Infectious Disease
Early-season influenza forecasting for the 2025-2026 influenza season and importance of evaluating accuracy Sarabeth M. Mathis* Sarabeth Mathis Mathis Mathis Mathis Centers for Disease Control and Prevention
Influenza-related hospitalizations are a major contributor to disease burden in the United States with CDC estimating 100,000 to 710,000 hospitalizations annually. Accurate predictions of influenza hospitalizations support public health planning and interventions.
CDC coordinated the FluSight forecasting challenge for the 2025-2026 influenza season beginning November 2025, and forecasting teams provided national and jurisdiction-specific probabilistic forecasts for weekly lab-confirmed influenza hospitalizations up to 3 weeks ahead of the current week. Each week, FluSight ensembled forecasts by calculating the unweighted median of each quantile among eligible forecasts and monitored the accuracy of the ensemble by evaluating coverage, a metric that summarizes how often the forecast prediction interval contains the observed value.
In the first four weeks of the FluSight challenge, the ensemble forecast included 34-36 models from 30 teams. National weekly influenza hospital admissions stayed below or around 5,000 admissions prior to December, and admissions then rapidly increased to exceed 30,000 by the end of December. Coverage values ranged from 92-96% for current week forecasts during the period of lower admissions but dropped substantially during the period of rapid influenza increases with weekly coverage values dropping to 60% for current week forecasts and 36% for 3-week ahead.
This decrease in coverage during periods of rapid increases has been observed previously. In 2024-2025, a high-severity season with multiple periods of increases in hospitalization, coverage values for the FluSight ensemble did not exceed 75% until early March 2025. Further efforts to identify the causes for the declines in forecast performance during these periods are needed to improve the utility of influenza forecasts. Continuous assessment of forecast performance is also critical to ensure performance remains in expected ranges and to inform transparent forecast communication.