Updating vaccine strain composition
Vaccine strain composition is updated to reflect changes in circulating viruses, using surveillance to guide manufacturers’ choices.
Surveillance and strain selection
Influenza viruses continually change, so vaccine strains need periodic replacement to maintain effectiveness. The annual influenza vaccine combines protection against several types, with year-round monitoring of high-risk areas informing which prevalent strains to include.
EMA issues annual EU composition recommendations based on WHO observations informed by worldwide monitoring of influenza prevalence and viral characteristics. Its human medicines committee endorsed the 2024/2025 recommendations in March 2024.
In May 2020, a commentator expected a similar monitoring-and-update approach ultimately to be needed for coronavirus vaccines, with candidates generally tested against the strains and types then described.
Removal of the B/Yamagata component
In March 2024, most authorised influenza vaccines were quadrivalent, targeting A(H1N1)pdm09, A(H3N2), B/Victoria and B/Yamagata. B/Yamagata had not been detected in circulation since March 2020, thought partly to reflect public-health measures against COVID-19. Because it no longer appeared to pose a public-health threat, EMA’s Emergency Task Force recommended transition to trivalent vaccines without it.
The task force recommended ideally removing B/Yamagata from all live-attenuated vaccines for 2024/2025 and completing the transition for other influenza vaccines by 2025/2026 to preserve supplies for the intervening campaign.
Composition for the 2024/2025 season
EMA’s ad hoc Influenza Working Group recommended the following strains for live-attenuated or egg-based trivalent vaccines.
- A/Victoria/4897/2022 (H1N1)pdm09-like virus.
- A/Thailand/8/2022 (H3N2)-like virus.
- B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
For cell-based trivalent vaccines, it recommended the following strains.
- A/Wisconsin/67/2022 (H1N1)pdm09-like virus.
- A/Massachusetts/18/2022 (H3N2)-like virus.
- B/Austria/1359417/2021 (B/Victoria lineage)-like virus.
Manufacturers could consider quadrivalent inactivated vaccines for that season, adding a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus to the relevant strains above. EMA recommended that marketing authorisation holders submit applications to change centrally authorised seasonal influenza-vaccine composition by 17 June 2024.