Evaluating chikungunya outbreak size and emergency vaccination strategies in Miami: an exploratory modeling study

The risk of vector-borne diseases such as chikungunya - caused by the chikungunya virus (CHIKV) and transmitted by Aedes mosquitoes - is increasing as globalization and climate change continue to expand the habitat of its primary vectors. In 2024 alone, nearly 500,000 chikungunya cases were reported worldwide. Miami-Dade County, Florida, is particularly vulnerable due to the endemic presence of Aedes aegypti and its role as a major entry point for international travel and trade. This study explores the possible impact of a CHIKV outbreak in urban Miami and the effectiveness of an emergency response vaccination program as a preventive measure.

We created a dynamic vector-host model with a SEIR-SEI structure including local temperature and demographic data. To capture uncertainty, we defined multiple scenarios with varying vector-host ratios (VHRs) and vaccination coverage levels. Emergency vaccination was modeled as an outbreak response strategy to explore its potential impact on outbreak size.

Our scenario-based analysis showed that CHIKV transmission in Miami can happen year-round, with peak outbreak potential from May to August. Outbreak likelihood strongly depended on the VHR, ranging from an annual average of 5% (VHR of 0.75) to 21% (VHR of 2). The VHR also had a major impact on outbreak size, with a median of 224 cases (IQR: 50-740) at a VHR of 1, and 21,053 cases (IQR: 8,900-36,990) at a VHR of 1.5. Outbreak response vaccination reduced outbreak size by a median of 19% (IQR: 2.5%-38%) at a VHR of 1 and 58% (IQR: 48%-66%) at a VHR of 1.5, assuming 10% population coverage.

While empirical validation is not yet possible due to the absence of large-scale outbreaks in Miami, our exploratory modeling provides forward-looking insights into how outbreaks might unfold and how emergency vaccination could mitigate their impact. This scenario-based framework should be viewed as an early evaluation to inform preparedness discussions, stimulate further research, and guide proactive public health strategies in Miami and other subtropical regions at risk for chikungunya.