Leveraging probabilistic forecasts for dengue preparedness and control: the 2024 Dengue Forecasting Sprint in Brazil

Forecast models are a key decision-support tool for public health authorities in managing epi- demics, feeding into early warning systems, scenario evaluations, and empirical basis for resource al- location. In Brazil, improving dengue forecasting became a priority in response to the unprecedented increase in cases, which surpassed the total of the previous decade and expanded to new regions. The Infodengue-Mosqlimate consortium launched the Brazilian Dengue 2024 Challenge (IMDC24), or Dengue Forecast Sprint, bringing together six international teams provided with cases and climate covariates data to generate actionable forecasts for 2024 and 2025 seasons in five diverse Brazilian states, leveraging advanced machine learning and classical statistical models. This paper outlines the structure and findings of the IMDC24. Model performance varied between years and locations, and no single model consistently excelled, especially during 2024s atypical, climate-change-driven con- ditions. This performance variability highlighted the need for ensemble approaches. The ensemble models developed are presented as the main results of this collaborative development. As intended, the ensemble models have been adopted by Brazilian public health authorities to help with planning and response to the forecasted 2025 dengue epidemics across the country. Significance StatementThe Dengue Challenge 2024 (IMDC24) was organized by the Mosqlimate-Infodengue consortium, which aims to provide forecasting models as decision support tools for early warning systems, scenario assess- ments, and empirical basis for resource allocation for mosquito-borne diseases. During IMDC24, six international teams, provided with dengue case, sociodemographic, and climate data, developed scenario forecasting models for the 2024 and 2025 dengue seasons in Brazil. In this study, we evaluated the per- formance of each model and built an ensemble model, considering the variation in performance of each model, especially during the atypical climate conditions of 2024. Among the main applications of this work, we highlight the incorporation of the results into the Brazilian Ministry of Healths nationwide dengue epidemic response agenda.