What is the CDC's estimated effectiveness of the 2025-2026 flu vaccine against H3N2 so far?
Executive summary
CDC-linked analyses of the 2025 Southern Hemisphere season and early Northern Hemisphere data put vaccine effectiveness (VE) against A(H3N2) in a mixed range: the multicountry Southern Hemisphere MMWR analysis estimated VE against H3N2 hospitalization at about 37.2% (adjusted) [1]. Other early national reports and summaries show VE against H3N2 varying by age and setting—roughly 30–40% in adults but substantially higher (70–75%) in children in some UK early-season estimates [2] [3].
1. What the CDC-linked MMWR found: a modest VE against H3N2 hospitalization
A CDC-collaborative MMWR analysis of eight Southern Hemisphere countries for the 2025 season reported adjusted VE against influenza A(H3N2)–associated hospitalization of 37.2% (among severe acute respiratory infection patients), with overall VE against any influenza hospitalization near 49.7% [1]. That MMWR is the clearest CDC-associated numeric estimate directly addressing 2025-season H3N2 effectiveness in the provided sources [1].
2. Multiple, setting-dependent estimates — outpatient versus hospital, adults versus children
CDC and affiliated networks publish VE that varies by surveillance network and clinical setting. For the 2024–25 season, CDC interim network reports showed outpatient VE for H3N2 ranging from non‑significant to around 42% and hospital VE as high as 55% in some networks [4] [5]. Separate early Northern Hemisphere and UK real-world analyses reported VE against hospital attendance/admission of 70–75% in children and roughly 30–40% in adults for the 2025/26 early season [3] [2].
3. Why the numbers differ: methods, populations and timing
VE estimates depend on study design (test-negative vs. case-control), outcome measured (any medically attended illness vs. hospitalization), age mix, vaccination coverage and timing, and circulating virus subclades. The eight-country MMWR used SARI and ILI datasets and adjusted estimates for confounders, producing the 37.2% H3N2 hospitalization VE figure [1]. U.S. network estimates and UK early-season reports used different networks, age groups and endpoints, producing higher or lower point estimates [4] [3].
4. The subclade story: antigenic drift and its possible impact on VE
Laboratory and genomic surveillance shows a rapidly expanding H3N2 "subclade K" (J.2.4.1) that has reduced reactivity to antisera raised against the vaccine reference strains, raising concern about reduced vaccine match [6] [7]. UK and WHO reports note antigenic differences between subclade K and the vaccine’s chosen J.2 reference strains; however, early real‑world data still shows meaningful protection, especially in children [6] [3].
5. What CDC recommends and the practical bottom line
CDC guidance for 2025–26 keeps vaccination as the primary prevention tool for eligible persons and notes vaccine composition choices for H3N2 in egg‑ and cell/recombinant formulations [8] [9]. The available reporting supports that vaccines reduce severe outcomes overall even when VE against a particular H3N2 subclade is reduced: the Southern Hemisphere analysis concluded vaccines reduced outpatient visits and hospitalizations by about half across settings [1].
6. Limitations and uncertainties in current reporting
Available sources do not provide a single definitive CDC “so far” point estimate for 2025–26 H3N2 VE in the U.S. population for the Northern Hemisphere season; the clearest CDC‑linked numeric estimate in these materials is the 37.2% adjusted VE against H3N2 hospitalization from the Southern Hemisphere multicountry MMWR [1]. Early Northern Hemisphere reports and UK analyses show differing magnitudes by age and setting, and antigenic drift (subclade K) introduces uncertainty about how VE will hold up as the season progresses [6] [3] [2].
7. How to read these numbers: policy and personal decisions
Public‑health value accrues even from partial VE: analyses show vaccines can cut medically attended and hospitalized influenza roughly in half across settings, even when VE against a specific H3N2 subclade is modest [1]. Clinicians and public health officials weigh that residual protection, the age‑specific VE patterns, and antiviral options when advising patients [1] [7].
Sources cited: CDC MMWR multicountry VE report and CDC network summaries [1] [4] [5], UK/WHO and journal analyses on subclade K and early-season VE [6] [3] [2], and CDC program pages on 2025–26 composition and recommendations [8] [9].