Challenges for Sustaining Measles Elimination: Post-Verification Large-Scale Import-Related Measles Outbreaks in Mongolia and Cambodia, Resulting in the Loss of Measles Elimination Status.

The Western Pacific Region's pursuit of measles elimination has seen significant progress and setbacks. Mongolia and Cambodia were the first two middle-income countries in the Western Pacific to be verified as having eliminated measles by the Western Pacific Regional Verification Commission for Measles and Rubella Elimination, in March 2014 and 2015, respectively. However, both countries experienced large-scale or prolonged importation-related measles outbreaks shortly afterwards, leading to the re-establishment of endemic transmission. We describe the path to initial elimination in both countries and explore these outbreaks' characteristics, factors contributing to the loss of elimination status, and implications for broader elimination efforts. Data sources include case-based epidemiological and laboratory surveillance reports, historical immunization coverage, genotype data, and published reports of in-depth outbreak investigations. In Mongolia, a single prolonged and large-scale outbreak revealed a hidden immunity gap among young adults and was driven in part by nosocomial transmission, leading to significant morbidity and mortality and loss of elimination status. Cambodia suffered multiple importations from neighboring endemic countries during the global measles resurgence in 2018-2019, complicated by cross-border mobility and significant nosocomial amplification, and the country was ultimately unable to sufficiently distinguish independent chains of transmission, leading to loss of elimination status. Our findings highlight the importance of broadening population immunity assessments beyond children to include adults and specific high-risk groups. Robust routine immunization programs, supplemented by tailored SIAs, are crucial for preventing and managing outbreaks. Additionally, strong outbreak preparedness plans, rapid response strategies, and cross-border collaboration and the global effort to prevent multiple resurgences and large-scale importation-induced outbreaks are vital for maintaining elimination status. The experiences of Mongolia and Cambodia underscore the challenges of sustaining measles elimination in the face of importation risks, shared borders with endemic countries, healthcare system gaps, and population movements. Strengthening the global coordination and synchronization of measles elimination activities is imperative to protect the gains achieved and prevent future setbacks.