Effects of Age and Birth Cohort on Influenza A Virus Subtype-Specific Hospitalization Rates, United States, 2010–2025

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Abstract
Background: Older individuals (≥65 years) are at greatest risk of severe influenza requiring hospitalization. Since 2009, influenza-associated hospitalization rates during A(H1N1)pdm09-predominant influenza seasons have been lower than during A(H3N2)-predominant seasons. Methods: Using laboratory-confirmed influenza hospitalization rates from US population–based surveillance, we investigated the relative A(H1N1)pdm09 to A(H3N2) hospitalization rate ratios by patient year of age and birth cohort from 2010 through 2025. Results: Results suggest partial protection against A(H1N1)pdm09 hospitalizations relative to A(H3N2) hospitalizations among patients born before 1945 (pre-1945 birth cohorts) and among individuals born during 1994 through 2009. Impact of partial protection against A(H1N1)pdm09-associated hospitalizations has diminished over time among older adults, contributing to elevated influenza hospitalization rates during the 2024–2025 influenza season. Conclusions: Our findings suggest that influenza A hospitalization rates may increase during future influenza seasons when influenza A(H3N2) and A(H1N1) viruses circulate or following the emergence of novel A(H1N1)pdm09-like viruses.
Funding Information
  • CDC's Emerging Infections Program
  • Council of State and Territorial Epidemiologists

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