“In Europe, we have a missed generation of unvaccinated children who are now adults, and we have no alternative” but find them and vaccinate them now, said Italian epidemiologist Vittorio Demicheli. “Otherwise, we’ll have measles outbreaks for decades.”

The median age of measles patients in Europe has risen to 17 from 10 in the past decade. Between January 2016 and March 2019, individuals between 10 and 29 made up about 29% of measles cases, with the majority in those 20-29 years old, according to the ECDC. Those older than 30 accounted for roughly 20% of cases.

“This is quite striking compared with other countries,” Mr. Bundle said. For example, in the U.S., in cases reported between January and April the median age was 5, according to the Centers for Disease Control and Prevention.