SciCheck Digest
The measles vaccine uses a weakened virus that’s never been shown to spread to others. Samples from the outbreak in Texas also show that a wild-type virus is responsible. Yet, social media posts have falsely claimed that the outbreak is due to a vaccine strain. Without evidence, other posts have blamed immigrants crossing the southern border illegally.
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In 2000, following widespread vaccination, the U.S. eliminated measles, a highly contagious disease that can cause serious health complications. With elimination, measles no longer circulates regularly in the country, but outbreaks still occur when travelers are infected abroad and then encounter pockets of susceptible people. Most often, the travelers are unvaccinated and returning to the U.S.
The measles vaccine, which is administered in two doses as part of the measles, mumps and rubella, or MMR, vaccine, is safe and highly effective. To generate robust protection, the vaccine uses a live but weakened version of the measles virus. There’s no evidence that a vaccinated person has ever spread the weakened measles virus to another person.
Despite this, social media posts have incorrectly claimed or suggested that the ongoing measles outbreak in Texas is due to a vaccine strain and that vaccine clinics are the reason why the outbreak has grown.
“‘Texas Measles Outbreak Began in the Vaccinated,’” reads a Feb. 20 X post from the anti-vaccine group Children’s Health Defense, which was shared on Instagram. “Isn’t it odd that in the same area where there was a mass measles vaccination campaign there is now an outbreak?”
“Measles cases in Gaines County, TX, exploded immediately after health officials handed out free measles vaccines — vaccines that contain live measles virus,” a Facebook post stated. “CDC studies show vaccinated children shed measles virus, which can spread to the unvaccinated. Did the vaccine fuel the outbreak?”
Misinterpreting a scientific finding from the 2015 Disneyland measles outbreak, another Facebook post claimed that “vaccine strain measles … can spread from person to person.” It added, “This is what happens with live virus vaccines.”
The Disneyland claim is similar to one Robert F. Kennedy Jr., the founder and former chairman of Children’s Health Defense who is now the Department of Health and Human Services secretary, has made before, including in a 2019 letter to the prime minister of Samoa. That year, the island nation experienced a large measles outbreak after vaccination rates fell perilously low, in part due to misinformation propagated by Children’s Health Defense. In the letter, Kennedy referenced the California outbreak to incorrectly posit that Samoan children who died of measles may have succumbed to a vaccine strain of the virus.
Even though the source of the measles importation is not yet known, posts have also made unsubstantiated claims that the Texas outbreak was sparked by immigrants illegally crossing the southern border.
No Evidence the Measles Vaccine Ever Transmits Virus
Because the measles vaccine is a live vaccine, it’s theoretically possible that it could in rare cases cause infections that then spread the vaccine virus. This would be most likely in an immunocompromised person whose immune system is not robust enough to respond to even the weakened virus and gets sick. Research does show that for some vaccinated individuals, the virus or parts of it can be found in small amounts in urine or respiratory secretions, as we’ve noted before.
But the vaccine is not recommended for immunocompromised people. And evidence of this so-called vaccine shedding is not the same as evidence of transmission. After many decades and billions of vaccinations, there is no indication that the vaccine virus is transmitted.
As the Centers for Disease Control and Prevention told us when we addressed this concern last year, not only is there “no history” of a measles outbreak ever occurring as a result of the vaccine, but even a single transmission of the vaccine virus to another person has “never been proven.”
A 2016 systematic review that included more than 770 scientific articles determined that there have been “no confirmed cases of human-to-human transmission of the measles vaccine virus.”
Multiple experts also reiterated to PolitiFact last month that the vaccine does not pose a transmission risk.
The claim that the current outbreak began with or expanded due to vaccination is contradicted by several other lines of evidence as well.
Lara Anton, a press officer for the Texas Department of State Health Services, told us in late February, when these claims first circulated, that all 30 samples the department genotyped and had results for showed the measles virus was the wild-type D8 strain, which circulates in many parts of the world. This included the first cases.
The first case in the Texas outbreak was unvaccinated, Anton said, along with many others. As of March 25, there have been 327 cases, of which only two have been in vaccinated people. The remainder are in people who are unvaccinated or have an unknown vaccination status.
In neighboring New Mexico, there have been 43 cases as of March 25. Of those, 31 were in unvaccinated people, eight were in people with unknown vaccination status and four were in people who had received one or more vaccine doses.
“The New Mexico Department of Health has no evidence of a measles outbreak beginning with a person that has been vaccinated,” David Morgan, a public information officer for the department, told us in an email. He added that CDC genotype testing shows that the measles strain found in New Mexico cases is the same one as in Texas and “that the New Mexico outbreak is not caused by the vaccine.” (Although the New Mexico health department told us it considers the cases in the two states to be a single regional outbreak, there isn’t yet a direct epidemiological link between any of the New Mexico cases and a Texas case.)
