Recent measles outbreaks in the U.S. reflect a troubling shift: the disease, long considered a childhood illness, is now increasingly affecting unvaccinated adults. While the current outbreak began among unvaccinated Mennonite children in Texas, it has spread to other states and now includes a notable number of adult cases, including fatalities. This trend is becoming more common—adults over 20 made up over a quarter of measles cases in 2023.

Historically, adults didn’t get measles because the virus spread so efficiently among children that nearly everyone was exposed early in life. Now, thanks to widespread but imperfect vaccination, unvaccinated children can grow into unvaccinated adults without ever encountering the virus—until outbreaks occur. Unlike vaccinated adults, who may experience mild breakthrough cases, unvaccinated adults are at much higher risk of severe complications, including pneumonia, encephalitis, and even death. Hospitalization rates for adult measles patients are two to three times higher than for school-age children.

Doctors today are often unprepared for adult measles cases due to their rarity in the post-vaccine era. During past outbreaks, such as the 1988–1990 surge in Los Angeles and the 2014–15 Disneyland outbreak, adult patients often went unrecognized until they were seriously ill. The pattern is clear: the older a person is when first exposed to measles, the more dangerous the illness becomes.

Adding to this concern is measles’ ability to induce “immune amnesia”—it can wipe out memory B cells that help the body fight previously encountered infections. This effect has been well-studied in children but likely affects adults too, possibly to an even greater extent given the severity of adult cases. Measles was declared eliminated in the U.S. in 2000, but vaccine coverage has dropped from over 95% to 92.7%, dangerously close to the herd immunity threshold.. As the Texas outbreak grows, it signals a broader public health risk: a generation of unvaccinated children aging into vulnerable adults.