In recent years resurgent measles epidemics have occurred in countries with consistent vaccination programs. After receiving a measles-elimination certificate from the World Health Organization in 2014, Mongolia experienced consecutive measles outbreaks in 2015 and 2016. Within two years almost 50,000 individuals became infected with over 150 deaths. The majority of cases occurred in children under five years and adults between 15-30 years. Age-targeted vaccination campaigns have been used, or proposed, in outbreaks of vaccine preventable disease (e.g. measles, influenza) to 1) limit incidence in groups with greater risk for severe disease and 2) to limit transmission from groups with high contact rates. We analyzed age-specific time series of reported cases during both outbreaks to assess the impacts of a child-targeted vaccination campaign in 2015 and an adult-targeted campaign in 2016. We use the dynamics of these outbreaks in target and non-target age classes to infer age-specific mixing structure in this population. We then compare the dynamics from the inferred age-specific mixing to the expected dynamics using simulations assuming conventional age-structured mixing matrices from the literature (e.g. POLYMOD).
Results/Conclusions
The outbreaks in 2015 and 2016 were characterized by a strongly bi-modal age distribution, with the majority of cases in young children and in adults older than 25 years. Of 2584 children under the age of 5 years who were infected in the 2016 outbreak, 30% were eligible for the 2015 vaccination campaign and 70% were children ineligible at the time of the 2015 vaccination campaign. This implies that there was significant direct protection of campaign eligible children in the 2015 outbreak. However, the occurrence of the 2016 outbreak implies that the immunization of children was not sufficient to reduce the overall reproductive rate below 1. We show that continued transmission in adults both facilitated the 2016 and resulted in high incidence among infants that had been ineligible for the 2015 outbreak. This adult to infant transmission is highly relevant as young children have high case fatality (>90% of deaths occurred in 2016). Thus, while the 2016 vaccination campaign targeting adult susceptibles was sufficient to end the outbreak, it was insufficient at providing indirect protection to at-risk infants. This observation has implications of the design of future outbreak response vaccination campaigns in populations with large populations of adult susceptibles.