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How effective have 2025–2026 flu vaccines been in preventing severe illness and hospitalizations across age groups?
Executive summary
Early evidence shows 2025–26 flu vaccines likely cut medically attended illness and hospitalizations roughly in half in settings where the same viruses circulated as in the Southern Hemisphere (about 50% VE against outpatient visits and 49.7% against hospitalizations) [1]. U.S. and other 2024–25 season networks found more variable age-specific protection—higher in children and lower in older adults—so final 2025–26 effectiveness by age will depend on circulating strains, vaccine match, and uptake [2] [3].
1. What the headline numbers say — “about 50%” protection against severe outcomes
An interim, multi-country analysis of the 2025 Southern Hemisphere season—whose vaccine composition matches the 2025–26 Northern Hemisphere formulation—estimated adjusted effectiveness of 50.4% against outpatient influenza-like illness and 49.7% against hospitalizations for severe acute respiratory infection (SARI), leading U.S. public-health authors to conclude vaccines reduced outpatient visits and hospitalizations by roughly one half in that setting [1] [4]. Media summaries and commentary echoed this midline result and used it as a reason to recommend vaccination before the Northern Hemisphere season [4].
2. Age matters: children generally show stronger protection; older adults less so
U.S. VE networks in 2024–25 reported wide variation by age: estimates for children (<18 years) ranged from modest outpatient protection (32%) to strong protection (59–60%) across different networks and showed 63% and 78% VE against hospitalization in two networks [2]. By contrast, broader 2023–24 analyses found VE substantially lower in older adults—VE against any flu as low as 26% in adults ≥65 years in one study—illustrating a recurring pattern of reduced effectiveness with age [3]. Available sources do not offer final, disaggregated 2025–26 age-breakdowns for the Northern Hemisphere yet; policymakers are therefore using past age patterns as a guide [1] [2].
3. Why effectiveness can swing year to year — strain match, surveillance, and production
VE depends on which strains circulate and how well the vaccine strains match them; the FDA and WHO select strains months ahead, and disruptions to global surveillance can reduce the accuracy of those choices [5] [6]. The 2024–25 season illustrated this volatility: different A subtypes (H1N1 vs H3N2) and B lineages yielded varying VE, and egg-adapted production issues can worsen mismatch [7] [3]. The Southern Hemisphere interim result is helpful because the 2025 vaccine composition was the same there and in the Northern Hemisphere, but success in the North depends on whether the same viruses arrive [1].
4. Conflicting and cautionary findings — one study found little or negative effectiveness in a specific population
Not all analyses agree. A Cleveland Clinic cohort study of working‑aged employees reported a higher risk of influenza among vaccinated persons in 2024–25, yielding a negative VE estimate; that preprint concluded the vaccine “was not effective” in that cohort [8] [9]. Public-health agencies and professional groups framed that result cautiously—advising continued vaccination—pointing to limitations of single‑system studies and to larger multi-network analyses that found moderate protection [9] [2]. These divergent findings highlight heterogeneity by population, exposure, and study design.
5. Newer vaccines and policy shifts that could change age-specific impacts
ACIP and national agencies are steering older adults toward higher-immunogenicity options (high-dose, adjuvanted, or recombinant vaccines), which aim to improve protection in the 65+ group [10]. Experimental platforms (mRNA vaccines) have shown promise in trials among adults 18–64 with higher relative efficacy than conventional shots, but most real-world 2025–26 rollout still relies on established platforms and targeted high‑dose/adjuvanted products for older adults [11] [12].
6. Practical takeaway — vaccines reduce severe outcomes but not uniformly
The most robust, multi-country interim data indicate roughly 50% reduction in outpatient visits and hospitalizations where the 2025 vaccine matched circulating viruses [1]. Historical and U.S. network data show stronger protection in children and diminished protection in older adults; single-site negative findings exist but are at odds with larger surveillance analyses, not yet decisive enough to change public recommendations [2] [3] [8]. Public-health authorities therefore continue to recommend vaccination for everyone ≥6 months and prefer enhanced vaccines for older adults while urging antivirals and other measures to reduce severe illness [13] [1].
Limitations and open questions: the current Southern Hemisphere interim estimate may not predict Northern Hemisphere outcomes if different strains circulate; age-specific 2025–26 VE for the U.S. is not yet published in full in available sources; and some single‑system studies report contradictory results that require peer review and reconciliation with larger, multi-site surveillance [1] [2] [8].