How did vaccine effectiveness against influenza hospitalisation differ between children, adults, and older adults in 2024-25?
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Executive summary
Interim U.S. estimates for 2024–25 show influenza vaccine effectiveness (VE) against hospitalization was substantially higher in children and adolescents—63% to 78%—than the broad inpatient VE range reported across age groups of about 39%–62% (CDC) [1] [2]. International and regional analyses show more variation: South American data found ~35% VE against hospitalization in high‑risk groups in early 2024 [3] [4], while European and multicountry analyses reported hospital VE in the mid‑30s to mid‑50s range [5].
1. What the principal U.S. analyses found — children clearly better protected
CDC interim analyses from four U.S. vaccine‑effectiveness networks reported that vaccination reduced risk of influenza‑associated hospitalization by 63%–78% among children and adolescents in 2024–25, a notably strong effect compared with overall inpatient estimates reported elsewhere in CDC summaries [1] [2]. The CDC’s season summary framed inpatient VE across age groups as roughly 39%–62%, indicating children stood out as a group with higher observed protection from severe outcomes [2].
2. Adults and older adults — middling, variable protection
Available U.S. reporting gives a range for inpatient VE across adults that overlaps with lower and midrange protection (about 39%–62% depending on age group) rather than the higher pediatric figures [2]. Separate reports and meta‑analyses emphasize that VE against hospitalization has varied by subtype and age historically, and that older adults often have lower vaccine response unless they receive enhanced formulations [5] [6]. The NEJM and other reviews note high‑dose or adjuvanted vaccines can improve protection for older adults, but precise 2024–25 age‑stratified comparative numbers beyond CDC ranges are not provided in the dataset [6].
3. International comparisons — striking heterogeneity by region and strain
Data from five South American countries covering March–July 2024 estimated a 35% reduction in influenza‑associated hospitalization among high‑risk groups—well below the pediatric U.S. figures and demonstrating regional heterogeneity that depends on circulating strains and vaccine match [3] [4]. Eight European studies covering September 2024–January 2025 reported hospital VE of 33%–56% overall, showing overlap with U.S. adult ranges but not matching the elevated pediatric VE reported in the U.S. networks [5].
4. Why these differences likely appeared — viruses, immunity, and vaccine type
Authors and public health agencies attribute between‑group VE differences to multiple, documented drivers: which influenza subtypes predominated (A(H3N2) often gives lower VE), how well vaccine strains matched circulating viruses, preexisting immunity patterns across age groups, and use of higher‑dose/adjunct vaccines for older adults [1] [5] [6]. The Pharmacy Times noted that recognition of circulating A(H3N2) by the 2024–25 vaccine strain was partial (~54.9%), a biologic explanation for uneven effectiveness by age and region [7].
5. What the numbers mean for public health action
Despite variability, CDC and commentators underline that vaccines reduced hospitalizations at population scale: U.S. estimates attribute prevention of roughly 170,000–360,000 hospitalizations in 2024–25 to vaccination, underscoring sizable population benefit even where VE is moderate [2]. Southern Hemisphere and European signals that VE can be as low as the mid‑30s for hospitalized patients reinforce guidance to combine vaccination with prompt antiviral treatment for high‑risk patients [3] [4].
6. Limitations, caveats, and competing perspectives
Interim estimates are subject to change as more data accrue and differ by study design, population, adjustments and calendar time; CDC caveats accompany their networks’ midseason figures [1] [8]. European and Southern Hemisphere networks report lower VE in some settings, which could reflect different predominant clades, vaccine formulations (trivalent vs quadrivalent) and healthcare‑seeking/testing patterns [3] [5]. Some clinical reviewers emphasize the benefit of high‑dose vaccines for older adults, but the sources here do not provide final age‑specific comparative VE for all formulations in 2024–25 [6].
7. Bottom line for readers
Children and adolescents in U.S. interim surveillance in 2024–25 experienced substantially higher vaccine effectiveness against influenza‑associated hospitalization (63%–78%) than the broader inpatient VE ranges reported for adults and older adults (about 39%–62% across age groups); international data show hospital VE ranging from roughly 33% to 56% or lower (South America ~35%), reflecting strain mix and vaccine match [1] [2] [3] [5]. Readers should note these are interim, regionally variable estimates and that public health authorities continue to recommend vaccination plus early antiviral treatment for high‑risk patients [3] [2].