A waning immunity model is most consistent with recently observed resurgent outbreaks of mumps in countries with high vaccine coverage, according to a study published in the Proceedings of the National Academy of Sciences. Noting that multiple countries with high vaccine coverage have experienced resurgent outbreaks of mumps, Aaron King, PhD, MA, and colleagues proposed two putative mechanisms of vaccine failure as driving observed trends: gradual waning of immunity and the introduction of novel viral genotypes capable of evading vaccinal immunity. The data were most consistent with the waning immunity model; by age 18, an estimated 32.8% of individuals had lost vaccine-derived immunity. The waning model reproduces features of the epidemiologic data, including the shift in mumps incidence toward older individuals, recent recurrence of mumps outbreaks, and the high proportion of mumps cases among previously vaccinated individuals. “Our results support, in principle, the administration of regular booster doses to achieve and maintain herd immunity,” Dr. King and colleagues wrote.