Utility of Rt epidemic trend categories for influenza surveillance: alignment with observed emergency department trends across two influenza seasons in the United States

CDC’s Rt epidemic trend categories for U.S. influenza show moderate, threshold-dependent alignment with weekly emergency department trends, peaking at 61% aligned under the 5% relative-change threshold.

• Why it matters: Rt categories are meant to signal infection dynamics ahead of care-seeking, but it is unclear whether direction-based categories match the magnitude-based changes seen in ED surveillance.
• What they did: Weekly Rt epidemic trend categories were compared with National Syndromic Surveillance Program ED trends across the 2024/2025 and 2025/2026 seasons, using observed-trend thresholds of ≥0.1% difference, ≥0.5% difference, and ≥5% relative change between consecutive weeks, with Rt compared to the following week.
• The result: Categorical alignment was strongest for growing and declining categories at the ≥5% threshold (61% aligned) and weakest at the ≥0.5% threshold (38.2% aligned), with calibration similarly best at ≥5%; misalignment clustered near inflection points and low-activity periods.

AI summary of the abstract · may contain errors · not medical advice · read the paper.