Immunization gaps and measles outbreaks in ASEAN (2015–2025): A systematic review and meta-analysis

Abstract

Background: Despite global elimination goals, measles remains a persistent regional health challenge in the Association of Southeast Asian Nations (ASEAN). This systematic review and meta-analysis characterized the distribution of cases and deaths during ASEAN measles outbreaks (2015–2025) and computed pooled proportions of cases stratified by age, vaccination history, migration status, and indigenous group membership. Methods: In this systematic review and meta-analysis (PROSPERO CRD420251149627), we searched Scopus, Medline, Embase, and Google Scholar through January 15, 2026, for studies on measles outbreaks in ASEAN (2015–2025) that stratified cases by age, vaccination history, or marginalized groups. A modified Joanna Briggs Institute/ROBINS-E tool was used to assess the risk of bias. Pooled proportions were calculated using random-effects generalized linear mixed models (GLMM). Heterogeneity and small-study effects were evaluated using I2 statistics and Luis Furuya-Kanamori (LFK) indices, respectively. Findings: Across 11 studies (15,679 cases), measles disproportionately affected children under five (n = 10,867). Vaccination history reporting was near-complete in children but negligible in adults (0·22%, 95% CI: 0·00–73·41%). Among cases with known history, 73·20% (95% CI: 59·37–83·61%) were unvaccinated. Age-ineligible infants (<9 months) comprised half the unvaccinated burden (49·98%, 95% CI: 24·85–75·13%). Among vaccinated cases, 74·84% (95% CI: 44·48–91·70%) were not fully vaccinated. Significant heterogeneity was observed throughout (I2 ≥ 89·0%). Risk of bias was generally low. Transmission was also documented among migrant and indigenous subpopulations. Interpretation: ASEAN measles outbreaks are driven by immunity gaps in unvaccinated children, partially vaccinated individuals, and age-ineligible infants. Achieving and sustaining regional measles elimination requires closing coverage gaps and addressing systemic drop-outs between doses. Future strategies must prioritize lifecycle immunity, including enforcement of the 2nd dose of measles containing vaccine (MCV2) and potential adult boosters, digitized vaccine registries, and cross-border collaboration.