would make up only a small fraction — less than 6%— of the total measles cases so far this year in the U.S. Settings like detention facilities and prisons are known to accelerate the spread of infectious diseases.
Eroding Vaccination Increased Measles Cases
The prevalence of measles in the U.S. has been closely tied to vaccination in U.S. communities, with lower vaccination rates leading to more measles cases. Receiving two doses of the vaccine against measles reduces a person’s risk of getting the disease by 97% if exposed.
Before the measles vaccine was released in the U.S. in 1963, almost all children eventually got measles, according to the CDC website. After the vaccine became available, measles cases fell but the virus continued to circulate. In 1989, the vaccine schedule incorporated a second dose of the vaccine.
After this, cases fell further, and by 2000, the U.S. had met the criteria for measles elimination “as a direct result of high 2-dose childhood coverage with the measles, mumps, and rubella (MMR) vaccine,” according to an April 2025 update on measles published by CDC scientists. A country can maintain its measles elimination status if it has limited outbreaks when measles is introduced from other countries, but it cannot have sustained local transmission lasting over a year.
The Pan American Health Organization will review data in November on whether the U.S. should keep its measles elimination status. Experts are doubtful that the country will retain it.
In an Aug. 6 opinion piece in STAT, Brewer cited as concerning, for example, the fact that there have been cases each week since early 2025. This was when a large outbreak began in Mennonite communities in West Texas that would ultimately lead to 762 cases in Texas and span multiple states and countries. He also mentioned an outbreak in Utah that seems to have continued for more than a year. Brewer is the chair of an independent committee that reviews the CDC’s annual report determining whether the country has maintained elimination status. He published his STAT opinion article before doing the review.
“Given the current epidemiological context, it appears highly likely that the USA will lose its measles elimination status in 2026,” another group of researchers wrote in a May 2 paper published in the Lancet.
Maimuna Majumder, a computational epidemiologist at Boston Children’s Hospital and Harvard Medical School and co-author of the Lancet paper, told us that she is “even less confident” that the U.S. will keep its measles elimination status than she was a few months ago when the study was published. “As outbreaks continue to pop up across the US due to under-vaccination, our risk of losing elimination status mounts further,” she said.
CDC data released Aug. 17 on the 2025-2026 school year show that 92.4% of kindergartners had received the MMR vaccine. This is down from 95.2% in the 2019-2020 school year. For the seven school years before that, vaccination rates had stayed between 94% and 95%. Meanwhile, CDC data show that 94% of people who have had confirmed measles in 2026 are unvaccinated or have unknown vaccination status. Another 3% got just one of two doses of the vaccine.
The erosion in vaccination rates at the national level doesn’t tell the whole story of why the U.S. is now more vulnerable to large outbreaks of measles, Ana Bento, an assistant professor in the department of public and ecosystem health at Cornell University College of Veterinary Medicine, told us. These outbreaks are taking hold in pockets of the U.S. with very low vaccination rates.
“A national average of 92 percent does not describe any actual classroom,” she said. “It describes the midpoint of a landscape that includes schools in the low twenties, and those are the schools where these outbreaks begin.”
Bento and a colleague put together a database looking at MMR vaccination coverage across most of the U.S. states and the District of Columbia, looking at the county, district and school level. In results published Aug. 18 in Nature Medicine, the researchers calculated whether and at what point communities reached a lower level of vaccination that would allow sustained transmission of measles. They found that schools on average crossed this threshold during the 2022-2023 school year.
Bento explained that these clusters of high vulnerability are not visible when monitoring vaccination coverage at the county level, because large schools tend to have higher vaccination rates than small schools and obscure the existence of pockets of unvaccinated children. She also noted that her group’s data does not cover children who are homeschooled, and that homeschooling has increased in recent years. Some recent outbreaks have been in communities with high rates of homeschooling, she said.
Infectious disease researchers have for years noted measles outbreaks in what CDC scientists have called “undervaccinated, close-knit” communities.
