Resurgence of mumps in Korea

Abstract

Routine vaccination against mumps has markedly reduced its incidence. However, the incidence of mumps continuously has increased since 2007. In 2013, a large mumps epidemic occurred in Korea, and this epidemic is still an ongoing problem. This epidemic occurred primarily in school settings and affected vaccinated adolescents, predominantly male students. The recent resurgence of mumps is caused by multiple factors: suboptimal effectiveness of the current mumps vaccines, use of the Rubini strain vaccine, waning immunity in the absence of natural boosting due to the marked reduction in the mumps incidence, genotype mismatch between the vaccine and circulating mumps virus strains, and environmental conditions that foster intense exposures. Containment of mumps outbreaks is challenging because the sensitivity of diagnostic tests is low among vaccinees and control measures are less efficient because of the inherent nature of the mumps virus. Despite the suboptimal vaccine effectiveness in outbreak settings, maintaining the high vaccine coverage is an important strategy to prevent mumps outbreaks, given that the routine use of mumps vaccines has substantially reduced the incidence of mumps and its complications as compared with that in the pre-vaccine era. In order to control the current mumps epidemic and prevent further outbreaks, we need to better understand the dynamics of mumps among vaccinated populations and the changing epidemiology in Korea. Concerted efforts should be made to systematically monitor the immunization status of the Korean population and to improve diagnosis efficiency. Furthermore, more effective mumps vaccines need to be developed in the future.

Keywords: Adolescents; Disease outbreaks; Genotype; Incidence; Male; Mumps; Mumps vaccine; Republic of Korea.

Figure 1. Number of reported mumps cases and the annual incidence of mumps in Korea since 1955, and the mumps vaccine strains used in Korea.

MMR, measles-mumps-rubella; MR, measles-rubella; NIP, National Immunization Program; mo, months; yr, years.

Figure 2. Number of reported mumps cases in Korea during 2013-2014 by age (A) compared with previous years (2007-2012) and by gender (B).

Figure 3. Changes in proportions of reported mumps cases in Korea by age between 2001 and 2014.

Figure 4. Cumulative number of reported mumps cases in Korea between 2010 and 2014, by region.

Figure 5. Monthly distribution of reported mumps cases and school terms in Korea between 2010 and 2014.

Figure 6. Changes in critical vaccination coverage according to vaccine effectiveness and the basic reproductive number.

VE, vaccine effectiveness.