From March to April 2006, an outbreak of
mumps occurred in Gyeonggi, Korea. The aim of this study was to describe and discriminate between primary and secondary
vaccine failure in a highly vaccinated population for
mumps using
IgG avidity testing. Fifteen clinical
mumps cases occurred among 41 students. Among these 15 patients, 11 vaccinated patients were considered secondary
vaccine failures with high
IgG titers and a high avidity index (AI, > or =32%); an unvaccinated patient was considered to have primary
infection with high
IgG titers and low AI, and three vaccinated patients were considered as other
infections with low
IgG titers and low AI. Among 26 unaffected students, 5 vaccinated patients were retrospectively diagnosed as sub-clinical
infection with high
IgG titers and high AI; the remaining students had low
IgG titers and low AI except for one previously infected student. The results of this study show that secondary
vaccine failure played an important role in this
mumps outbreak. Therefore, booster immunization for
mumps should be considered in immunized adolescents to prevent further outbreaks.