More than 800 people in the U.S. have had measles since the beginning of the year, according to NBC News data. The majority of cases are in West Texas, where an outbreak that shows no signs of abating began in January.
Nearly all of the cases are among people who haven’t been vaccinated, but 3% of the identified cases are so-called breakthrough infections. People got sick despite being either partially or fully vaccinated with the measles, mumps and rubella (MMR) shot.
Infectious diseases doctors are at pains to emphasize that the MMR vaccine is one of the most effective vaccines on the market, but as Rodney Rohde, a professor at Texas State University explains, a small number of people who are fully vaccinated may get sick during a large outbreak.
One dose of the MMR vaccine is 93% effective at preventing measles, with the second dose increasing that to 97%.
“The vaccine is highly effective,” said Rohde. “But it means that after two doses, while 97 out of 100 people will develop strong immunity and be protected if exposed to measles, the remaining three out of 100 could still be vulnerable.”
The latest outbreak has seen claims circulating on social media from people who say they are fully vaccinated and still contracted a breakthrough infection, making them seriously unwell, something Rohde said is “slightly possible.” “They could fall into one of those small percentages that got the full vaccine but just didn’t respond,” he says.
However, he said, it’s also plausible that those people may not have realized they weren’t fully vaccinated against measles. Research suggests that when breakthrough infections occur, the MMR vaccine still tends to provide some partial protection. “If someone vaccinated does get measles, it’s often a milder version, sometimes called modified measles,” says Rohde. “A rash can still occur, but it may be less widespread [across the body], fainter or atypical, meaning it doesn’t follow the classic pattern of starting at the hairline and spreading downwards. Fever is also less common or lower-grade. In classic measles, fevers can spike over 104 degrees Fahrenheit, but in modified cases, it’s often milder or absent.”
According to Rohde, the same is true for other common measles symptoms:
- Cough
- Runny nose
- Conjunctivitis, or red eyes
“They may still occur, but are often less intense,” he says. “Koplik spots, which are tiny white spots in the mouth, are less commonly seen. Breakthrough cases are also generally less contagious, but still a potential source of transmission.”
People are contagious four days before and four days after the rash begins, according to the Texas Department of State Health Services. The rash usually appears 14 days after exposure.
Rohde also suggests that people experiencing severe symptoms from a breakthrough infection may have just had a single vaccine dose, which still offers significant protection, but not quite as much as the full two-dose regimen. “These cases can be closer to classic measles than in two-dose breakthroughs, with a high fever which may last longer,” he says. “The rash is usually a more typical measles rash and can be more extensive than in fully vaccinated breakthrough cases. They are also more likely to get possible complications.”
There’s still the question of why these infections happen.
As Rohde and others explain, there may be a number of reasons, ranging from the timing of vaccination to genetic variability in the immune response.
Measles vaccine immunity is mostly stable
While Health and Human Services Secretary Robert F. Kennedy Jr. has called for the public to get vaccinated against measles, he also claimed in an interview with CBS News that “the vaccine wanes very quickly.”
Vaccine experts counter that the MMR shots offer lifelong protection.
“If immunity waned a lot, you would see a lot of cases in vaccinated people in older age groups, and we’re not seeing that in this Texas outbreak or any others in the recent past,” said Scott Weaver, a microbiology and immunology professor at the University of Texas Medical Branch, who also works with the Global Virus Network coalition of experts. “People seem to be solidly protected, and almost everybody for life, as far as we can tell.”
Unlike influenza or Covid, measles is a very stable virus that has evolved little to evade vaccine-induced immunity over time. In 2021, Mayo Clinic researchers published a study concluding that there’s a near-zero probability of a novel measles virus naturally emerging with the capability of circumventing the MMR vaccine.
While the virus isn’t changing, it appears that in some rare cases, the body’s own immune system can slowly stop producing antibodies against the virus. Based on international studies, scientists have concluded that it is possible for some children vaccinated against measles to lose their protection over the course of two decades.
“Because measles is highly infectious, this waning is expected to cause some breakthrough infections,” said Alexis Robert, a researcher at the London School of Hygiene and Tropical Medicine who authored a paper on long-term vaccine protection in 2024.
All people respond to either vaccines or natural infections a little bit differently, according to Weaver.
The MMR vaccine, which works by presenting the immune system with a live but weakened version of the measles virus, trains the body’s immune cells to recognize different parts of the virus’ proteins called epitopes and generate antibodies against them in the future, he said.
An epitope refers to small segments of amino acids on a protein’s structure that the immune system’s antibodies learn to recognize and bind to, should the virus attempt to invade again in future.
It’s not unexpected that a few people respond poorly to any vaccine, Weaver said.
“Some people have a very robust response because they respond to many different [viral] epitopes, and some people, just by chance, have immune cells that can’t recognize as many of those epitopes,” he said. “Differences in our genetic makeup determine how many different epitopes we respond to and develop immune memory against.”
After measles was declared eliminated in the U.S. in 2000 and no longer considered a public health threat, research into the virus slowed. With vaccine hesitancy rising and the U.S. close to losing its measles elimination status, some scientists see the need to study and monitor the rare cases of breakthrough infections.
“While transmission patterns show measles outbreaks are driven by groups of unvaccinated individuals, especially among children, a potential risk of infection years after vaccination still impacts what strategy is most effective and the coverage needed to eradicate measles,” Robert said.
How quickly does the measles vaccine take effect?
Timing can also make a difference. A child who had just received the MMR vaccine will still be vulnerable to measles within the first couple of weeks, especially during an intense outbreak where a lot of virus is circulating, Rohde said.
“In most cases, the human immune system takes roughly two weeks to develop enough protection against the measles virus and many other pathogens,” he said. “If you were exposed just before or after getting the vaccine, your body may not be at full strength.”
In some cases, for example people who are either born with a weakened immune system or develop conditions such as autoimmune diseases or blood cancers, the body may not mount as effective an immune response against measles, resulting in breakthrough infections.
“There are people who just have an overall immune system that’s just not as effective against many different infectious agents,” Weaver said. “We know about people who are immune compromised for certain reasons, like they’re HIV-infected, they’re under chemotherapy for cancer, things like that, but there are some people who are just born with immune systems that don’t function quite as well.”
Another plausible explanation is that people may have received a defective MMR vaccine, although Weaver said this is unlikely in the United States, where quality controls tend to be fairly stringent.
“In the cases of measles, it’s a live virus, so that virus could become less infectious if it’s not stored properly and that could lead to less [immune] response,” he said.
When you might need a measles booster
Whether you’re more vulnerable to developing a breakthrough measles infection could also depend on when you were born. Before the first measles vaccines were developed in the 1960s, virtually everyone contracted the virus during childhood, providing those who survived with lifelong immunity.
However, people who were born between 1957 and 1968 received a first-generation measles vaccine made using an inactivated virus that did not work so well. The Centers for Disease Control and Prevention recommends that anyone born within this time frame should get at least one MMR dose.
Between 1968 and 1989, most people received only a single MMR dose, making them slightly less well protected compared with people who got the standard two doses in the past 36 years.
Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine, suggested that if someone who got only a single dose is near a current measles outbreak or is traveling internationally to a country where the virus is endemic, it could be worth checking with with a doctor about getting another dose.
“Even if you’re truly immune and you get another dose of MMR, there’s no harm to that,” Schaffner said.
