Dengue transmission risk in California under climate and land-use change: a semi-mechanistic modelling study

SummaryBackground Dengue cases are increasing in non-endemic regions due to environmental change and increasing travel and trade. For these non-endemic regions, estimating dengue risk is challenging as transmission is driven by both local environmental conditions and the introduction of viremic travelers. In this study, we aimed to estimate current and future dengue risk in California, USA—a region that has recently experienced its first cases of locally-acquired dengue. Methods We modeled dengue risk as the product of three key components needed for local transmission—vector presence, temperature-suitability for pathogen transmission, and viral introductions via travel-associated cases—estimated using vector and case surveillance, sociodemographic, and environmental data. We estimated risk for locations and months where local transmission was reported in 2023–2024 to define a ‘threshold' level of risk. We then projected monthly, census tract-level risk under both current conditions and future scenarios of climate warming and urban expansion. Findings Approximately 18.2 million (95% CI: 17.9–18.3) California residents—primarily in the Central Valley and the Los Angeles and San Diego metropolitan areas—currently live in areas where peak monthly dengue risk exceeds levels estimated during observed local transmission. Under moderate scenarios of climate warming and urban expansion, an additional 4.1 million (95% CI: 3.7–4.6) residents may be at risk by mid-century. Outside the summer months and beyond the Central Valley and southern California, current and future risk remains low due to one or more major bottlenecks to transmission. Interpretation Our study identifies the specific regions and months conducive to dengue transmission in the non-endemic setting of California. At present, this covers a substantial portion of the state and is projected to expand under ongoing climate warming and urbanization. Our results underscore the need for sustained vector control, and timely detection of travel-associated cases. Funding National Science Foundation, National Institute of Food and Agriculture.