A flu shot contains inactivated (killed) pieces of influenza virus, along with a small number of supporting ingredients that keep the vaccine stable and effective. The active ingredient is a protein called hemagglutinin, harvested from specific flu strains selected fresh each year. Everything else in the vial, from stabilizers to trace manufacturing residuals, is present in tiny quantities measured in micrograms.
Here’s a breakdown of what’s actually in each dose and why it’s there.
The Active Ingredient: Flu Virus Proteins
The core of every flu shot is hemagglutinin, a protein found on the surface of influenza viruses. Your immune system learns to recognize this protein so it can fight the real virus later. A standard-dose flu shot contains 15 micrograms of hemagglutinin per strain. The high-dose version made for adults 65 and older contains 60 micrograms per strain, four times as much, to generate a stronger immune response in aging immune systems.
The virus used to produce these proteins is grown, then killed and broken apart. You’re not injected with a live or whole virus. Only the surface proteins make it into the final product.
Which Flu Strains Are Included
For the 2025-2026 season, all U.S. flu vaccines are trivalent, meaning they protect against three strains: two influenza A viruses (an H1N1 and an H3N2) and one influenza B virus from the Victoria lineage. The specific strains are updated annually based on global surveillance of which viruses are circulating.
Egg-based vaccines and cell-based vaccines actually use slightly different reference strains. Egg-based shots for this season use an A/Victoria/4897/2022 (H1N1) virus, an A/Croatia/10136RV/2023 (H3N2) virus, and a B/Austria/1359417/2021 virus. Cell-based and recombinant vaccines use an A/Wisconsin/67/2022 (H1N1) and an A/District of Columbia/27/2023 (H3N2), paired with the same B strain. The slight differences reflect how viruses can change when grown in eggs versus mammalian cells.
One wrinkle worth knowing: the CDC identified a new H3N2 subclade (nicknamed “subclade K”) in August 2025 that has drifted genetically from the H3N2 component chosen for this season’s vaccines back in February. This kind of mismatch happens periodically and can reduce how well the vaccine works against that particular strain, though it still offers some cross-protection.
Preservatives
Thimerosal is a mercury-based preservative used only in multi-dose vials, where the same bottle is punctured repeatedly and needs protection against bacterial contamination. A multi-dose flu shot contains about 25 micrograms of mercury per 0.5 mL dose. Single-dose prefilled syringes, which are the most commonly administered format today, contain no thimerosal at all.
The form of mercury in thimerosal (ethylmercury) is processed and cleared by the body much faster than the methylmercury found in fish. Decades of safety studies have found no link between thimerosal in vaccines and harm. Still, if you prefer to avoid it entirely, single-dose options are widely available.
Stabilizers and Buffers
Vaccines need to survive storage and transportation without breaking down. Stabilizers protect the viral proteins from heat, light, and freezing. Common stabilizers in flu shots include sugars like sucrose and lactose, amino acids like glycine, and occasionally gelatin. Buffers (salts and pH-adjusting compounds) keep the solution at the right acidity so it doesn’t degrade or irritate tissue at the injection site. These ingredients are present in very small amounts and are substances your body already encounters through food.
Manufacturing Residuals
Some ingredients aren’t added to the final vaccine on purpose. They’re used during production and remain in trace amounts too small to have a biological effect.
- Formaldehyde is used to inactivate (kill) the virus. Not all flu vaccines contain residual formaldehyde, but those that do have between less than 0.005 and 0.1 milligrams per dose. For context, an infant’s bloodstream naturally contains about 1.1 milligrams of formaldehyde at any given time, 10 to 220 times more than a single vaccine dose.
- Egg proteins (ovalbumin) remain in trace amounts in egg-based vaccines, since the virus is grown inside fertilized chicken eggs. The quantity is extremely small. People with egg allergies can receive any flu vaccine, egg-based or otherwise, that is appropriate for their age and health status.
- Antibiotics like gentamicin are sometimes used during manufacturing to prevent bacterial contamination of the cell cultures. Only trace residuals remain. Notably, penicillin and related antibiotics (the most common drug allergies) are never used in vaccine production.
- Detergents and inactivating agents like polysorbate 80 and cetyltrimethylammonium bromide help break the virus apart so only the relevant proteins remain. Residual amounts in cell-based vaccines like Flucelvax are measured at less than a milligram per dose.
How Ingredients Differ by Vaccine Type
Not all flu shots are made the same way, and the manufacturing method determines which residuals you’ll find inside.
Egg-based vaccines are the traditional method. The virus is grown in fertilized eggs, then harvested, killed, and purified. These contain trace egg protein and may contain small amounts of formaldehyde.
Cell-based vaccines like Flucelvax grow the virus in mammalian kidney cells instead of eggs. This eliminates egg protein entirely, which matters for people with severe egg allergies or those who simply prefer an egg-free option. The trade-off is different residuals: trace amounts of cell protein and cell DNA from the manufacturing line, in quantities well under the safety thresholds set by regulators.
Recombinant vaccines like Flublok skip the virus entirely. They use insect cells programmed to produce hemagglutinin protein directly. No flu virus, no eggs, and no formaldehyde are involved. This is the “cleanest” option in terms of residual ingredients.
The high-dose vaccine (Fluzone High-Dose) uses the same egg-based process as standard shots but concentrates the hemagglutinin to 240 micrograms total across all strains, compared to roughly 45 micrograms in a standard trivalent dose.
The Adjuvant in Senior Vaccines
One flu vaccine, Fluad, includes an ingredient called MF59 that most flu shots don’t have. MF59 is an adjuvant, a substance that boosts the immune response. It’s an oil-in-water mixture made primarily of squalene, a naturally occurring oil found in human skin, the liver, and many plants. The adjuvant helps older adults (65 and up) mount a stronger defense, since immune function declines with age. Fluad is also used for certain organ transplant recipients between 18 and 64 who take immune-suppressing medications. Standard flu shots given to most adults and children do not contain any adjuvant.