Nationwide Measles Seroprevalence in the Republic of Korea, 2023

Although measles-mumps-rubella vaccination coverage remains above 95% in the Republic of Korea, imported measles cases continue to occur. This study aimed to assess the population-level immunity to measles to inform the evaluation of the national immunization program and to identify potential immunity gaps relevant to outbreak risk.

Serum specimens were collected from 1,719 individuals aged 0-93 years (birth cohorts 1930-2023) across six regions. Anti-measles virus IgG binding antibodies were measured using an enzyme-linked immunosorbent assay (ELISA). A subset of 286 specimens was further analyzed using plaque reduction neutralization test (PRNT). Antibody concentrations were standardized to the World Health Organization 3rd International Standard for Anti-Measles (NIBSC 97/648; 3,000 mIU/mL).

Overall, ELISA seropositivity was 75.9% across all ages and increased to 77.4% after exclusion of infants aged <1 year. Among participants aged ≥1 year, ELISA geometric mean titers (GMTs) were lowest in adolescents and young adults (approximately 10-29 years), ranging from 154.1 to 273.5 mIU/mL. PRNT seropositivity was ≥70% across all 10-year age groups among participants aged ≥1 year, and the age-group-specific pattern of neutralizing antibody GMTs was broadly consistent with the susceptible age groups identified in the binding antibody analysis.

This national serosurvey identified reduced binding and neutralizing antibody levels in parts of the adolescent and early-adult populations, indicating a possible immunity gap (immune pocket). Given the age distribution of measles cases reported during 2024-2025, these findings highlight the need for strengthened surveillance and systematic review of vaccination history in these age groups to help reduce secondary transmission at the community level.