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 U.S. 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 both influenza A(H3N2) and A(H1N1) viruses circulate or following the emergence of novel A(H1N1)pdm09-like viruses.