vaccine has been reported as approximately 80%, which is considered inadequate to provide population protection. Previous studies have shown that the effectiveness of 2 doses of vaccine is from 88% to 95%. 8,9 The estimated herd immunity threshold for mumps ranges from 88% to 92%. 10Although there was no single explanation for this outbreak, multiple factors may have contributed; these factors include waning immunity, vaccine failure, high population density and high contact rates in colleges, and incomplete vaccineinduced immunity to the wild virus. The relatively advanced age of the majority of infected patients points toward the waning immunity hypothesis. However, more research is needed to study the long-term vaccine effectiveness.In our study, all the subjects had received 2 doses of MMR vaccine, and yet 16 HCWs were found to be seronegative. In a recent measles outbreak, an unvaccinated HCW became infected in a hospital. Of 64 people with confirmed cases of measles, 17 became infected while visiting the healthcare facility."A limitation of our study is the small sample size; we did not include all the HCWs employed. Therefore, the results may underestimate the number of susceptible HCWs already employed.Mumps should be considered a reemerging yet vaccinepreventable disease, with transmission occurring in both healthcare and community settings. Future studies should include all HCWs, to better assess mumps seroprevalence in healthcare institutions. In view of the possible waning immunity, it is essential to carry out periodic serological surveillance and to vaccinate susceptible HCWs.