Mumps is resurging. And it may be because the immune response provoked by the mumps vaccine weakened 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 ongoing resurgence, which began in 2006, has left about a third of children 10 to 14 at risk.

The researchers estimate that about 25% of vaccination people will lose their immunity in 8 years, 50% in 19 years, and 75% in 38 years. The study is in science transitional medicine. We’ve seen the outbreaks of mumps in vaccinated populations said the lead author.