What strains of influenza are included in the current influenza vaccine?

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Last updated: December 10, 2025 • View editorial policy

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Current Influenza Vaccine Strains

All 2024-2025 seasonal influenza vaccines in the United States are trivalent formulations containing three viral strains: one influenza A(H1N1), one influenza A(H3N2), and one influenza B/Victoria lineage virus. 1

2024-2025 Season Composition

The current influenza vaccines contain hemagglutinin derived from the following strains:

  • Influenza A(H1N1) component: Unchanged from previous season 1
  • Influenza A(H3N2) component: Different this season compared to last season 1
  • Influenza B/Victoria lineage component: Unchanged from previous season 1

Important Change: Removal of B/Yamagata Lineage

Influenza B/Yamagata lineage virus has been removed from all seasonal influenza vaccines for 2024-2025. 1 This represents a shift from the quadrivalent vaccines used in recent years back to trivalent formulations, as B/Yamagata has not been detected globally since 2020. 1

Historical Context of Vaccine Formulations

Evolution from Trivalent to Quadrivalent

Prior to 2024-2025, quadrivalent vaccines were the standard and contained four strains:

  • Two influenza A strains (H1N1 and H3N2) 1
  • Two influenza B strains (one from Victoria lineage and one from Yamagata lineage) 1

The rationale for quadrivalent vaccines was that the two influenza B lineages (Victoria and Yamagata) are antigenically distinct, and immunization against one lineage provides only limited cross-protection against the other. 1 This created challenges in predicting which B lineage would predominate each season, leading to potential mismatches when trivalent vaccines contained only one B strain. 1

Specific Historical Examples

For the 2021-2022 season, quadrivalent vaccines contained:

  • A/Victoria/2570/2019 (H1N1)pdm09-like virus (egg-based) or A/Wisconsin/588/2019 (H1N1)pdm09-like virus (cell/recombinant-based) 1
  • A/Cambodia/e0826360/2020 (H3N2)-like virus 1
  • B/Washington/02/2019-like virus (Victoria lineage) 1
  • B/Phuket/3073/2013-like virus (Yamagata lineage) 1

For the 2017-2018 season, trivalent vaccines contained:

  • A/Michigan/45/2015 (H1N1)pdm09-like virus 1
  • A/Hong Kong/4801/2014 (H3N2)-like virus 1
  • B/Brisbane/60/2008-like virus (Victoria lineage) 1

Platform-Specific Variations

Different but antigenically related influenza A strains may be included in egg-based versus cell-based or recombinant vaccines within the same season, though all are matched to expected circulating strains. 1 This occurs because optimal viral characteristics differ by manufacturing platform. 1

Clinical Implications

The shift back to trivalent formulations does not represent reduced protection, but rather reflects the global disappearance of the B/Yamagata lineage. 1 The Advisory Committee on Immunization Practices and American Academy of Pediatrics make no preferential recommendation for any specific vaccine product when multiple age-appropriate options are available. 1, 2

Vaccination should not be delayed to obtain a specific product or formulation—timely vaccination with any available age-appropriate vaccine is the priority. 1, 2

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

2
Guideline

Influenza Vaccination Guidelines

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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