Rural-to-Urban Migrant Worker Mobility Shaped Measles Epidemics in China

Despite sustained high routine childhood vaccination coverage, measles outbreaks have persisted across Provincial-Level Administrative Divisions (PLADs) in China. Epidemiological evidence suggests that migrant workers substantially contribute to these outbreaks. In this study, we investigated the role of inter-PLAD rural-to-urban migrant workers--a migrant worker group originating from less developed rural regions with potentially lower vaccination coverage--in driving measles epidemics in China from 2005 to 2014. We developed a networked metapopulation Susceptible-Exposed-Infectious- Recovered model incorporating detailed migrant worker mobility and traveler mobility patterns. By simulating measles transmission dynamics within migrant worker subpopulations, we identified key epidemiological connections between origin and host PLADs. In northern China, migrant workers from Hebei and Shandong were the key contributors to outbreaks in two northern host PLADs, Beijing and Tianjin. In southern China, migrant workers from Anhui and Sichuan were the key contributors across multiple southern host PLADs. Counterfactual modeling suggests that measles epidemics in host PLADs were driven by susceptibility replenishment from under-vaccinated migrant workers during the Chinese New Year migration periods. Moreover, epidemics in origin PLADs might have been synchronized and facilitated by case importation from migrant workers returning from endemic host PLADs, and the strength of this seeding effect depended on the volume of migrant worker flows. Traveler mobility showed minimal impact on measles epidemics. Counterfactual modeling of pre-migration vaccination showed a 50.9% incidence reduction nationally, with significant reduction in host PLADs, and in turn in origin PLADs due to weakened seeding effect. Our findings provide mechanistic insights into the epidemiological role of rural-to-urban migrant workers in measles epidemics, which could support targeted vaccination strategies for improved measles control in China and regions with similar migration dynamics.