Yes, the flu vaccine protects against influenza A. Every seasonal flu vaccine includes two influenza A strains: one H1N1 and one H3N2. How well it works in any given year depends on how closely those vaccine strains match the viruses actually circulating, but even in lower-effectiveness seasons, vaccination reduces your risk of getting sick, being hospitalized, and dying from influenza A.
Which Influenza A Strains the Vaccine Covers
Starting with the 2024-2025 season, flu vaccines in the United States are trivalent, meaning they protect against three viruses: an influenza A(H1N1) strain, an influenza A(H3N2) strain, and one influenza B strain. Two of the three components target influenza A directly.
The specific strains are updated each year based on global surveillance of which viruses are spreading. For the 2025-2026 season, the FDA selected an A/Victoria/4897/2022-like virus for H1N1 coverage and an A/Croatia/10136RV/2023-like virus for H3N2 coverage in egg-based vaccines, with slightly different reference strains for cell-based and recombinant versions. These selections aim to give your immune system the best possible preview of what it will encounter during flu season.
How Effective It Is Against Each Subtype
The vaccine doesn’t work equally well against all influenza A subtypes. A large meta-analysis of 76 studies from both hemispheres found that flu vaccines performed best against H1N1, with a pooled effectiveness of 56% in the Northern Hemisphere. Protection against H3N2 was considerably lower, at just 22%. Influenza B fell in between at 42%.
Recent Canadian data from the current season illustrates a similar pattern: the vaccine showed 31% effectiveness against H1N1 and 40% against H3N2 for medically attended illness. These numbers shift each year depending on how well the vaccine strains match circulating viruses.
The gap between H1N1 and H3N2 protection is consistent across many seasons. H3N2 viruses mutate faster than H1N1, making them harder to match accurately months in advance when vaccine production decisions are made.
Why Protection Varies From Year to Year
Influenza A viruses constantly accumulate small mutations in their surface proteins, a process called antigenic drift. These are the proteins your immune system learns to recognize after vaccination. When the circulating virus drifts far enough from the vaccine strain, your antibodies become less effective at neutralizing it.
The difference can be dramatic. During the 1997-1998 season, when the vaccine strain didn’t match the dominant H3N2 virus well, effectiveness dropped to about 50%. The following season, with a good match, it climbed to 86%. French surveillance data from the same period showed a similar swing: 70-80% effectiveness in matched years versus roughly 40% in mismatched ones. During the 2003-2004 season, when a drifted H3N2 variant emerged, vaccine efficacy fell to around 50-56% against lab-confirmed cases, compared to the 70-90% expected in well-matched years.
This is why the vaccine is reformulated annually. Even when the match isn’t perfect, vaccination still provides partial protection, and the antibodies you develop can reduce the severity of illness even if they don’t prevent infection entirely.
How the Vaccine Works Against Influenza A
The flu vaccine teaches your immune system to recognize two key proteins on the surface of influenza A: hemagglutinin and neuraminidase. Hemagglutinin is what the virus uses to latch onto and enter your cells. Neuraminidase helps newly made virus particles break free from infected cells to spread further. Antibodies targeting hemagglutinin block the virus from getting in, while antibodies against neuraminidase limit its ability to spread once inside your body.
It takes about two weeks after vaccination for your body to build enough of these antibodies to provide meaningful protection. If you’re exposed to the flu during that window, you can still get sick. This is one reason health officials recommend getting vaccinated before flu season picks up rather than waiting until cases are surging.
Protection Against Hospitalization and Severe Illness
Even when the vaccine’s ability to prevent any flu illness looks modest, its impact on serious outcomes is meaningful. A meta-analysis of randomized clinical trials found that flu vaccination reduces the odds of hospitalization by 29% and all-cause mortality by 18%.
For adults 65 and older, who face the highest risk of severe influenza A complications, high-dose vaccines offer an additional edge. In a double-blind trial, the high-dose vaccine was 24% more effective than the standard dose at preventing lab-confirmed influenza in older adults. More strikingly, it reduced influenza-related hospitalizations by nearly 44% compared to the standard-dose vaccine. Hospitalization rates were roughly half: 0.06% in the high-dose group versus 0.11% in the standard-dose group.
Why Annual Vaccination Still Matters
Your immune protection from last year’s flu shot fades over time, and the circulating viruses are likely different from what last year’s vaccine targeted. Both of these factors make annual vaccination necessary. Even in seasons when effectiveness against influenza A hovers in the 30-40% range, that still means roughly one in three flu infections is prevented among vaccinated people. For a virus that hospitalizes hundreds of thousands each year in the U.S. alone, that translates to a substantial reduction in suffering at the population level.
The vaccine also provides a buffer if you do get infected. Vaccinated people who still catch the flu tend to have shorter, milder illnesses and are less likely to develop complications like pneumonia. This partial protection is especially valuable for people with chronic health conditions, young children, pregnant individuals, and older adults.