Oklahoma has also now reported nine measles cases (seven confirmed and two probable) in the northeastern part of the state, all in unvaccinated people. The initial cases “reported exposure to the measles outbreak in West Texas and New Mexico,” according to the state’s Department of Health.
Kansas, too, has identified 23 measles cases in the southwest part of the state as of March 26, with a suspected link to the cases in Texas and New Mexico. Only one of the cases is in a person who has been fully vaccinated for their age.
All of this is consistent with how measles outbreaks occur in the U.S. The outbreak was first identified in January in undervaccinated Mennonite communities in Gaines County, Texas, where many children are home schooled or attend smaller private schools and are not vaccinated.
Even in the public schools there, two of the three districts in the county had kindergarten MMR vaccination rates well below the 95% needed to prevent outbreaks in 2023-2024 — including one with fewer than half of students vaccinated. (Overall, the vast majority of parents vaccinate their children against measles, but when unvaccinated people are clustered together in the same area, a measles introduction can lead to large outbreaks.)
Notably, the measles outbreak has also led to the first measles death in the U.S. in a decade, in an otherwise healthy child — something there’s no evidence the vaccine strain could do. Another person in New Mexico also tested positive for measles after death.
“There’s no data and the biological plausibility of it being a vaccine strain … it’s just not even in the cards,” Dr. Michael Mina, a former Harvard School of Public Health professor who studied measles, told us.
Given all this, vaccination clinics are not a plausible source of measles. It’s also completely expected to see clinics pop up precisely when cases are continuing to rise because vaccination is an important tool for stopping the spread of measles. Vaccination, however, does not provide immediate protection, nor do enough people necessarily go to the clinics to get vaccinated to stop the spread of an outbreak, so it’s common to continue to see clinics as an outbreak develops. (Local health officials in West Texas have said that there hasn’t been much demand for vaccines.)
Uncommonly, vaccination can temporarily contribute to the case numbers if someone has a reaction to the vaccine that looks similar enough to measles to be counted as a case. This is not the same, however, as the vaccine causing a real case of measles.
The one Facebook post, for example, pointed to the 2015 Disneyland outbreak, incorrectly claiming that “73 of the 194 cases were determined to be vaccine strain measles.” But as Science Feedback has explained, the post was likely misinterpreting a 2017 paper that discussed methods for distinguishing between “measles cases and vaccine reactions to avoid unnecessary outbreak response measures such as case isolation and contact investigations.”
The paper did not say that 73 of 194 measles cases in that outbreak were caused by the vaccine. Rather, the paper referred to sequences and suspected cases. In other words, precisely because vaccine reactions are not infectious and do not require the same public health response, it is useful to quickly differentiate between those instances and a bona fide measles case.
Indeed, by early March, there initially had been five measles cases among vaccinated people in the Texas outbreak count. That tally is now down to two. One case was removed, Anton said, because it was “reclassified as a vaccine reaction and not a case.” She said the state does additional testing when someone develops symptoms within six to 21 days of being vaccinated to check whether it’s a measles case or a vaccine reaction.
Two other cases were changed to being unvaccinated since the vaccinations had occurred after being exposed to measles. The individuals had received a first MMR dose a day or two before becoming symptomatic, Anton said. It’s recommended that unvaccinated people who have measles exposure get the vaccine within 72 hours to either prevent infection or reduce illness severity, but the prophylaxis does not always work.
Original Measles Source Unknown
Although it’s no surprise that measles is spreading in an undervaccinated community, it’s not known how measles first infected people in Gaines County. The first measles case of the outbreak had not traveled internationally, according to the Texas health department.
People online have taken advantage of this information vacuum, blaming “Biden’s open borders” and illegal immigration.
“If the vaccines worked and it was eradicated, where did it come from?” asked one Threads post from last month, misunderstanding the difference between elimination and eradication, the latter of which means a disease is no longer present anywhere in the world. “Hmm. In Texas, mind you. Right near the border. Right where people are coming over, unvaccinated, but yet it’s the Americans you’re worried about??”
As we said, it’s not known how measles made its way to Texas. However, it’s worth noting that the cases are in the northwestern part of the state and not particularly close to the border. There’s no evidence at this time that people in the country illegally are responsible, PolitiFact has written.
Measles importations usually occur when U.S. residents return from visiting places where measles is common. To a lesser extent, foreign travelers introduce the disease.
Mina told us that it’s possible measles could have originated in Mexico, but Europe is a more typical source. The countries with the highest number of reported measles cases are generally in Europe, Africa and Asia — not the Americas, according to World Health Organization data.
Last November, the Pan American Health Organization reverified that Brazil was measles-free, restoring measles elimination status — first given in 2016 — to the entire Americas region.
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