Spatiotemporal dynamics of chikungunya transmission in Foshan City, China: A modelling analysis

A large-scale chikungunya outbreak occurred in Foshan, China, in July and August 2025. This study aimed to quantify the temporal and spatial variation in chikungunya virus transmission across the city's five districts.

Using case surveillance data, we constructed a Bayesian model to estimate the instantaneous effective reproduction number (R_t) for each district between 16 June and 30 August 2025, explicitly accounting for time-varying reporting delays.

A general non-increasing trend in R_t was inferred across Foshan from mid-July onward. The estimated R_t in Shunde spiked on 8 July at 13.84 (95% credible interval [CrI]: 11.10-17.10), followed by Chancheng and Nanhai on 18 July at 4.16 (95% CrI: 3.26-5.31) and 3.64 (95% CrI: 2.80-4.71), respectively. In these districts, R_t was estimated to fall below the epidemic threshold of 1 by early August, while Gaoming and Sanshui, located farther from Shunde, maintained values near 1.

The substantial variation in R_t estimates may have reflected changes in rainfall and intensified vector control measures post-outbreak. Spatial heterogeneity in transmission may have been attributed to both population density and proximity to the outbreak epicentre, Shunde.