Response timing and the cost of delay: a model-based policy counterfactual of the 2025 Foshan chikungunya outbreak in China
Year: 2026
Type: preprint
Abstract: Background Between June and December 2025, Guangdong Province, China, experienced the largest chikungunya outbreak ever recorded in the country, with Foshan City as its epicentre. The emergency response was unprecedented in scale, but the contribution of response timing and intensity to the outcome has not been quantified. We developed a model-based policy counterfactual to ask how the outbreak would have differed under alternative response scenarios. Methods We coupled a weather-driven mechanistic model of Aedes albopictus density with a two-zone host-vector transmission model in which temperature-dependent extrinsic incubation and vector competence followed published trait surfaces. The response was encoded as a two-step schedule matching the documented timeline (case finding from 8 July 2025; citywide vector control from 19 July 2025). Parameters were estimated against post-detection-wave surveillance anchors only, with uncertainty represented by a six-start envelope. Five scenarios were compared: the fitted actual response, no response, the actual response advanced or delayed by two weeks, and a Breteau-index-triggered response. Results The fitted model reproduced the observed outbreak (11,988 cases; envelope 10,695 to 13,018) against approximately 10,819 officially reported for Foshan. Growth of reported cases between 15 July and 9 August 2025 was dominated by a detection wave from active case finding rather than by incidence; by 15 July true infections exceeded reported cases by a factor of 5.1 to 14.5. Removing the response gave a median of 95,182 reported cases (78,084 to 127,785), 7.9 times the fitted actual level. Advancing the response by two weeks reduced final size by 27%, a two-week delay increased it by 42%, and a Breteau-index threshold trigger reduced it by approximately half. Conclusions Within this reconstruction, response timing determined the scale of the 2025 Foshan outbreak more than any other single factor. Entomological thresholds were crossed weeks before the epidemic was detected, and acting on them could have substantially reduced the outbreak. The framework offers a reusable instrument for evaluating response policy in cities facing Aedes-borne outbreak risk.
Source: Research Square
Author: Tian-Yu Wang
Institution: Guangdong Peizheng College
Language: English
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W7213322166