Influenza Vaccine Strain Changes from 2023-2025
No, the influenza vaccine strains have not remained the same from 2023 to 2025, with notable changes occurring in the 2024-2025 season, including the removal of the B/Yamagata lineage component and updates to the influenza A (H3N2) component. 1
Vaccine Composition Changes
2024-2025 Season Updates
The 2024-2025 influenza vaccines in the United States have undergone significant changes:
- All U.S. vaccines are now trivalent (containing three virus strains), whereas previous seasons used quadrivalent vaccines 1
- B/Yamagata lineage removal: This component has been removed as this virus has not circulated since 2020 1
- H3N2 component updated: The influenza A (H3N2) component is new for the 2024-2025 season 1
- Unchanged components: The influenza A (H1N1)pdm09 and influenza B Victoria lineage components remain the same as the previous season 1
Specific Strains for 2024-2025
For egg-based vaccines:
- H1N1: A/Victoria/4897/2022 (H1N1)pdm09-like virus (unchanged) 1, 2
- H3N2: A/Thailand/8/2022 (H3N2)-like virus (new) 1, 2
- B Victoria: B/Austria/1359417/2021-like virus (unchanged) 1, 2
For cell culture-based or recombinant vaccines:
- H1N1: A/Wisconsin/67/2022 (H1N1)pdm09-like virus 1, 2
- H3N2: A/Massachusetts/18/2022 (H3N2)-like virus 1, 2
- B Victoria: B/Austria/1359417/2021-like virus 1, 2
Previous Season (2022-2023)
For comparison, the 2022-2023 season vaccines were quadrivalent and contained:
- H1N1: A/Victoria/2570/2019 (H1N1)pdm09-like virus (egg-based) or A/Wisconsin/588/2019 (H1N1)pdm09-like virus (cell-based) 1
- H3N2: A/Darwin/9/2021 (H3N2)-like virus (egg-based) or A/Darwin/6/2021 (H3N2)-like virus (cell-based) 1
- B Victoria: B/Austria/1359417/2021-like virus 1
- B Yamagata: B/Phuket/3073/2013-like virus 1
Clinical Implications
Importance of Annual Updates
The influenza virus undergoes continuous antigenic changes, necessitating regular updates to vaccine strains to match circulating viruses. This evolutionary pattern is why vaccine composition is reviewed annually 3.
Effectiveness of Current Formulation
Preliminary estimates for the 2024-2025 season indicate that the current vaccine formulation has been effective:
- 32-60% effective against outpatient influenza visits in children and adolescents
- 36-54% effective against outpatient influenza visits in adults
- 41-78% effective against influenza-associated hospitalizations 4
Key Considerations for Healthcare Providers
- Vaccination timing: The CDC and AAP recommend administering influenza vaccines as soon as they become available, ideally by the end of October 1
- No preference between products: For most individuals, any age-appropriate influenza vaccine may be used 1
- Two-dose schedule: Children 6 months through 8 years receiving influenza vaccine for the first time or who received only one dose before July 1,2024, should receive two doses at least 4 weeks apart 1
Common Pitfalls to Avoid
- Delaying vaccination to obtain a specific product when an appropriate one is available
- Assuming vaccine strains remain constant between seasons
- Overlooking the change from quadrivalent to trivalent vaccines in the U.S. for the 2024-2025 season
- Failing to recognize that egg-based and cell culture-based vaccines contain slightly different but antigenically related strains
The ongoing evolution of influenza viruses necessitates regular updates to vaccine composition to ensure optimal protection against circulating strains. Healthcare providers should stay informed about these changes to provide accurate information to patients.