Current Influenza Vaccine Strains

Vaccine Composition

The American Academy of Pediatrics notes that 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
The current influenza vaccines contain hemagglutinin derived from the following strains: Influenza A(H1N1) component, which is unchanged from the previous season, Influenza A(H3N2) component, which is different from the previous season, and Influenza B/Victoria lineage component, which is unchanged from the previous season 1

The Advisory Committee on Immunization Practices and American Academy of Pediatrics previously recommended quadrivalent vaccines, which 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, 3, 4
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 2

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, as optimal viral characteristics differ by manufacturing platform 1, 4

The American Academy of Pediatrics states that 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, and vaccination should not be delayed to obtain a specific product or formulation—timely vaccination with any available age-appropriate vaccine is the priority 4

The degree of antigenic similarity between vaccine and circulating strains varies by season, significantly impacting clinical outcomes, with well-matched seasons resulting in 70-90% effectiveness in healthy adults under 65 years, according to the Centers for Disease Control and Prevention (CDC) 5
Mismatched seasons, such as the 2014-2015 season, can result in poor match, with 81% of H3N2 viruses being drifted strains, leading to 145 pediatric deaths and hospitalization rates of 57.2 per 100,000 in children 0-4 years, as reported by the American Academy of Pediatrics 6
Partial match scenarios, like the 2019-2020 season, showed 82.5% match for A(H1N1)pdm09 and 59.7% match for B/Victoria, but only 46.5% match for A(H3N2), according to the American Academy of Pediatrics 7, 8

Even in mismatched seasons, vaccination provides substantial protection against severe outcomes, such as hospitalization and death, with 50-60% effectiveness in preventing hospitalization/pneumonia and 80% effectiveness in preventing death in nursing home residents, as recommended by the CDC 5, 9
Vaccination is 30-70% effective in preventing hospitalization for pneumonia and influenza in elderly persons outside nursing homes, according to the CDC 5, 9
The American Academy of Pediatrics recommends that vaccination remains essential regardless of match, as it is the single best preventive measure available, with most pediatric deaths (90% historically) occurring in unvaccinated children 6