Early Warning Systems and Emergency Declarations During Brazil's Largest Dengue Outbreak

OBJECTIVES: Early warning systems (EWSs) are essential for alert, detection, and response to potential public health emergencies. Brazil's 2024 dengue epidemic, the largest ever recorded, offers a critical case for assessing correspondence between timely epidemic risk detection and health governance responsiveness. METHODS: We collected data on dengue cases, deaths, and public health emergency declarations (PHEDs) at federal, state, and municipal levels. We evaluated the timeliness of 2024 emergency declarations by retrospectively mapping observed risk levels across all states and capitals, utilizing the 2023 federal endemic channel guidelines and the updated 2025 risk-level criteria. Using logistic regressions, we investigated whether epidemic severity was associated with the enactment of a PHED, accounting for primary health coverage and political dynamics that influence local government declarations. RESULTS: The 2024 epidemic escalated rapidly. Brazil remained at an alert level for 19 consecutive weeks and at an epidemic level for 11, with epidemic risk thresholds reached unusually early. Eleven states and seven capitals issued PHEDs after epidemic thresholds were reached, and an additional five capitals were covered by overlapping state declarations. Despite prolonged epidemic conditions across 18 states and 20 capitals, only 11 states and 7 capitals issued emergency declarations. High-burden jurisdictions such as Bahia and Belém did not declare emergencies. Logistic regression models show that only the duration at elevated risk levels significantly predicted emergency declarations, while political factors and primary health care coverage did not. That declarations followed prolonged epidemic exposure rather than early detection underscores the need for institutional mechanisms that translate surveillance signals into timely governance responses. CONCLUSIONS: The EWS framework was insufficiently responsive to the rapid dynamics of the 2024 dengue epidemic. Strengthening responsive governance requires adaptive surveillance tools and institutional mechanisms that link early detection to timely, coordinated responses, while accounting for decentralized decision-making and uneven subnational capacities.