Risk Factors for Measles Virus Infection Among Adults During a Large Outbreak in Postelimination Era in Mongolia, 2015.

Background: In 2015, a large nationwide measles outbreak occurred in Mongolia, with very high incidence in the capital city of Ulaanbaatar and among young adults. Methods: We conducted an outbreak investigation including a matched case-control study of risk factors for laboratory-confirmed measles among young adults living in Ulaanbaatar. Young adults with laboratory-confirmed measles, living in the capital city of Ulaanbaatar, were matched with 2-3 neighborhood controls. Conditional logistic regression was used to estimate adjusted matched odds ratios (aMORs) for risk factors, with 95% confidence intervals. Results: During March 1-September 30, 2015, 20 077 suspected measles cases were reported; 14 010 cases were confirmed. Independent risk factors for measles included being unvaccinated (adjusted matched odds ratio [aMOR] 2.0, P < .01), being a high school graduate without college education (aMOR 2.6, P < .01), remaining in Ulaanbaatar during the outbreak (aMOR 2.5, P < .01), exposure to an inpatient healthcare facility (aMOR 4.5 P < .01), and being born outside of Ulaanbaatar (aMOR 1.8, P = .02). Conclusions: This large, nationwide outbreak shortly after verification of elimination had high incidence among young adults, particularly those born outside the national capital. In addition, findings indicated that nosocomial transmission within health facilities helped amplify the outbreak.

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.

Estimation of the unvaccinated among those aged less than 25 years according to aimag and its association with incidence of measles outbreak 2015-2016 in Mongolia.

Mongolia had an epidemic of measles in 2015-2016, even though more than 90% of the population have been vaccinated since 1997. This study aimed to examine the associations between unvaccinated proportion and measles incidence according to aimag. Mongolia has 21 provinces (aimag) with Ulaanbaatar as the capital city. Vaccination coverage between 1991 and 2014 and measles incidence according to aimag were obtained from the National Center for Communicable Diseases of Mongolia database. Accumulated unvaccinated proportion (AUP) among those aged 1 to 24 years in 2015 was estimated from the unvaccinated at the 1st dose of 1991 to 2014. From 1991 to 2014, unvaccinated proportion among those aged 1 to 24 years in the whole country has been reducing from 28.0% in 1991 to 1.8% in 2014. The AUP in 2015 varied from 2.7% (Selenge) to 21.8% (Govisumber). The incidence was remarkably higher in only two aimags with a large density of the unvaccinated aged 1 to 24 years (Ulaanbaatar and Darkhan-Uul) than in the other aimags. The incidence had no significant correlation with the AUP, although the correlation between the incidence and the density of unvaccinated aged 1 to 24 years was significant when the two aimags were included. In conclusion, the AUP between 2.7% and 21.8% had no correlation with the incidence according to aimags in Mongolia measles epidemic 2015-2016.