Current Influenza Vaccine Strains (2024-2025 Season)
All influenza vaccines available in the United States for the 2024-2025 season are trivalent formulations containing three viral strains: influenza A(H1N1), influenza A(H3N2), and influenza B/Victoria lineage. 1
Specific Strain Composition
The current seasonal influenza vaccines contain hemagglutinin derived from the following three strains 1:
- Influenza A(H1N1) component - unchanged from the previous season 1
- Influenza A(H3N2) component - updated and different from the previous season 1
- Influenza B/Victoria lineage component - unchanged from the previous season 1
Important Change from Previous Years
The shift from quadrivalent to trivalent vaccines represents a significant change from recent influenza seasons. Previously, quadrivalent vaccines contained four strains: two influenza A strains (H1N1 and H3N2) and two influenza B strains (one from Victoria lineage and one from Yamagata lineage) 2.
The rationale for quadrivalent formulations was that the two influenza B lineages are antigenically distinct, with immunization against one lineage providing only limited cross-protection against the other 2. However, the return to trivalent formulations does not represent reduced protection but rather reflects the global disappearance of the B/Yamagata lineage 1.
Manufacturing Platform 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 variation exists because optimal viral characteristics differ by manufacturing platform 1.
Clinical Recommendation
The American Academy of Pediatrics and Advisory Committee on Immunization Practices make no preferential recommendation for any specific vaccine product when multiple age-appropriate options are available 1. Vaccination should not be delayed to obtain a specific product or formulation—timely vaccination with any available age-appropriate vaccine is the priority 1.