Predominant Influenza Viruses: 2024–25 vs 2025–26
• 2024–25: Predominantly influenza A, with roughly equal A(H1N1)pdm09 and A(H3N2) circulation; B/Victoria was minority. 2025–26: Early A(H3N2) predominance with emerging antigenic variants driving activity. 1, 2, 3
2024–25 season (U.S., pediatric-focused guideline framing)
89% influenza A; subtyped isolates split ~53% H1N1pdm09 and ~47% H3N2; B/Victoria much less common 1.
- Research corroboration: outpatient surveillance showed A(H3N2) and A(H1N1)pdm09 predominated; B/Victoria limited 4. Global/WHO lab data in 2024 also A(H3N2) and A(H1N1)pdm09 >> B/Victoria 5.
- Clinical relevance (morbidity/mortality/QOL): This A-dominant, high-severity season aligned with higher pediatric hospitalizations and deaths; vaccine match was generally good for H1N1 and B, mixed for H3N2, informing prevention impact 1.
2025–26 season (early signals)
- ACIP/AAP 2025–26 guidance updates vaccine composition toward H3N2 drift strains, reflecting expected A(H3N2) pressure 6, 2.
- England modeling and surveillance: unusually early, rapid rise of A/H3N2 (subclade K) with antigenic change, indicating H3N2 predominance early in 2025–26 and potential immune escape concerns 3.
- Nuance/caveat: While U.S. pediatric guidelines pre‑season do not yet quantify subtype shares for 2025–26, the vaccine composition change and international surveillance strongly indicate early H3N2 dominance; continue to monitor CDC FluView for U.S.-specific proportions as the season progresses 6, 2, 3.
Algorithmic takeaway for practice (to reduce morbidity, mortality, QOL loss):
- Expect mixed A(H1N1)pdm09 and A(H3N2) in 2024–25 (A-dominant overall); prioritize vaccination and early antivirals, with awareness of H3N2 severity in children 1.
- For 2025–26, plan for A(H3N2)-leading circulation early; ensure use of updated trivalent vaccines and prompt treatment in high-risk pediatric patients; watch for local H3N2 variant activity that could affect severity and vaccine protection 6, 2, 3.