U.S. Rep. Byron Donalds, a Florida Republican gubernatorial candidate, has attributed domestic measles flare-ups to illegal immigration.
Appearing on CBS News’ “Face the Nation” on August 23, host Margaret Brennan pointed out that Florida’s immunization levels currently fall short of the threshold required for measles herd immunity. She then asked Donalds whether he would advise Florida parents to vaccinate their children.
“No,” Donalds responded, asserting instead that the resurgence of measles in specific regions stems from high levels of unlawful immigration permitted during the prior administration.
Highlighting recent outbreaks in southwest Florida and El Paso, Texas, Donalds claimed that individuals residing in the country unlawfully fail to adhere to standard measles, mumps, and rubella (MMR) vaccination protocols.
“So the key thing is, you number one, you got to control immigration into your country,” he added. “You can’t have an entire segment of people, more than 10 million, coming to your country without that vaccination schedule.”
The figure of 10 million unauthorized border crossers during Joe Biden’s presidency is incorrect; federal records indicate the administration permitted approximately 4.3 million individuals to enter the U.S. while awaiting immigration hearings.
Furthermore, infectious disease specialists emphasize there is no empirical link connecting recent measles resurgences to migrant populations. Experts point to declining domestic MMR vaccination rates as the primary catalyst, with roughly 94% of confirmed 2026 cases occurring in individuals who are either unvaccinated or whose vaccination status remains unverified. Donalds’ campaign did not provide evidence supporting his assertions.
Similar political claims blaming immigrants for viral outbreaks have surfaced during previous measles spikes stretching back well before the Biden administration.
Dr. George Rust, an epidemiologist and professor at the Florida State University College of Medicine, explained that the U.S. historically experienced isolated measles events triggered by domestic travelers contracting the virus abroad and displaying symptoms upon return.
“Maybe one or two additional individuals would catch it from them, but because of high immunization rates and rapid public health response, there would be no second wave of cases,” Rust noted. He added that outbreaks are now more prevalent because those initial exposures encounter clusters of unvaccinated residents, fueling wider transmission.
The Current State of Measles in the U.S.
The ongoing 2026 measles season has already exceeded the historic case counts recorded in 2025, which were the highest seen since 1991.
Measles is an exceptionally contagious respiratory illness capable of causing severe medical complications, including pneumonia, brain swelling, and death. Following the first U.S. fatalities attributed to the virus in over a decade during 2025—which claimed the lives of two children and one adult—the Pennsylvania Department of Health reported on August 25 that two additional unvaccinated individuals had died from the infection.
Administering the two-dose MMR regimen offers approximately 97% protection against infection.
“Ninety percent of unvaccinated people who are exposed are likely to catch it,” Rust warned. “Because of the high rate of infectivity, measles outbreaks can spread exponentially in segments of the population that are unimmunized or not immune.”
Is the Spread Driven by Immigrants?
Because the Centers for Disease Control and Prevention does not track the immigration status of measles patients, connecting specific outbreaks directly to undocumented immigrants is not supported by official data. Standard contact tracing indicates that the virus typically enters the country via unvaccinated American citizens returning from international destinations, particularly Europe and the Western Pacific.
The U.S. achieved official measles elimination status in 2000, meaning continuous domestic transmission had been halted.
“Any given chain begins with someone who traveled,” explained Dr. Paul Spiegel, director at the Johns Hopkins Center for Humanitarian Health. “An introduction becomes an outbreak only when the virus reaches enough susceptible, unvaccinated people, and the importation data do not point to migrants in any case.”
Dr. William Schaffner, an infectious diseases specialist at Vanderbilt Health, concurred, noting that the overwhelming majority of cases stem from unvaccinated American children traveling abroad, contracting the virus, and bringing it back. Only a fraction of cases involve foreign nationals entering the country.
While West Coast outbreaks frequently trace back to unvaccinated residents returning from Asia, central and eastern U.S. cases usually originate from travel to Europe. Schaffner noted he has never encountered data showing significant virus introductions originating from south of the border.
Regarding the early 2026 outbreak in El Paso, Texas, infections emerged within an Immigration and Customs Enforcement detention facility and a separate federal facility holding both migrants and U.S. citizens. Public health authorities blamed the spread on substandard facility conditions and a lack of baseline vaccination tracking for detainees, noting that ICE frequently holds individuals who arrived long before their detainment.
In Southwest Florida, the state’s largest cluster centered around Ave Maria University in Collier County. State health department tracing concluded that the outbreak most likely began following a student’s holiday travel out of state.
Similarly, several of the largest recent outbreaks occurred in geographic pockets with large Amish and Mennonite populations characterized by lower immunization rates—mirroring previous outbreaks clustered within similarly vaccine-hesitant communities.
Global MMR Vaccination Standards
Nations that serve as the primary points of origin for many undocumented immigrants—including Mexico, Guatemala, and Honduras—incorporate the MMR vaccine into their standard pediatric immunization schedules.
Experts point out that the underlying issue is not an absence of national vaccination programs, but rather a broader dip in vaccine coverage below the 95% target required to maintain herd immunity—a vulnerability shared by the United States.
While El Salvador and Honduras reported isolated cases, Guatemala and Mexico experienced more substantial flare-ups largely concentrated among unvaccinated individuals. Notably, Mexico’s recent epidemic traced its origins to the U.S., beginning in Chihuahua in February 2025 after an unvaccinated nine-year-old from a Mennonite community contracted the virus while visiting relatives in Seminole, Texas, during that state’s outbreak.
“Discussions of migration and infectious disease often get the causal relationship backward,” Spiegel said. “Human mobility of every kind—tourism, business travel, migration—can move a virus from one place to another, but movement does not by itself create an outbreak; susceptibility does.”
Conclusion
Donalds attributed localized measles resurgences to unauthorized immigration during the Biden administration.
However, public health data shows measles is typically imported by U.S. residents returning from international travel, with experts identifying declining domestic vaccination rates as the primary driver of current outbreaks. Analysts found no data linking undocumented immigrants to the spread, and the countries of origin frequently cited in these debates maintain routine childhood MMR schedules.
Because Donalds provided no evidence to substantiate his claim, the assertion is rated False.