As measles has spread across the United States, the overwhelming majority of cases have been among unvaccinated people or those with unknown vaccination status.
Of the 483 cases reported by the Centers for Disease Control and Prevention (CDC), 97% have been confirmed in those who are unvaccinated or with unknown status.
However, a few cases have been confirmed among individuals fully vaccinated with the measles, mumps, rubella (MMR) vaccine.
In Texas, two of the 400 confirmed cases have been among people fully vaccinated. Additionally, a fully vaccinated measles patient was found to have traveled on an Amtrak train in Washington, D.C. -- potentially exposing others while contagious.
Infectious disease experts and vaccinologists told ABC News that breakthrough infections of measles can occur but are rare.
"We know that the measles vaccine is highly effective," Dr. Dan Barouch, the William Bosworth Castle professor of medicine and professor of immunology at Harvard Medical School, told ABC News.
"However, it's not 100%, so a small percentage of people can still develop measles, even if they receive a measles vaccine," he continued. "In most cases, such cases of measles is less severe than in an unvaccinated individual."
How do breakthrough cases happen
The CDC currently recommends that people receive two doses of the MMR vaccine, the first at ages 12 to 15 months and the second between 4 and 6 years old.
One dose is 93% effective, and two doses are 97% effective, the CDC says. Most people have lifelong immunity against measles when they are fully vaccinated, according to the federal health agency.
One year-old River Jacobs is held by his mother, Caitlin Fuller, while he receives an MMR vaccine from Raynard Covarrubio, at a vaccine clinic put on by Lubbock Public Health Department on March 1, 2025 in Lubbock, Texas.
Jan Sonnenmair/Getty Images
Generally, an unvaccinated person is more likely to be infected with measles than a vaccinated person. One study found an unvaccinated individual was 35 times more likely to contract measles than a vaccinated individual.
However, 97% efficacy means if 100 vaccinated people are exposed to measles, about 3 might still catch it, but 97 will be protected.
Dr. Amy Arrington, division chief of global biologic preparedness at Baylor College of Medicine/Texas Children's Hospital in Houston, said there are two different types of breakthrough cases. The first is called primary vaccine failure, she said.
"That is that small subset of people that just don't form an immune response to the measles vaccine," she told ABC News. "So, after your first dose of the vaccine, it's thought that maybe 7% of people won't have an immune response, and the second shot is thought to really cover most of those people."
Then there is secondary vaccine failure, which is when somebody has an insufficient response to the vaccine over time or waning of the immune response, Arrington said.
This may occur in older people, who have immune systems that weaken over time.
Dr. Gregory Poland, a vaccinologist and co-director of The Atria Research Institute -- which focuses on disease prevention -- said other reasons for breakthrough cases include someone being genetically predisposed or the MMR vaccine not being stored at the right temperature.
"The reason that people who got one or maybe two doses of vaccine and yet still got infected, I've written extensively about this, is the vaccine might not have been handled properly," he said. "You have to maintain a cold chain, so if it got warmed up or sat out of the refrigerator, they might have gotten ineffective vaccine."
Breakthrough cases are less severe
Fully vaccinated people who contract measles are less likely to spread the disease to other people and typically have milder illness, according to the CDC.
A 2020 study.&text=Of%202676%20cases%20with%20verified%20vaccination%20status%2C,of%20complications%20was%20known%20for%202563%20(96%).) looking at measles outbreak in the Netherlands in 2013-14 found that the measles vaccine protected against serious illness.
Skin of a patient after three days with measles rash.
CDC
Of more than 2,400 unvaccinated cases, 14.5% had complications and/or were hospitalized, the study found. Among measles patients who had received two vaccine doses, none developed complications and/or were hospitalized.
Another 2024 study looking at measles cases in the U.S. from 2001 to 2022 also found that breakthrough measles cases tended to have milder disease with fewer complications.
Poland, who was not an author of the paper, said the study found the "classical measles presentation" was not present in about two-thirds of measles patients who received two vaccine doses.
"Complications reduced by 60%, hospitalization reduced by 70%, severe disease reduced by 50%," he said. "If you have gotten vaccinated and you're one of the rare people who develop breakthrough infection, that infection is substantially more mild and less severe with significant decreases in the risk of a complication of hospitalization or even severe disease."
Importance of getting vaccinated
The experts reiterated that it is very important to get vaccinated because measles is highly contagious.
Measles is one of the most contagious viruses known to humans. Just one infected patient can spread measles to up to nine out of 10 close contacts who aren't immune, according to the CDC.
Additionally, measles virus can linger in the air and live on surfaces for up to two hours after an infected person has left a room.
"Measles is one of the most contagious viruses that we know about, so it will spread like wildfire in an unvaccinated population," Barouch said. "Whenever population immunity is less than 95%, then we see outbreaks. … The outbreak will continue to spread as long as a fraction of the population is unvaccinated."
Arrington said it's likely that there will be more breakthrough cases seen in the U.S. as the overall number of cases grow, but it's not a cause for concern.
"As we see case numbers grow, as that denominator gets bigger, we would expect to see breakthrough cases that are occurring in either primary or secondary vaccine failure cases," she said. "It's nothing to be alarmed about, and it's expected because no vaccine is perfect."