Alicia Fry

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.

Background: Adult studies have shown that quadrivalent recombinant influenza vaccines (RIV4) induce a higher antibody response than quadrivalent egg-based inactivated influenza vaccines (IIV4). Methods: Healthcare personnel (HCP) from a 2018–2019 cohort study that assessed IIV4 immunogenicity in Israel were recruited into a 2019–2020 randomized trial of RIV4 and IIV4. 2018–2019 low titer low responders (LTLRs) had pre-vaccination hemagglutination inhibition antibody titers ≤1:40 and <4-foldrise post-vaccination. We assessed whether RIV4 versus IIV4 receipt in the 2019–2020 season improved response among 2018–2019 LTLRs using logistic regression adjusted for employment hospital. Results: Of 228 HCP classified as 2018–2019 LTLRs, 2019–2020 RIV4 recipients were 3.6 (95% CI: 1.2–11.4) and 8.3 (95% CI: 3.0–26.3) times as likely to become a non-LTLR against influenza A(H1N1)pdm09 and cell-derived A(H3N2) vaccine components compared to IIV4 recipients. Conclusions: RIV4 had improved immunogenicity against influenza A viruses among previously classified low vaccine responder HCP, highlighting how recombinant influenza vaccines could improve antibody responses against influenza A for HCP at risk for poor influenza antibody response.