A new systematic review found documented instances in which people vaccinated against measles contracted and transmitted the virus, including transmission chains involving hundreds of subsequent cases, while the researchers maintained that unvaccinated infections remain the primary drivers of outbreaks.
By yourNEWS Media Newsroom.
People who have received measles vaccines can still contract the disease and transmit the virus to others, according to a new peer-reviewed analysis that examined decades of outbreak investigations and other evidence involving vaccinated measles cases.
The systematic review and meta-analysis, published Aug. 14 in Expert Review of Vaccines, identified 890 vaccinated people who contracted measles across the studies meeting researchers’ inclusion criteria. Seventy of those patients — 7.9% — were documented transmitting measles to at least one other person.
Read the full report: Do vaccinated cases transmit measles A systematic review and meta-analysis
Those 70 vaccinated patients generated 237 directly linked secondary cases, with a median of two secondary infections for each transmitting case. Individual vaccinated patients transmitted measles to between one and 69 people. When researchers examined later generations of infection, they identified 812 cases that could ultimately be traced to transmission originating with vaccinated individuals.
“Measles transmissions from vaccinated cases, although relatively uncommon, must be considered in public health investigations, as such transmissions can contribute to outbreaks,” the authors wrote.
The findings, also detailed in a report by The Defender, establish that vaccination does not make onward measles transmission impossible. The researchers simultaneously emphasized that such cases remain uncommon and concluded that outbreaks continue to be “critically driven by unvaccinated cases.”
The distinction is significant because public discussion of measles frequently centers on vaccination status. Media coverage of measles outbreaks often emphasizes cases among unvaccinated populations, reflecting public health data showing that most reported cases occur among people without documentation of measles-mumps-rubella vaccination.
Reuters reported earlier this year on declining vaccination rates and the medical freedom movement while covering the increase in U.S. measles cases. During Texas outbreaks in 2025, The New York Times described measles as having “hopscotched through unvaccinated communities.”
Coverage of recent measles-associated deaths in Pennsylvania has likewise highlighted that the deceased were Amish, a population in which vaccination rates are typically lower.
The new review does not dispute that unvaccinated people account for the majority of infections. Instead, it examines a narrower question: what happens when a vaccinated person nevertheless contracts measles, and can that person pass the virus to others?
Researchers Narrowed Nearly 12,000 Records to 33 Studies
The Public Health Ontario researchers conducted an extensive search of five major scientific databases and supplemented that work with gray literature and resources from the World Health Organization and European Centre for Disease Prevention and Control.
Their search produced 11,911 peer-reviewed records and 22 gray-literature records. After applying eligibility requirements, only 33 studies were included in the final analysis. Most were outbreak investigations, and 12 involved outbreaks in the United States.
Thirty of the 33 studies contained enough information to examine later generations of infection. Across those investigations, researchers ultimately counted 812 measles infections attributable to transmission chains beginning with a vaccinated case.
The review also examined whether receiving one or two doses was associated with differences in onward transmission. Researchers found no statistically significant difference between measles patients who had received one vaccine dose and those who had received two.
Information about disease severity among vaccinated transmitters was more limited. Only a small number of studies provided sufficient clinical details. Of 19 transmitting patients whose symptoms were described, more than half reportedly experienced mild measles.
Children’s Health Defense Senior Research Scientist Karl Jablonowski said one of the most notable findings was how little published research directly quantified vaccination’s effect on transmission once infection occurred.
“The paper is a scathing review of the popular narrative, by the very people who are credited for writing the narrative,” Jablonowski said.
“Of the 11,911 papers and 22 gray-literature reports, only one attempted to estimate the efficacy of the vaccine preventing transmission,” he said. “The estimate was not statistically different from the unvaccinated.”
That limited evidence meant the meta-analysis could not determine from the included literature how much more or less likely a vaccinated person with measles was to transmit the virus compared with an infected unvaccinated person.
