Delayed introduction, contact variation, and susceptible dynamics explain spatial asynchrony during Korea’s large pertussis outbreak
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Infectious disease outbreaks typically exhibit a large-scale spatial synchrony, reflecting coupling through shared environmental forcing and human mobility. During South Korea’s unprecedented large pertussis outbreak in 2024—2025, however, asynchronous epidemic patterns were observed throughout the country: some regions experienced two epidemic waves, while others regions experienced only the first or second wave. 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 temporal variation in transmission is spatially homogeneous. In particular, contact variation can drive differential levels of susceptible depletion, thus shaping the potential for multiple epidemic waves. This analysis offers a perspective on spatiotemporal epidemic dynamics that depart from classical epidemic theory.
Significance Statement
When infections spread across space, resulting outbreaks among neighboring regions share similar patterns. This spatial synchrony has been documented across a wide range of pathogens, but we still have limited understanding of when epidemic dynamics deviate from this expectation. Here, I used a mathematical modeling approach to analyze a large pertussis outbreak in South Korea, which exhibited asynchronous epidemic patterns throughout the country. The analysis showed that seasonal variation in transmission does not need to differ across regions to explain this pattern. Instead, variation in baseline transmission level and delayed introduction alone are sufficient to produce strong spatial asynchrony. Analyses further show that high transmission levels can prevent a second wave by rendering susceptible hosts immune during the first wave.
