A multi-level spillover risk assessment for the largest chikungunya virus outbreak in China

Chikungunya virus (CHIKV) has posed growing threats in recent years, with increasing reports of both local transmission and multi-level spillover in regions historically considered low-risk for local CHIKV transmission. These trends underscore the urgency for proactive, scalable spillover risk assessment frameworks, we therefore develop a transferable multi-level framework to evaluate spillover potential across city, provincial, and interprovincial levels. At the city level (L1), we estimate street-scale spillover risk using population-density-adjusted gravity models. At the provincial level (L2), we quantify spillover risk from the epicenter to other cities by adjusting population mobility flows with a road-network-based accessibility. At the interprovincial level (L3), we construct a composite Spillover-Establishment Risk Index (SERI) that combines climate-driven viral suitability, outbreak intensity, and human mobility. Applying this framework to the 2025 CHIKV outbreak, the most extensive ever reported in China, we present for the first time in China a multi-level spillover risk assessment framework that successfully recapitulates spillover heterogeneity. All top five highest risk streets overlapped with reported local outbreaks, and model calibration at the provincial level re-ranked the top two destinations to better align with provincial-level risk. Moreover, over 99% of predicted spillover risk was captured within a 50 km road-network buffer from the epicenter, highlighting its utility as a first-ring surveillance zone. This modular design supports rapid adaptation to diverse epidemic settings and improves early-warning strategies against arboviral spillover.