Vaccine skeptics misconstrue studies to falsely claim that measles vaccine causes measles outbreaks

In February 2025, social media posts appeared alleging that the current Texas measles outbreak, which began in January 2025, was caused by people who received the measles vaccine. One example is this article from The People’s Voice, published on 17 February 2025. The People’s Voice, formerly NewsPunch, has a record of publishing misinformation and conspiracy theories.

The headline of the article was shared on Facebook by Marcella Piper-Terry, an activist who’s campaigned against vaccination and published vaccine misinformation. Piper-Terry wrote that “In the 2015 measles outbreak in California, 73 of the 194 cases were determined to be vaccine strain measles, which can spread from person to person. This is what happens with live virus vaccines”.

Another example is this tweet from the anti-vaccine organization Children’s Health Defense, formerly headed by the current secretary of the U.S. Department of Health and Human Services, Robert F. Kennedy Jr. The tweet claimed the Texas outbreak “began in the vaccinated” and that “the current ‘outbreak’ of measles in Texas is a vaccine-induced outbreak”. The person who made the claim, Brian Hooker, previously misleadingly claimed that unvaccinated children are healthier than vaccinated children. The tweet provided no evidence to substantiate Hooker’s allegations.

Generally, these posts allege that because the measles vaccine uses a live virus, vaccinated people can transmit the vaccine virus to others, thus causing measles. More specifically, these posts alleged that vaccinated people “shed” the live measles virus from the vaccine and transmitted the virus to others, thus causing a measles outbreak in Texas.

However, the claim is incorrect and unfounded. Both The People’s Voice and Piper-Terry’s Facebook post misconstrue information in scientific publications to create this potentially dangerous misconception, as we explain below.

Cited studies didn’t show that weakened virus in measles vaccine causes outbreaks

The combination measles-mumps-rubella (MMR) vaccine contains live, but attenuated (weakened), measles virus. Unlike the wild measles virus, this weakened virus doesn’t cause disease in people with a properly functioning immune system. The MMR vaccine is highly effective at preventing disease: two doses are more than 90% effective against measles.

It’s true that those with immunodeficiency disorders, such as people with inherited genetic defects in the immune system or people receiving immunosuppressive treatments, may experience complications from receiving live vaccines, as even the weakened virus can cause disease in this group^[1-3].

However, there’s no evidence that such vaccine-associated infections have been transmitted to others with a normal immune system.

The People’s Voice article claimed that three published studies support its hypothesis that the measles vaccine caused the Texas measles outbreak.

One is a 1995 study by the U.S. Centers for Disease Control and Prevention (CDC), which reported that PCR could be used to detect measles virus RNA in urine specimens from people who recently received the MMR vaccine^[4]. This indicates that the vaccine virus strain can be shed in urine. However, it didn’t show that vaccinated individuals were capable of transmitting the virus to others.

Another is a 2012 study about an investigation into a child who developed a rash shortly after receiving the MMR vaccine during a local measles outbreak.

It’s important to note that rash is a symptom of measles, but is also a common side effect of the MMR vaccine. Indeed, the question that the 2012 study sought to address was how to distinguish between a rash caused by measles infection and one caused by measles vaccination.

As the study showed, certain tests can help differentiate the two. One such test was PCR analysis of a urine sample, which detected the measles vaccine strain instead of the wild measles virus. This indicated that the rash that the child was experiencing was a side effect of the measles vaccine, rather than a symptom of measles infection, and that the child was therefore not a measles case.

Like the 1995 CDC study mentioned earlier, this shows that the vaccine strain can be shed in urine. But it doesn’t show that the vaccine strain can be transmitted to others, nor that it causes disease.

Furthermore, the People’s Voice article left out other information in the study that contradicted its hypothesis. For example, the study cited another study by Katz et al. showing that no live measles virus could be grown from recently vaccinated children—data that indicates that the vaccine strain isn’t contagious. The 2012 study also cited another study that found that “[w]hile the attenuated virus can be detected in clinical specimens following immunization, it is understood that administration of the MMR vaccine to immunocompetent individuals does not carry the risk of secondary transmission to susceptible hosts” [emphasis added].

The third study is from 2014, documenting measles cases in people who were vaccinated against measles^[5]. This study reported how a vaccinated person contracted the wild measles virus and transmitted it to several others.

This isn’t unexpected. The MMR vaccine is highly effective at protecting people from disease: up to 99 in 100 people are protected after receiving two doses of the vaccine. But there are rare cases in which people who receive two doses still don’t develop sufficient immunity to measles and therefore are vulnerable to infection if exposed to the virus.

The key takeaway here is that the study didn’t report transmission of the vaccine strain to others, but of the wild measles virus that causes disease. Therefore, it’s not evidence of the vaccine strain being transmitted and causing disease. So it doesn’t show that the vaccine can cause a measles outbreak either.

In summary, none of the three published studies substantiate the hypothesis that the Texas measles outbreak was caused by a recent MMR vaccination drive. A spokesperson for the Texas health authority told NBC News that “[a]ll samples tested have come back as a known strain of the wild measles virus—not the vaccine”.

