Delayed introduction and susceptible variation drive spatial asynchrony in pertussis epidemics

Infectious disease outbreaks typically exhibit a large-scale spatial synchrony, reflecting coupling through shared environmental forcing and human mobility. A few exceptions to this theoretical expectation have been reported, but the conditions under which synchrony breaks down remain poorly understood. South Koreas 2024-2025 pertussis outbreak provides a striking example, during which epidemic trajectories diverged substantially throughout the country: notably, the decline in spatial synchrony with distance was more pronounced than that reported in many previous outbreaks. Integrating high-resolution surveillance data from 252 municipalities and a Bayesian transmission model, I show that heterogeneity in introduction timing and differences in local contact levels can drive such fine-scale asynchrony even when all locations share identical seasonal forcing for transmission rates. In particular, regional variation in baseline contact levels can drive differential levels of susceptible depletion, thus shaping the potential for multiple epidemic waves. This analysis offers a unique perspective on spatiotemporal epidemic dynamics that depart from classical epidemic theory.