Fact Check: Did Undocumented Immigrants Cause U.S. Measles Outbreaks? No Evidence Supports That Claim

The claim

Florida gubernatorial candidate Byron Donalds said undocumented immigration under the Biden administration was the reason measles outbreaks appeared in parts of the United States. The statement makes a specific causal claim: that immigration status, rather than broader vaccination and transmission patterns, explains the outbreaks.

PolitiFact reviewed the assertion with infectious-disease experts and found no data supporting it. The Centers for Disease Control and Prevention tracks confirmed cases, vaccination status, geography and outbreak links, but does not report patients' immigration status in a way that would substantiate Donalds' allegation.

A lack of immigration-status data does not prove no undocumented person has ever contracted measles. It does mean a politician cannot responsibly attribute national outbreaks to that population without case evidence, contact tracing or another valid epidemiological basis.

The evidence points to gaps in vaccination

Measles spreads extremely easily among people who lack immunity. PolitiFact reported that about 94 percent of confirmed 2026 cases were among people who were unvaccinated or whose vaccination status was unknown. Public-health specialists identified declining MMR coverage as the primary driver of renewed outbreaks.

International travel can bring the virus into the United States because measles continues circulating globally. A returning U.S. resident, tourist, citizen or immigrant can become an initial case; what determines whether the virus produces an outbreak is whether it reaches a community with enough susceptible people.

Countries that are major sources of migration to the United States—including Mexico, Guatemala and Honduras—include measles vaccination in their routine childhood schedules. That does not guarantee every person is vaccinated, but it contradicts the suggestion that immigrants uniformly come from systems without MMR vaccination.

Verdict: False

The available evidence does not support blaming undocumented immigrants for U.S. measles outbreaks. Surveillance identifies low or unknown vaccination status as the dominant shared factor, while the federal dataset does not provide the immigration evidence Donalds would need for his claim.

Public officials can legitimately debate border policy and health screening. They should not turn an infectious disease into a claim about a disfavored group without data. That can stigmatize families and distract from the practical intervention that is proven to reduce transmission: maintaining high two-dose MMR coverage.

People exposed to measles or uncertain about immunity should follow current public-health and medical guidance. The fact check addresses a political attribution; it does not replace advice from a clinician or local health department during a confirmed exposure.

Limits and what to watch

Travel can introduce measles into a community, but available national data do not identify undocumented immigrants as the cause of the outbreaks and the CDC does not publish patients' immigration status.

The next development to watch is updated CDC outbreak totals, state contact-tracing reports and vaccination-coverage data.