Demographic transition and the dynamics of measles in China
Industrialization and demographic transition generate non-stationary dynamics in human populations that can impact the transmission and persistence of infectious disease. Decades of increasing vaccination and development have led to dramatic declines in the global burden of measles, but the virus remains persistent in much of the world. The demographic and economic transition in China over the last several decades, coupled with dramatic declines in the incidence of childhood diseases, has been exemplary of this phenomenon. However, despite an orders of magnitude reduction in measles incidence since 1980, this disease remains endemic at both the national and provincial scale. Here analyze more than 30 year of provincial-level, age-specific measles incidence data from 6 provinces in China to evaluate the impact of vaccination on measles distribution and persistence. We develop a novel application of the classic catalytic model to account for non-stationary epidemic dynamics and evaluate evidence for shifting patterns of age-specific force of infection in China. We further propose a novel method to estimate the impact of targeted vaccination interventions based on the resulting change in the age-distribution of cases rather than the conventional administrative measure of doses delivered.
We show that a combination of demographic transition, as a result of declining birth rates, and reduced prevalence, due to improved vaccination, has shifted the age distribution of susceptibility to measles throughout China. The force of infection of measles has declined dramatically in the industrialized eastern provinces in the last decade, driving a concomitant increase in adult cases while central and western provinces exhibit dynamics consistent with endemic persistence. These dramatic declines in the force of infection in eastern provinces are consistent with a transition of the effective reproductive ratio to near or below 1. By contrast, the central and western provinces, exhibit super-critical dynamics and may provide a source of measles infections that permit national-scale persistence through metapopulation dynamics. The shift in the age distribution of susceptibility and regional heterogeneity in the progress towards elimination emphasizes the importance of progressive control strategies to evaluate program success and prioritize local interventions to achieve national-scale objectives.