Jablonowski summarized the research gap more simply: “We cannot know what we do not study.”
The review’s authors nevertheless cautioned against interpreting that absence of comparative evidence as proof that vaccination has no effect on transmission. They said their findings provided “no evidence to contradict” the prevailing understanding that vaccinated people who develop breakthrough measles are less likely to transmit the virus than unvaccinated cases.
Earlier Outbreaks Documented Breakthrough Infections
Cases of vaccinated people contracting and spreading measles have been reported previously.
In 2011, a fully vaccinated person in New York developed measles and subsequently infected other people. The case became notable because it demonstrated that transmission from a vaccinated individual, while unusual, could occur.
During Italian measles outbreaks between 2017 and 2021, vaccinated people were also among those infected.
Similar questions emerged during a 2025 outbreak in Colorado, where multiple measles cases occurred among people reported as fully vaccinated.
Breakthrough infections are also part of the historical development of U.S. measles vaccination policy. Measles outbreaks at schools and universities during the late 1980s and early 1990s occurred despite high levels of vaccination in some settings. The CDC subsequently adopted a two-dose recommendation for measles-containing vaccines.
Researchers Examine Why Measles Vaccination Can Fail
The authors identified two principal explanations for why a vaccinated person can still develop measles.
Primary vaccine failure occurs when vaccination does not produce an adequate immune response after administration. Secondary failure occurs when protection established following vaccination decreases over time.
The review found that comparatively few of the included studies contained the laboratory information needed to determine which type of failure explained individual breakthrough infections.
Health writer Sayer Ji said the most likely explanation, consistent with possibilities discussed by the authors, was that vaccinated patients experienced breakthrough infections involving wild-type measles virus.
Ji also raised the possibility that some illnesses could involve vaccine-strain virus. Published medical literature has documented rare instances in which vaccinated individuals developed measles associated with vaccine strains, and other scientific reports have described vaccine-associated measles cases. Such reports do not establish that vaccine-strain illness explains the transmission events examined across the broader meta-analysis.
Authors Stress Breakthrough Transmission Remains Uncommon
Despite documenting hundreds of infections connected to vaccinated transmitters, the researchers emphasized the rarity of the phenomenon relative to overall measles transmission.
Only a small share of vaccinated people exposed to measles develop the disease, the authors said, and an even smaller proportion of those breakthrough cases transmit the virus to someone else. On that basis, they concluded that measles outbreaks remain “critically driven by unvaccinated cases.”
Their analysis nevertheless indicates that public health investigators cannot automatically rule out vaccinated measles patients as possible sources of transmission. The documented chains included individual vaccinated cases responsible for multiple secondary infections, with one person linked directly to as many as 69 subsequent cases.
Ji argued that the institutional backgrounds of the researchers make the findings noteworthy, describing the authors as “not outsiders raising doubts,” but rather “the people who spent their careers building the measles elimination framework.”
The study’s conflict-of-interest disclosures also identify professional ties among some researchers. Co-author Dr. Walter A. Orenstein, a former director of the U.S. Immunization Program at the CDC and former assistant surgeon general, disclosed consulting relationships with Merck, Sanofi and Moderna. Orenstein is also co-editor of “Plotkin’s Vaccines,” a major textbook in the field.
Other authors are affiliated with the WHO or prominent Canadian medical and public health organizations.
Taken together, the systematic review provides evidence for two findings that are not mutually exclusive: measles vaccination substantially reduces the likelihood of disease at the population level, according to the authors’ interpretation, while vaccinated people who experience breakthrough infections can, in documented circumstances, transmit measles and contribute to larger transmission chains.
The researchers said those breakthrough transmissions are uncommon and should not displace the role of unvaccinated cases in driving outbreaks. At the same time, the 70 documented vaccinated transmitters and 812 infections ultimately traced to those cases led the authors to conclude that vaccination status alone should not cause investigators to exclude an infected person as a potential source during a measles outbreak.
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