Introduction: In 2013, several clusters of mumps cases were reported in communities of young adults, 72% of whom were fully vaccinated (2 doses). The objective of this study was to estimate the decline in vaccine efficacy over time. Method: A multicenter case-control study was conducted using clusters of mumps cases that occurred between January and June 2013 at French universities or military barracks. Only confirmed or probable cases (parotitis lasting more than two days or less than two days with an epidemiological link to a case) were included. Controls, drawn from the same population, were matched to cases based on place of residence and the year of the study/shared event. Individuals with a history of mumps or without reported vaccination status were excluded. Analyses were conducted using conditional logistic regression, explaining the onset of the disease by the time since the last dose and age (modeled using fractional polynomials), sex, and number of doses. Results: Sixty cases and 227 controls were included in the study; 77% of cases and 76% of controls had received two doses of the vaccine (p = 0.89). Our results show a significant association between the risk of developing the disease and the increase in time elapsed since the date of the last vaccine injection (p = 0.03). Discussion: This study confirms the waning of immunity against mumps over time. It supports the recommendation for a third dose of vaccine during clusters of mumps cases in a community. (R.A.)