“The practical implication is that outbreaks are not landing randomly,” Bento explained. “They land in the specific places where susceptible children have accumulated into dense pockets.”
A Falling Percentage of Cases from Overseas
The CDC does not track the immigration status of people with measles. However, the agency does keep track of whether measles cases in the U.S. were imported internationally, meaning a person was likely infected overseas and then carried the virus to the U.S.
In a setting where measles has been eliminated, a greater proportion of cases are imported, whereas when measles circulates more within a country, this percentage falls. These data indicate a growing proportion of measles cases in the U.S. are acquired locally, and not brought in from abroad.
Between 2001 and 2011, 41% of measles cases were imported, according to CDC researchers. This proportion subsequently fell. A CDC analysis of cases between 2013 and 2019 found that 12% were imported, with an increase to 31% for the pandemic years of 2020 through 2023. By Jan. 1 through April 17, 2025 — the last period for which the CDC released data on imported cases — internationally imported measles made up just 6% of cases. Since 2000, most imported cases have been brought back to the country by unvaccinated U.S. residents who travel overseas, according to the CDC website, and this remained true as of early 2025. Agency scientists reported that 92% of imported cases in early 2025 were in U.S. residents.
The Johns Hopkins University measles tracking project, which collects data from a variety of sources including state and county health departments, also categorizes confirmed cases based on whether they are locally acquired or imported from out of the country or out of state. By this count, 5% of cases since the beginning of 2025 have been imported. “Of the imported cases, the vast majority occur in travelers returning from other countries with ongoing measles outbreaks,” the project’s website says.
It’s worth noting that most imported measles cases in the recent past have involved unvaccinated U.S. residents who traveled to Europe, Asia or Africa — and not the Americas. Between 2001 and 2019, for U.S. measles outbreaks with at least 50 cases, the known source countries were Israel, the U.K., Belgium, Ukraine and the Philippines.
What’s Known About Recent Large Outbreaks
It’s also not clear what evidence Donalds was considering when linking the largest recent outbreaks in the U.S. to illegal immigration during Biden’s presidency.
We first wrote more than a year ago about unsupported claims connecting the large Texas outbreak, first identified in Mennonite communities in Gaines County in January 2025, to illegal immigration. The source of the outbreak is unknown, according to the CDC.
“We don’t know exactly how the Gaines County outbreak [began],” Chris Van Deusen, director of media relations at the Texas Department of State Health Services, told us. “It spread through a community with low immunization rates.”
Majumder said that the “most concerning” element of the Texas outbreak is that rather than “staying localized to a single under-vaccinated community, the outbreak spread — not only throughout the state, but to other states, too.”
Once in other communities, measles continued to spread. “These outbreaks demonstrate that under-vaccination across the US is now widespread enough that one under-vaccinated community can cause an entirely ‘homegrown’ outbreak in another,” Majumder said.
Mennonites who had been living in Mexico since the 1920s first attempted to establish a community in Gaines County, Texas, in 1977, the nonprofit Texas State Historical Association explains. The initial group struggled with their immigration status until 600 members of the community attained legal status in 1980, and the community maintains ties to Mennonite communities abroad.
The Gaines County outbreak in fact started a large measles outbreak in Chihuahua, Mexico. “Mexico’s outbreak in Chihuahua was seeded by an unvaccinated child who had visited Seminole, Texas, and every one of the more than one hundred cases Mexican authorities sequenced carried the same lineage,” Bento said. “We exported this epidemic.”
Bento said that while it is not established how measles arrived in Gaines County, genomic data indicate that the measles lineage was the same as one that caused an outbreak in Canada with links to Mennonite communities beginning in October 2024, suggesting possible spread from Canada to the U.S.
“The US, Canada, and Mexico have been trading measles cases over the last year and a half,” Brewer said. “It’s one of the things that’s made it hard for any of these countries to fully stop ongoing transmission.”
