Flu vaccines contain a small number of active ingredients designed to train your immune system, plus a handful of inactive ingredients that keep the vaccine stable, sterile, and effective. The exact formula depends on which type of flu vaccine you receive: egg-based, cell-based, recombinant, or nasal spray. Here’s what’s in each one and why.
The Active Ingredients: Flu Virus Strains
The core of every flu vaccine is material from influenza viruses. For the 2024-2025 season, vaccines target an H1N1 strain (A/Victoria/4897/2022), an H3N2 strain (A/Thailand/8/2022), and a B/Victoria lineage strain (B/Austria/1359417/2021). Quadrivalent vaccines add a fourth strain from the B/Yamagata lineage. The WHO updates these recommendations each year based on which viruses are circulating.
In standard injectable vaccines, these viruses are inactivated, meaning they’ve been killed and can’t cause infection. Some contain the whole inactivated virus, while others use only fragments of the virus surface. The nasal spray (FluMist) is different: it contains live viruses that have been weakened so they can replicate briefly in the cool temperatures of your nasal passages but can’t cause actual flu illness.
How Manufacturing Changes the Ingredient List
The biggest factor shaping what’s in your flu shot is how it was made. There are three main production methods, and each leaves different trace ingredients behind.
Egg-Based Vaccines
Most flu vaccines are still grown in fertilized chicken eggs. This process works well but means the final product can contain trace amounts of egg protein, specifically ovalbumin. Among licensed vaccines that report ovalbumin content, quantities are generally 1 microgram or less per dose. That’s a tiny amount, and reviews of studies involving egg-allergic people who received egg-based flu vaccines have found no cases of anaphylaxis or serious allergic reactions. The CDC now recommends that people with egg allergies can receive any flu vaccine appropriate for their age, with no extra safety precautions needed.
Egg-based production also uses antibiotics like neomycin and gentamicin to prevent bacterial contamination in the eggs during processing. These are reduced to very small or undetectable levels during purification, but residual traces may remain. Formaldehyde is used to inactivate the virus so it can’t cause disease. It’s diluted heavily during manufacturing, though residual quantities can be present in the final vaccine.
Cell-Based Vaccines
Cell-based vaccines (like Flucelvax) grow the virus in animal cells instead of eggs. The practical advantage for you is that these vaccines contain no egg proteins. There’s also a potential effectiveness benefit: growing viruses in eggs can introduce small genetic changes that make the vaccine virus slightly different from the flu strains actually circulating in the population. Cell-grown viruses may match circulating strains more closely, which could improve how well the vaccine works.
Recombinant Vaccines
Recombinant vaccines (like Flublok) skip the virus entirely. Instead of growing flu virus in eggs or cells, manufacturers use insect cells and a genetic instruction set to produce just the hemagglutinin protein, the part of the flu virus your immune system needs to recognize. The result is a vaccine that contains no egg proteins, no antibiotics, and no preservatives. Each dose includes sodium chloride, sodium phosphate salts, and a small amount of polysorbate 20 as a stabilizer, along with residual traces of insect cell proteins (no more than about 14 micrograms) and cellular DNA (no more than 10 nanograms).
Preservatives: Thimerosal
Thimerosal is the ingredient that generates the most questions. It’s a mercury-containing preservative used only in multi-dose vials, the bottles a clinic draws multiple shots from. A standard 0.5 mL dose from a multi-dose vial contains about 25 micrograms of mercury. That’s ethylmercury, which the body clears quickly, not the methylmercury that accumulates from fish consumption.
Thimerosal’s job is straightforward: it prevents bacteria and fungi from contaminating the vial each time a needle is inserted. Single-dose vials and prefilled syringes don’t need this protection, so they’re either thimerosal-free or contain only trace amounts left over from manufacturing. If you want to avoid thimerosal entirely, ask for a single-dose vial or prefilled syringe, or choose a recombinant or nasal spray vaccine, none of which contain it.
Adjuvants: The Immune Booster in One Vaccine
Most flu vaccines contain no adjuvant, but one does. Fluad, designed for adults 65 and older, includes an adjuvant called MF59. It’s an oil-in-water emulsion made primarily of squalene (a naturally occurring oil also found in your own liver and in olive oil) at a concentration of 4.3%, stabilized with two surfactants.
MF59 works by creating a controlled burst of immune activity at the injection site. It triggers the release of chemical signals that recruit waves of immune cells to the area, essentially amplifying the body’s response to the vaccine. This matters for older adults because the immune system weakens with age, making standard vaccines less effective. The adjuvant helps compensate by producing stronger and broader antibody responses, including against flu strains that don’t perfectly match the vaccine.
Stabilizers, Buffers, and Salts
The remaining ingredients keep the vaccine physically and chemically stable from the factory to your arm. Phosphate-buffered saline, a combination of sodium chloride, potassium chloride, and phosphate salts, maintains the solution at the right pH so the active ingredients don’t break down. The amounts are small: a typical dose contains around 4 milligrams of sodium chloride and microgram-level quantities of the various phosphate salts.
The nasal spray vaccine has a slightly different stabilizer profile. Each 0.2 mL dose of FluMist includes sucrose (about 14 mg) and hydrolyzed porcine gelatin (2 mg) to protect the live weakened viruses during storage. It also contains arginine (an amino acid), monosodium glutamate (0.188 mg, used here as a stabilizer rather than a flavor enhancer), and potassium phosphate salts. FluMist contains no preservatives.
What’s Not in Flu Vaccines
No flu vaccine contains aluminum, which is an adjuvant used in some other vaccines but not these. No flu vaccine contains fetal cell lines; the cell-based version uses animal kidney cells. And despite persistent rumors, flu vaccines do not contain microchips, antifreeze, or any form of live flu virus capable of causing the flu (the nasal spray’s weakened virus replicates only in the nose and does not cause systemic illness).
The formaldehyde present in some flu vaccines as a manufacturing residual exists in far smaller quantities than what your body produces naturally. Your blood contains about 2.5 micrograms of formaldehyde per milliliter at any given time as a normal byproduct of metabolism, considerably more than the trace found in a vaccine dose.