The flu shot covers three strains of influenza virus: two type A strains (H1N1 and H3N2) and one type B strain (B/Victoria lineage). This is a change from recent years, when flu vaccines included four strains. For the 2024–2025 season, all U.S. flu vaccines are trivalent, meaning they target three viruses instead of four.
The Three Strains in This Season’s Vaccine
Every year, the World Health Organization and national regulators select which specific virus strains go into the flu vaccine based on global surveillance data. For the 2024–2025 U.S. season, the CDC lists three components. The exact strains differ slightly depending on how the vaccine is manufactured.
Vaccines made using the traditional egg-based process contain an A/Victoria/4897/2022 (H1N1) strain, an A/Thailand/8/2022 (H3N2) strain, and a B/Austria/1359417/2021 (B/Victoria lineage) strain. Vaccines made using newer cell-based or recombinant technology contain slightly different versions of the two A strains, an A/Wisconsin/67/2022 (H1N1) and an A/Massachusetts/18/2022 (H3N2), paired with the same B/Victoria strain. These small differences exist because viruses can change slightly when grown in eggs versus lab-cultured cells, and regulators want each formulation to match circulating viruses as closely as possible.
Why the Vaccine Dropped From Four Strains to Three
Until recently, flu vaccines were quadrivalent, covering two A strains and two B lineages: B/Victoria and B/Yamagata. The B/Yamagata lineage hasn’t been detected in human circulation since the early pandemic years, and scientists now consider it effectively extinct. Because that virus is no longer spreading, there’s no reason to include it in the vaccine. The U.S. switched entirely to trivalent vaccines for the 2024–2025 season. A few countries, like Canada, still use a quadrivalent formulation in their nasal spray vaccine, but the global direction is moving toward three components.
What the Flu Shot Does Not Cover
The flu vaccine protects only against influenza viruses. It does not protect against the common cold (caused by rhinoviruses and other agents), RSV, COVID-19, or the many other respiratory viruses that circulate alongside flu each winter. Many people use the word “flu” loosely to describe any bad cold or stomach bug, which creates the impression that the vaccine didn’t work when they were never dealing with actual influenza in the first place.
The vaccine also can’t guarantee a perfect match against every influenza strain in circulation. Flu viruses mutate constantly, and sometimes the strains selected months before flu season drift enough that the match is imperfect. Even when this happens, vaccination still provides partial cross-protection and tends to reduce the severity of illness if you do get infected.
How Well the Vaccine Works
Effectiveness varies from season to season depending on how well the vaccine strains match what’s actually circulating. Data from the 2023–2024 season gives a useful snapshot. In children aged 6 months to 17 years, the vaccine reduced the risk of outpatient flu visits by roughly 56 to 65 percent and cut the risk of hospitalization by about 58 to 64 percent. In adults 18 and older, the numbers were lower: outpatient effectiveness ranged from 35 to 47 percent, and protection against hospitalization was around 39 to 40 percent.
Those adult numbers may look modest, but a 40 percent reduction in hospitalizations translates to tens of thousands fewer severe cases across the population. Protection also tends to be stronger in younger, healthier people and weaker in older adults, which is why specialized vaccine formulations exist for seniors.
Different Formulations for Different Groups
All flu vaccines this season cover the same three strains, but the way those strains are delivered varies. Standard-dose inactivated vaccines are the most common and are available for most people 6 months and older.
For adults 65 and older, three vaccines are preferentially recommended because they produce a stronger immune response in aging immune systems. One is a high-dose vaccine (Fluzone High-Dose) that contains four times the antigen of a standard shot. Another uses an immune-boosting additive called an adjuvant (Fluad) to help the body mount a more robust response. The third is a recombinant vaccine (Flublok), which a review of existing studies suggested may be more effective than standard-dose options in this age group.
For people with egg allergies, both the recombinant vaccine and the cell-culture-based vaccine (Flucelvax) are completely egg-free. That said, current guidance allows people with egg allergies to receive any age-appropriate flu vaccine, including egg-based versions.
How Quickly Protection Kicks In
Your body doesn’t build full protection the moment the needle goes in. Most studies measure peak antibody levels about one month after vaccination, and that initial response is a strong predictor of how durable your protection will be through the season. This is why health officials encourage getting vaccinated in September or October: it gives your immune system time to ramp up before flu activity typically peaks in December through February.
Protection does wane over the course of a season, particularly in older adults. Getting vaccinated too early (July or August) can mean your antibody levels have already started declining by the time flu peaks. The timing sweet spot for most people is early to mid-fall.