Age-stratified serosurvey reveals immunity patterns against measles in Youyang, China

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Objective: To evaluate age-specific measles antibody dynamics and identify population immunity gaps in Youyang County, China, approximately two decades after the introduction of a two-dose measles-containing vaccine (MCV) schedule at 8 and 18 months of age. Methods: A cross-sectional serosurvey was conducted among residents without rash in 2025. Measles-specific IgG antibodies were quantified using enzyme-linked immunosorbent assay (ELISA), with seroprevalence defined as >200 mIU/mL. Seroprevalence and geometric mean concentrations (GMCs) were analyzed across eight age strata. Results: < 0.001). The lowest levels were observed in infants aged <8 months (36.5, 95%CI: 26.2-47.9; GMC: 120.47 mIU/mL). Following two vaccine doses, peak immunity was achieved in children aged 18 months to <6 years (>95%; GMC: 730.99-1044.61 mIU/mL). Seroprevalence then reached a distinct nadir in adults aged 18-<40 years (65.6, 95% CI: 50.9-77.9; GMC: 330.48 mIU/mL), before rebounding in older adults aged ≥40 years (90.1-93.5%). Conclusion: This investigation revealed a distinct immunity gap, with measles adjusted seroprevalence at 36.5% (95% CI: 26.2-47.9) in infants under 8 months and 65.6% (95% CI: 50.9-77.9) in reproductive-aged adults, despite robust post-vaccination responses in young children. This serological pattern may be a common characteristic in populations that complete two-dose MCV immunization by 18 months of age and experience persistently low measles incidence.

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