What are the CDC and WHO mid-season estimates of 2025–2026 flu vaccine effectiveness against hospitalization?
Executive summary
CDC and partner analysis of eight Southern Hemisphere countries found 2025 vaccines reduced influenza-associated outpatient visits by 50.4% and hospitalizations by 49.7% during March–September 2025, and CDC says the same vaccine composition is being used for the 2025–26 Northern Hemisphere season and “might be similarly effective” if the same viruses circulate [1]. Available sources do not provide a definitive mid‑season CDC or WHO VE estimate specifically for 2025–2026 hospitalizations in the Northern Hemisphere; reporting instead points to Southern Hemisphere interim data and prior U.S. interim estimates from 2024–25 for context [1] [2].
1. What the CDC’s interim Southern Hemisphere numbers say — near 50% protection
CDC-led surveillance pooling data from eight Southern Hemisphere countries reported adjusted vaccine effectiveness (VE) of 50.4% against outpatient visits and 49.7% against hospitalizations for the 2025 Southern Hemisphere season (March–September 2025) and explicitly frames that as “reducing influenza‑associated outpatient visits and hospitalization by approximately one half” [1]. Media reporting and public‑health commentary repeated those point estimates and associated confidence intervals in coverage of what Northern Hemisphere authorities might expect [3] [4].
2. Why those Southern Hemisphere estimates matter to the 2025–26 season — but are not the same as a Northern Hemisphere mid‑season estimate
CDC authors note the 2025–26 Northern Hemisphere vaccine composition matches the 2025 Southern Hemisphere composition and “might be similarly effective if the same viruses circulate,” which is an inference, not a Northern Hemisphere mid‑season VE estimate [1]. In other words, Southern Hemisphere VE is the best available interim signal in these sources, but it is not a direct measure of 2025–26 Northern Hemisphere hospitalization VE [1].
3. What the WHO says — not found in current reporting provided
Available sources do not mention a World Health Organization mid‑season VE estimate for 2025–2026 hospitalizations. The provided material references WHO’s role in strain selection and candidate vaccine viruses but contains no WHO VE point estimate for 2025–26 hospitalizations [5] [6]. Therefore, a WHO mid‑season hospitalization VE number is not present in the current reporting.
4. U.S. CDC prior practice and other interim estimates — context from 2024–25 and surveillance networks
CDC routinely publishes interim VE estimates from its U.S. networks; for 2024–25 those interim reports indicated vaccines reduced medically attended influenza and hospitalizations, and CDC maintains pages with preliminary VE tables and past-season VE charts [2] [7] [8]. Media outlets cited CDC preliminary ranges for the preceding season (e.g., 41–78% effectiveness against hospitalization last season, as reported by The New York Times), demonstrating that U.S. mid‑season ranges can vary substantially by season and by subtype [9].
5. Subgroup and strain details that color VE — higher for some groups and B lineage
Analyses highlighted variation by age and virus type: in the Southern Hemisphere report, adjusted VE against influenza B was notably higher (77.6%), VE among priority groups for hospitalization was lower (~45.7%), and effectiveness tended to be lower in older adults compared with children and people with underlying conditions [4]. Such heterogeneity explains why a single headline VE number can mask important differences across populations [4].
6. Limitations, alternative viewpoints, and potential agendas
Limitations: the Southern Hemisphere estimates are interim, from surveillance networks pooling different data systems and countries, and apply to March–September 2025; they are not a formal Northern Hemisphere mid‑season VE estimate [1]. Alternative viewpoints: some reporting warned that vaccine-match uncertainty (especially with shifts in circulating strains such as H3N2) can reduce VE and that regional data (e.g., Canada, earlier seasons) sometimes show different VE—examples noted in media coverage of previous seasons [10]. Possible agendas: public‑health agencies emphasize vaccination to prevent severe outcomes and may highlight favorable VE estimates to support uptake; media organizations frame VE ranges to advise readers on personal risk and vaccine choice [1] [9].
7. Bottom line for readers seeking a 2025–26 hospitalization VE number
If you want a specific mid‑season 2025–26 hospitalization VE for the Northern Hemisphere, available reporting does not provide one; instead the best-cited interim signal in these documents is that 2025 Southern Hemisphere vaccination reduced hospitalizations by roughly half (49.7%), and CDC states the Northern Hemisphere vaccine composition is the same and “might” perform similarly if viruses match [1]. Watch CDC and WHO channels for formal Northern Hemisphere mid‑season VE releases and for subgroup and subtype breakdowns that will determine how that headline number applies to different ages and risk groups [1] [8].