Strengthening measles immunity in Thailand: Rationale for a third vaccine dose to avert outbreaks.

Achieving measles elimination in Thailand requires restoring population immunity to levels sufficient to interrupt transmission. Recent outbreaks indicate widening immunity gaps despite long-standing national immunization programs. We assessed measles-specific immunity using 3675 age-representative serum samples collected in 2024 from four provinces. IgG concentrations were measured by ELISA with a protective threshold of ≥200 IU/L and interpreted alongside national serosurveys conducted in 2004 and 2014 to evaluate temporal changes in immunity across vaccine-era birth cohorts. Overall seropositivity in 2024 was 61.3% and showed a pronounced U-shaped age pattern. Adults older than 40 years had high seroprotection above 95% consistent with natural infection. Adolescents and young adults aged 14 to 31 years had much lower immunity ranging from 27% to 46%. Birth-cohort analysis revealed progressive declines in seroprotection among individuals born between 1985 and 2008, indicating low seroprevalence of vaccine-induced immunity. National case data from 2024 reflected these findings, with most infections outside deep southern provinces occurring in individuals older than 15 years. Thailand's current immunity levels fall below the herd immunity threshold needed to halt measles transmission, estimated at 92% or higher. The high susceptibility in adolescents and adults suggests that two routine doses are inadequate under current epidemiologic conditions. These results suggest the findings warrant further evaluation of targeted booster strategies, including potential role of a third measles-containing vaccine dose for individuals aged 10 to 40 years. Integrating this dose into adolescent or adult booster programs, along with strengthened surveillance and targeted catch-up vaccination in low-coverage regions, offers a practical pathway to closing immunity gaps and accelerating progress toward measles elimination.