The flu vaccine for the 2024–2025 U.S. season covers three strains: an influenza A(H1N1), an influenza A(H3N2), and an influenza B/Victoria lineage virus. This is a change from previous years, when the vaccine included four strains. The B/Yamagata lineage was dropped starting in 2024–2025 because it hasn’t been detected in global surveillance since March 2020.
The Three Strains in This Season’s Vaccine
Every flu vaccine available in the United States for the 2024–2025 season is trivalent, meaning it targets three virus strains. The exact strains differ slightly depending on how the vaccine is manufactured, but all versions cover the same three flu subtypes.
For egg-based vaccines (the most common type), the strains are:
- 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 and recombinant vaccines, the H1N1 and H3N2 components use slightly different reference viruses: A/Wisconsin/67/2022 for H1N1 and A/Massachusetts/18/2022 for H3N2. The B/Victoria component is identical across all platforms. This distinction exists because flu viruses can mutate slightly when grown in eggs, so cell-based and recombinant vaccines use reference strains that more closely match the virus circulating in humans.
Why the Vaccine Went From Four Strains to Three
From the 2013–2014 season through 2023–2024, flu vaccines in the U.S. were quadrivalent, covering two influenza A subtypes and two influenza B lineages (Victoria and Yamagata). That changed because the B/Yamagata lineage appears to have vanished from circulation. No confirmed wild-type B/Yamagata viruses have been detected anywhere in the world since March 2020, likely wiped out by the public health measures used during the COVID-19 pandemic.
In October 2023, the FDA’s advisory committee unanimously voted to recommend removing B/Yamagata from flu vaccines as soon as possible. By March 2024, the committee confirmed that all 2024–2025 vaccines should be trivalent. Dropping a strain that no longer circulates removes an unnecessary component and simplifies manufacturing.
How Vaccine Strains Are Chosen Each Year
The World Health Organization coordinates the strain selection process twice a year: once in February for the Northern Hemisphere season and once in September for the Southern Hemisphere. Scientists from over 100 national influenza centers collect and analyze circulating viruses year-round, tracking how they mutate and which variants are gaining ground. Based on that surveillance, the WHO recommends specific reference viruses for each component.
National regulatory agencies like the FDA then finalize the composition for their country. For the upcoming 2025–2026 U.S. season, the FDA has already announced updated strains. The H3N2 component shifts to an A/Croatia/10136RV/2023-like virus (egg-based) or A/District of Columbia/27/2023-like virus (cell-based), reflecting the continued evolution of that subtype. The H1N1 and B/Victoria components remain similar to the current season.
This annual update is why you need a new flu shot each year. The virus drifts genetically from season to season, and the vaccine has to keep pace.
High-Dose and Senior Vaccines Use the Same Strains
If you’re 65 or older and receive a high-dose or adjuvanted flu vaccine, the viral strains inside are the same as those in standard-dose shots. The difference is in how the immune system is stimulated, not which viruses are targeted. High-dose vaccines contain more of the active ingredient to provoke a stronger immune response in older adults, whose immune systems typically respond less robustly to standard doses. All of these options are trivalent for the current season.
How Well the Vaccine Matches Circulating Strains
Even in a well-matched year, flu vaccine effectiveness varies by strain. Data from the 2023–2024 season illustrates this clearly. Against influenza A viruses (which include both H1N1 and H3N2), the vaccine reduced outpatient illness by about 35% in adults and 50% in children. Against influenza B, it performed considerably better: 70% effectiveness in adults and 64% in children.
The gap between A and B protection is common. Influenza A subtypes, particularly H3N2, mutate faster and are harder to match precisely. H3N2 is also more prone to changing during egg-based manufacturing, which can reduce how well the vaccine-generated antibodies recognize the actual circulating virus. Cell-based and recombinant vaccines sidestep this egg-adaptation problem, though no single manufacturing method guarantees a perfect match.
Even moderate effectiveness translates to meaningful protection at a population level. A vaccine that’s 35% effective against influenza A still prevents millions of illnesses, hundreds of thousands of medical visits, and thousands of hospitalizations each season. Protection against severe outcomes like hospitalization and death tends to be higher than protection against any symptomatic infection.
What Changes for the 2025–2026 Season
The 2025–2026 U.S. flu vaccines will remain trivalent. The most notable update is a new H3N2 component, reflecting how quickly that subtype evolves. The egg-based version will use an A/Croatia/10136RV/2023-like virus, while cell-based and recombinant vaccines will use an A/District of Columbia/27/2023-like virus. The H1N1 component stays in the same family as the current season, and the B/Victoria strain remains A/Austria/1359417/2021-like for the third consecutive year, a sign that this lineage has been relatively stable.
The Southern Hemisphere, which has its flu season during the Northern Hemisphere’s summer, is already shifting further ahead. For its 2026 season, the recommended H1N1 has moved to an A/Missouri/11/2025-like virus. These southern hemisphere selections often preview what may appear in the following Northern Hemisphere vaccine, since scientists can observe which strains dominate in one hemisphere before finalizing the other’s composition.