Detecting vaccine strain in suspected measles cases doesn’t mean that an outbreak was caused by the vaccine

Piper-Terry’s post alleged that during “the 2015 measles outbreak in California, 73 of the 194 cases were determined to be vaccine strain measles, which can spread from person to person”.

She didn’t provide a source for this claim, but we were able to trace these figures to this published study from 2017^[6]. More specifically, this quote from the study: “During the measles outbreak in California in 2015, a large number of suspected cases occurred in recent vaccinees. Of the 194 measles virus sequences obtained in the United States in 2015, 73 were identified as vaccine sequences”.

When removed entirely from the study’s context, this sentence may seem to support Piper-Terry’s claim. However, the key phrase the study used is “suspected cases”, which Piper-Terry’s post transformed to “cases”, implying that these were confirmed measles cases. This is incorrect. To understand why, we need to look at the study more broadly to understand what it was trying to accomplish.

The goal of the study was to develop a method to distinguish between measles cases (measles infection) and people who were recently vaccinated against measles, but not actually infected.

The reasons why both groups are easily confused is twofold. One, the symptoms of measles infection and the common side effects of the MMR vaccine are quite similar: both include a rash, fever, and runny nose, although unlike infection, vaccination doesn’t come with the same risks of complications. Two, a measles outbreak also tends to be associated with a rise in measles vaccine uptake, as people seek to protect themselves from the disease. Therefore, it’s possible for recently-vaccinated people to be categorized as suspected cases during an outbreak—even if they aren’t infected.

Being able to tell the difference between the two is critical to the public health response to a measles outbreak. First, it enables resources to be better targeted at actual measles cases. Second, it avoids unnecessary isolation of recently-vaccinated people displaying measles-like symptoms but who aren’t actually infected with measles.

The first sentence of the study’s abstract makes its goal clear: “During measles outbreaks, it is important to be able to rapidly distinguish between measles cases and vaccine reactions to avoid unnecessary outbreak response measures such as case isolation and contact investigations”.

Piper-Terry’s allegation that what happened in Samoa was also a vaccine-associated outbreak is baseless. There’s no evidence that the most recent Samoa measles outbreak, which occurred in 2019, was due to the vaccine. On the contrary, evidence suggests that the source of the outbreak was a traveler to Samoa who’d been infected with measles.

MMR vaccine coverage in Samoa had plummeted prior to the outbreak, after two babies died following MMR vaccination. The deaths weren’t caused by the vaccine itself, but by healthcare staff incorrectly using a muscle relaxant instead of water to dilute the vaccine. Nevertheless, this fatal healthcare error led to a fear of the MMR vaccine and subsequent low vaccine uptake, which enabled the spread of measles once it was introduced into the community.

The first report on the outbreak by Samoa’s Ministry of Health noted that specimens from suspected cases were sent to a clinical laboratory for testing and were confirmed to be measles. This type of testing is able to distinguish between the wild measles virus and a vaccine strain. There were no reports of the vaccine strain being implicated in the 2019 Samoa outbreak.

Pediatrician Vincent Ianelli wrote on Vaxopedia that the idea that the MMR vaccine is the real cause of measles outbreaks isn’t credible, because if this were true, we’d see outbreaks of measles a lot more often, particularly in environments where very young children congregate:

“[J]ust think about what happens in a typical daycare or school. All of the kids don’t get vaccinated on the same day, so if shedding was an issue, wouldn’t the kids who had gotten their vaccines first shed on the ones who weren’t yet vaccinated, getting them sick?

Of course, this doesn’t happen.”

Conclusion

The measles vaccine contains live, attenuated (weakened) measles virus, which means it’s unable to cause disease in people with a properly functioning immune system. Those with immunodeficiency disorders, such as people with inherited genetic defects in the immune system, may be recommended to avoid receiving live vaccines, as even the weakened virus can cause disease in this group.

But there’s no evidence that such vaccine-associated infections have been transmitted to other people with a normal immune system. Social media claims to the contrary misconstrued information in published studies, lending the impression that there’s credible evidence to support such claims when the studies don’t support these claims or even contradict them.

The Texas Department of State Health Services conducts testing of measles cases to establish which type of measles virus is responsible for the outbreak. All tested cases so far have been linked to the wild measles virus, not the measles vaccine strain. To date, there has been no evidence implicating the vaccine strain as the origin of a measles outbreak.

REFERENCES

• 1 – Monafo et al. (1994) Disseminated measles infection after vaccination in a child with a congenital immunodeficiency. Journal of Pediatrics.
• 2 – Angel et al. (1998) Vaccine-associated measles pneumonitis in an adult with AIDS. Annals of Internal Medicine.
• 3 – Bitnun et al. (1999) Measles inclusion-body encephalitis caused by the vaccine strain of measles virus. Clinical Infectious Diseases.
• 4 – Rota et al. (1995) Detection of Measles Virus RNA in Urine Specimens from Vaccine Recipients. Journal of Clinical Microbiology.
• 5 – Rosen et al. (2014) Outbreak of Measles Among Persons With Prior Evidence of Immunity, New York City, 2011. Clinical Infectious Diseases.
• 6 – Roy et al. (2017) Rapid Identification of Measles Virus Vaccine Genotype by Real-Time PCR. Journal of Clinical Microbiology.