The 2024-2025 flu vaccines in the United States protect against three influenza strains: an A(H1N1), an A(H3N2), and a B/Victoria lineage virus. This is a notable change from recent years, when vaccines targeted four strains. The B/Yamagata lineage has been dropped entirely because it hasn’t been detected anywhere in the world since March 2020.
The Three Strains in This Year’s Vaccine
The exact virus strains depend on how your vaccine was manufactured. If you received an egg-based flu shot (still the most common type), your vaccine contains viruses similar to A/Victoria/4897/2022 (H1N1), A/Thailand/8/2022 (H3N2), and B/Austria/1359417/2021 (B/Victoria). If you received a cell-based or recombinant vaccine, the H1N1 and H3N2 components are slightly different: A/Wisconsin/67/2022 (H1N1) and A/Massachusetts/18/2022 (H3N2). The B/Victoria component is the same across all vaccine types.
The reason for two sets of strains comes down to how the vaccines are produced. Most flu vaccines are still grown in chicken eggs, a process that’s been used for over 70 years. As viruses adapt to grow in eggs, they pick up mutations that can make them slightly different from the viruses actually circulating in people. Cell-based vaccines grow the virus in mammalian cells instead, which avoids those egg-adapted changes. Recombinant vaccines skip the virus entirely, building the key protein synthetically.
Why the Vaccine Went From Four Strains to Three
For years, flu vaccines were quadrivalent, covering two A strains and two B lineages (Victoria and Yamagata). Starting this season, every flu vaccine in the U.S. is trivalent. The FDA confirmed in March 2024 that only trivalent vaccines would be available.
The reason is straightforward: B/Yamagata appears to have gone extinct. Global flu surveillance has not confirmed a single case of B/Yamagata since March 2020, likely a consequence of the public health measures during the COVID-19 pandemic. With no circulating virus to protect against, including it in the vaccine would serve no purpose. Removing it also eliminates any theoretical risk from live-attenuated vaccines (like the nasal spray) containing a virus that no longer circulates naturally.
Egg-Based vs. Cell-Based: Why It Matters
The distinction between egg-based and cell-based vaccines isn’t just a manufacturing detail. It can affect how well the vaccine works. Detailed genetic analyses of egg-based vaccine viruses used between 2010 and 2024 have found egg-adapted mutations in every single season, across all strains. Most of these mutations occur in the parts of the virus that your immune system learns to recognize, which is exactly where you don’t want changes.
H3N2 has historically been the worst offender. During the 2023-2024 season, researchers found that all four egg-based vaccine viruses had picked up mutations in key areas of the surface protein, while none of the corresponding cell-based viruses had. A study published in Infectious Diseases and Therapy found that cell-based vaccines were more effective than egg-based versions in both children and adults that season, likely because of these mismatches.
If you have a choice, cell-based (marketed as Flucelvax) or recombinant (Flublok) vaccines avoid this problem entirely. Not every pharmacy or clinic carries them, but they’re increasingly available.
Vaccine Options for Older Adults
Adults 65 and older have three enhanced options beyond the standard flu shot. Fluzone High-Dose contains four times the antigen of a standard vaccine, which helps overcome the weaker immune response that comes with aging. FLUAD adds an adjuvant, a compound that boosts the immune system’s reaction to the vaccine. Flublok, the recombinant option, uses three times the antigen of standard doses and avoids egg-adapted changes altogether.
Observational studies suggest each of these enhanced vaccines is more effective than standard-dose shots in older adults, with the most evidence available for the high-dose version. No head-to-head randomized trials have compared the enhanced options directly against each other, so there’s no clear winner among the three.
How Strains Get Selected Each Year
The World Health Organization coordinates the strain selection process twice a year: once for the Northern Hemisphere (in February) and once for the Southern Hemisphere (in September). In February 2024, an advisory group of experts met for four days to review surveillance data collected through the WHO’s Global Influenza Surveillance and Response System, a network of laboratories in over 100 countries that tracks which flu viruses are circulating and how they’re evolving.
The goal is to pick vaccine strains that most closely match what’s expected to circulate during the upcoming flu season. It’s an educated prediction made months in advance, since manufacturers need lead time to produce hundreds of millions of doses. Some years the match is excellent. Others, the virus drifts between the selection date and flu season, reducing effectiveness. This built-in lag is one of the biggest limitations of current flu vaccines, and it’s why the composition changes nearly every year.
The Nasal Spray Vaccine
FluMist, the nasal spray option, is also trivalent this season, covering the same three virus types: A(H1N1), A(H3N2), and B/Victoria. Unlike injectable vaccines, FluMist uses live but weakened viruses that can replicate in the cooler temperatures of your nasal passages but not in your lungs. It’s approved for people ages 2 through 49 who aren’t pregnant and don’t have certain immune conditions. For the 2024-2025 season, a self-administered version for ages 2 through 49 also became available, meaning some people can use it at home without a healthcare visit.