Mumps is resurging. And it may be because the immune response provoked by the mumps vaccine weakens significantly over time, and not because people are avoiding vaccination or because the virus has evolved to develop immunity to the vaccine, a new study has found.

The mumps resurgence has been largely in people 18 to 29, most of whom received the recommended two shots in early childhood, and not in older people who gained immunity through natural infection before the vaccine was developed.

Using data from epidemiological studies and mathematical models, researchers found that the resurgence, which began in 2006, has left about a third of children 10 to 14 at risk. The researchers estimate that about 25 percent of vaccinated people will lose their immunity in 8 years, 50 percent in 19 years, and 75 percent in 38 years. The study is in Science Translational Medicine.

“We’ve seen the outbreaks of mumps in vaccinated populations,” said the lead author, Joseph A. Lewnard, a research associate at the Harvard T.H. Chan School of Public Health, “in contrast to measles, where it’s only been in unvaccinated pockets.”

A third shot for mumps is recommended during outbreaks, Lewnard said.