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, who originate from less developed rural regions with potentially lower vaccination coverage and are employed in urban centers, in contributing to measles epidemics in China from 2005 to 2014. We developed a networked metapopulation Susceptible–Exposed–Infectious–Recovered model that incorporated migrant worker mobility around Chinese New Year (CNY) migration periods and year-round general-purpose traveler mobility. 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 sustained by susceptibility replenishment through inflows of under-vaccinated migrant workers during the CNY migration periods. Moreover, epidemics in origin PLADs might have been synchronized and facilitated by case importation of exposed and infectious 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 with coverage ranging from 25% to 100% showed national incidence reduction from 33.0% to 50.9%, 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.Author summary: Measles transmission persisted across Provincial-Level Administrative Divisions (PLADs) in China from 2005 to 2014 despite high routine childhood vaccination coverage. We tested our hypothesis that the mobility of inter-PLAD rural-to-urban migrant workers contributed to measles outbreaks. These migrant workers (5.7% of the national population during the study period), often from less developed rural regions with potentially lower vaccination coverage, sought employment in urban centers, with the largest mobility flows occurring around the Chinese New Year. We found that migrant worker mobility shaped epidemics through two mechanisms. Epidemics in migrant worker host PLADs (urban destination regions) were sustained by inflows of under-vaccinated migrant workers that increased the proportion of people susceptible to measles. Epidemics in origin PLADs (rural home regions) might have been synchronized and facilitated by infected or exposed migrant workers returning from endemic host PLADs. In counterfactual simulations, vaccinating migrant workers before migration with coverage ranging from 25% to 100% reduced national incidence by 33.0% to 50.9%. These results suggest that targeted vaccination for migrant workers could strengthen measles control in China.