Abstract
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 3,675 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
percent and showed a pronounced U-shaped age pattern. Adults older than 40 years had high
seroprotection above 95 percent 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 levels 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 percent 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 measlescontaining 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.
