Delayed introduction, transmission variation and susceptible dynamics explain spatial asynchrony during Republic of Korea's large pertussis outbreak.

Infectious disease outbreaks typically exhibit a large-scale spatial synchrony, reflecting coupling through shared environmental forcing and human mobility. Exceptions to this theoretical expectation have been reported, but the conditions under which synchrony breaks down remain poorly understood. Republic of Korea's 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, we show that heterogeneity in introduction timing and differences in local transmission levels, which may reflect differences in either contact patterns or susceptibility, can drive such fine-scale asynchrony even when all locations share identical seasonal forcing for transmission rates. In particular, regional variation in baseline transmission levels can drive differential levels of susceptible depletion, thus shaping the potential for multiple epidemic waves. This analysis offers a unique perspective on spatio-temporal epidemic dynamics that depart from classical epidemic theory.