Game Over for the Baseline: Influenza Hospitalization Patterns Before, During, and After the COVID-19 Pandemic...

Area of Science:

• Epidemiology
• Public Health
• Infectious Disease Dynamics

Background:

• Influenza hospitalization burden pre- and post-COVID-19 pandemic is not well-defined nationally.
• Characterizing phase-stratified burden and seasonal structure is crucial.
• Assessing racial/ethnic disparities and anomalous departures from pre-pandemic expectations is needed.

Purpose of the Study:

• To characterize phase-stratified influenza hospitalization burden and seasonal structure.
• To quantify racial and ethnic disparities in influenza hospitalizations.
• To determine if post-pandemic seasons deviate from pre-pandemic expectations.

Main Methods:

• Analysis of 16 influenza seasons (2009-2024) using FluSurv-NET data.
• Application of OLS regression, bootstrapped rate ratios, STL decomposition, Prophet forecasting, and Isolation Forest anomaly detection.
• Stratification across pre-pandemic, disruption, and recovery phases.

Main Results:

• Mean peak hospitalization rate nearly doubled in recovery (5.1 to 11.1 per 100,000).
• Cumulative seasonal burden increased significantly, with earlier peak timing (MMWR week 9 to 50).
• Stronger seasonality observed in recovery, with notable racial/ethnic disparities (RR 1.72-2.24) and identified anomalous seasons.

Conclusions:

• Post-pandemic influenza recovery features intensified and restructured seasonality.
• Persistent racial and ethnic disparities in influenza hospitalization burden continue.
• Anomalous burden exceeding pre-pandemic projections was identified using forecasting and anomaly detection.