Changing epidemiology and transmission dynamics of measles in Ho Chi Minh City: a comparison of the 2018-2020 and 2024-2025 outbreaks

Introduction Measles remains a major public health threat worldwide. Following a resurgence of cases, Ho Chi Minh City (HCMC) experienced a severe outbreak in 2024-2025 that differed from the 2018-2020 epidemic. We compared the epidemiology and transmission dynamics of these two outbreaks to inform future surveillance and vaccination strategies. Methods Line-list data for measles cases reported in HCMC between 2018 and 2025 were extracted from the infectious disease surveillance system. Demographic changes were assessed using cross-sectional age distributions and longitudinal birth-cohort analyses. Transmission dynamics were characterized by estimating the effective reproduction number (Rt) using the Cori and Wallinga-Teunis methods. Age-stratified Who-Infected-Whom transmission matrices were constructed to quantify within- and between-age-group transmission before and after interventions. Results The 2018-2020 outbreak primarily affected children under five years of age (median age 3.75 years, IQR 0.9-9.1), whereas the 2024-2025 outbreak shifted toward older age groups (median age 8.4 years, IQR 1-14). Birth-cohort analyses identified two major immunity gaps: children born during the COVID-19 immunization disruption (2019-2023) and adolescents and young adults born before 2014 who were not eligible for the 2018 outbreak response immunization campaign. The 2024-2025 outbreak also showed broader spatial spread and a reversed geographic trajectory compared with the earlier epidemic. Transmission patterns changed substantially, with the 2018-2020 outbreak sustained mainly by transmission among young children, whereas the 2024-2025 outbreak was driven by older age groups, with individuals >15 years acting as an important source of infection for infants. Conclusion The epidemiology of measles in HCMC has shifted from predominantly pediatric transmission to increasing involvement of adolescents and adults, reflecting accumulated immunity gaps across multiple birth cohorts. These findings suggest that age-restricted outbreak response immunization may leave residual susceptible populations that contribute to future outbreaks. Achieving sustainable measles elimination will require vaccination strategies that address historical immunity gaps across all vulnerable age groups, in addition to maintaining high routine childhood vaccination coverage.