The World Health Organization (WHO) has recently announced the updated influenza vaccine strains for the 2025-2026 northern hemisphere flu season. This update, which was released on February 28, 2025, is part of WHO’s ongoing effort to maintain the efficacy of seasonal influenza vaccines. Each year, the global health body evaluates the evolving epidemiological, virological, and serological data to provide recommendations for the coming flu season. These recommendations reflect the dynamic nature of influenza viruses, particularly their tendency to mutate and evolve.
Key Changes in the 2025-2026 Vaccine Strain Recommendations
As influenza viruses continue to evolve, the WHO regularly adjusts the vaccine components to ensure that the public is protected from the most recent circulating strains. The major adjustments to the 2025-2026 vaccine strains are primarily focused on the H3N2 strain, with other strains remaining largely unchanged. The recommended strains for different vaccine types are as follows:
- Trivalent Egg-based Vaccines
- A/Victoria/4897/2022 (H1N1) pdm09-like virus
- A/Croatia/10136RV/2023 (H3N2)-like virus
- B/Austria/1359417/2021 (B/Victoria lineage)-like virus
- Trivalent Cell Culture or Recombinant Protein-based Vaccines
- A/Wisconsin/67/2022 (H1N1) pdm09-like virus
- A/District of Columbia/27/2023 (H3N2)-like virus
- B/Austria/1359417/2021 (B/Victoria lineage)-like virus
- Quadrivalent Egg- or Cell Culture/Recombinant-based Vaccines (unchanged from 2024)
- B/Phuket/3073/2013 (B/Yamagata lineage)-like virus
Substitution of H3N2 Strains in the Vaccines
One of the most significant updates for the 2025-2026 season is the substitution of the H3N2 strain in the trivalent and recombinant vaccines. The previous strain, A/Thailand/8/2022 (H3N2), will be replaced by A/Croatia/10136RV/2023 (H3N2) in the egg-based vaccines. Similarly, the A/Massachusetts/18/2022 (H3N2) strain will be replaced by A/District of Columbia/27/2023 (H3N2) in the cell culture or recombinant protein-based vaccines.
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The H3N2 strain is particularly notable for its rapid mutation rate. These substitutions were made to ensure that the vaccines reflect the most current and widely circulating strains. By updating these strains, WHO ensures that the vaccines remain effective against the evolving virus, helping to prevent widespread influenza outbreaks.
Unchanged Strains in the 2025-2026 Vaccine
While the H3N2 strain has been updated, other components of the influenza vaccine remain unchanged. Specifically, the H1N1 strains and the B/Victoria lineage strains have not seen any alterations. These strains are considered to be stable and continue to circulate at significant levels in the global population, thus remaining effective in their current form.
For the H1N1 component, the A/Victoria/4897/2022 (egg-based) and A/Wisconsin/67/2022 (cell culture/recombinant) strains will continue to be used. For the B/Victoria lineage, the B/Austria/1359417/2021 strain will also remain the same.
However, the B/Yamagata lineage component, which is included in the quadrivalent vaccines, remains unchanged as well. The B/Phuket/3073/2013 strain continues to be the recommended strain for the Yamagata lineage in these vaccines.
Comparing the 2024-2025 and 2025-2026 Vaccine Strains
In comparison to the 2024-2025 strains, the most notable update is the substitution of the H3N2 strains. Below is a comparison of the two flu seasons’ vaccine components:
| Vaccine Type | 2024-2025 Strains | 2025-2026 Updated Strains | Key Changes |
|---|---|---|---|
| Trivalent Egg-based | A/Thailand/8/2022 (H3N2) | A/Croatia/10136RV/2023 (H3N2) | H3N2 strain substituted |
| Trivalent Cell/Recombinant | A/Massachusetts/18/2022 (H3N2) | A/District of Columbia/27/2023 (H3N2) | H3N2 strain substituted |
| Quadrivalent | Tri-valent + B/Phuket/3073/2013 | Tri-valent + B/Phuket/3073/2013 | B/Yamagata lineage retained |
As evident from the table, while the H3N2 strain has been updated, there are no changes to the H1N1 or B/Victoria lineage components. These strains remain stable and effective, allowing for continued protection against these variants of the flu virus.
The Importance of Updated Strains
The flu virus is known for its ability to mutate rapidly, making it necessary for the WHO to update the recommended vaccine strains every year. The H3N2 virus, in particular, has a high mutation rate, which is why adjustments are made annually to ensure the vaccines target the most relevant strains.
The ongoing changes in vaccine composition reflect the broader issue of viral evolution. As influenza viruses evolve, vaccines must adapt to provide continued protection. This year’s updates are part of a longer-term effort to maintain flu vaccine efficacy across the globe.
Recombinant Antigens: Critical Materials for Vaccine Development
For researchers working on influenza vaccines, recombinant antigens play a vital role in accelerating the development of effective vaccines. Recombinant hemagglutinin (HA), neuraminidase (NA), and nucleoprotein (NP) proteins are essential in the creation of both vaccines and antiviral drugs. These proteins are key to understanding the influenza virus’s structure and function, and they aid in the development of diagnostic kits, antiviral therapies, and more.
Sino Biological, a leader in biological reagents and services, has unveiled a new portfolio of influenza research reagents to support the development of vaccines for the 2025-2026 northern hemisphere flu season. This new offering includes high-quality recombinant HA, NA, and NP proteins designed to assist researchers in accelerating their studies and contribute to global influenza prevention efforts.
Looking Ahead: The Future of Influenza Vaccines
The continuous development of vaccines is essential in the fight against influenza. With the ongoing mutation of influenza viruses, it is crucial to adapt vaccine formulations to keep up with viral changes. The 2025-2026 northern hemisphere flu season marks another step in this effort, with updated strains reflecting the most recent data on circulating viruses.
The future of influenza vaccines will likely see further improvements, driven by advances in vaccine technology, faster testing methods, and better global collaboration. As the flu virus continues to evolve, so too will our strategies for combatting it.
In conclusion, the 2025-2026 influenza vaccine updates are critical for maintaining the effectiveness of vaccines worldwide. The substitution of H3N2 strains and the continuation of other strains reflect the dynamic nature of influenza viruses and the necessity for timely updates to ensure public health is safeguarded during each flu season.