The Utah outbreak, which has lasted since mid-2025, early on involved communities with ties to the Fundamentalist Church of Jesus Christ of Latter-Day Saints. A large outbreak in South Carolina, meanwhile, centered on evangelical churches and schools in Spartanburg County, spanning October 2025 to March 2026.
Bento said that no link tying any of these outbreaks to unauthorized immigration had been identified.
She and her colleagues wrote in an Aug. 13 commentary in Vaccine that the South Carolina outbreak spread in an area with a high rate of religious exemptions from school vaccine requirements, which rose sixfold in Spartanburg County between the 2013-2014 and 2024-2025 school years.
In Spartanburg County, the researchers found, 73% of schools whose MMR vaccination rates were below 80% had measles exposures, while 5% of schools with 95% or higher vaccination rates did. “That is a local exemption-policy story, not a migration story,” Bento said.
Nor were we able to find a link to immigration in measles outbreaks in Southwest Florida, even though Donalds mentioned cases in this area as an example when asked to justify his claim that illegal immigration had caused measles outbreaks.
There was a 2026 outbreak of measles in Collier County that started at Ave Maria University, a private Catholic school. The county reported 107 measles cases between January and April. However, there isn’t evidence this outbreak was related to illegal immigration. The university’s dean of students reported in a letter to students that Florida’s health department “has indicated these measles cases most likely originated with a student’s holiday travel from another state,” according to NBC News.
Cases in Texas Detention Facilities
Donalds also referred to an outbreak in El Paso, Texas. There were measles cases in federal detention facilities in Texas this year, including at an ICE facility in El Paso. The legal status of those involved and how the outbreak began have not been established. ICE did not reply to our request for comment, nor did the El Paso department of public health.
The El Paso department of public health lists 16 cases this year in congregate settings and eight cases in the community. The local news outlet El Paso Matters reported that all 16 congregate cases were at the ICE detention facility Camp East Montana. El Paso’s lead epidemiologist told the outlet that all eight cases in the community were among people who worked for the federal government or worked at or had ties to the West Texas Detention Facility, in nearby Hudspeth County, where a larger measles outbreak has been reported.
Hudspeth County has had 136 confirmed cases of measles in 2026, according to the Texas Department of State Health Services. Van Deusen, the spokesperson from the state health department, told us that all 136 cases were in people housed in federal detention.
The West Texas Detention Facility houses inmates in custody of the U.S. Marshals, and these can include immigrant detainees, according to the Texas Tribune. It’s unclear how many immigrant detainees are at the facility, what the immigration status was of people who got measles, and how measles got into the facility.
Regardless of the source of the initial infection, a factor in the spread of disease is the nature of detention centers themselves.
“Measles is extremely contagious and tends to cause explosive outbreaks in congregant settings like detention centers and prisons,” Jennifer Nuzzo, director of the Pandemic Center at Brown University, told Scientific American when a measles outbreak at Camp East Montana was first reported. “Anytime you have a group of unvaccinated or undervaccinated people crammed into a common indoor space for an extended period, you can expect outbreaks if an infection is introduced.”
In the case of the Texas outbreak, emails obtained by El Paso Matters suggest that delays in reporting cases and “limited information sharing by ICE and by a hospital hindered El Paso’s ability to trace and contain infections,” Bento said. Meanwhile, the emails show public health workers contending with a lack of information about vaccination coverage in the detention centers.
“The failure documented in Texas was screening, vaccination, and reporting inside federal custody,” Bento said.
She added that in contrast to the large Texas outbreak from the year prior, the 2026 El Paso area outbreak did not grow rapidly in the community. “El Paso County and Gaines County both had measles introduced in the same state in the same period,” Bento said. “El Paso, at about 96 percent kindergarten [vaccination] coverage, had eight community cases. Gaines County, at 77 percent, had 414. The variable that mattered was local vaccination coverage, not proximity to the border.”
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