A vaccine contains a small number of ingredients, each with a specific job. The main component is an active ingredient that teaches your immune system to recognize a particular germ. Everything else in the vial, typically just a few additional substances, exists to keep that active ingredient stable, effective, and safe from contamination. Here’s what each ingredient does and why it’s there.
The Active Ingredient: Training Your Immune System
The core of every vaccine is something that mimics part of a virus, bacterium, or toxin just enough to trigger an immune response without causing the disease. Different vaccines use different strategies to accomplish this, and the approach determines what the active ingredient actually looks like.
Killed whole viruses or bacteria. Some vaccines contain the entire germ, but it’s been killed with heat or chemicals so it can’t replicate. The polio shot, hepatitis A vaccine, and rabies vaccine all work this way. Your immune system sees the dead germ, learns its shape, and builds defenses against the live version.
Pieces of a germ. Rather than including the whole organism, many vaccines contain just one protein or sugar molecule from its surface. Flu shots and pneumococcal vaccines use this approach. The selected piece is the part your immune system needs to see in order to mount a protective response.
Inactivated toxins. The diseases caused by tetanus and diphtheria aren’t from the bacteria themselves but from the toxins they produce. These vaccines contain the toxin after it’s been chemically deactivated, so it’s harmless but still recognizable to your immune system.
Lab-grown proteins. The hepatitis B and HPV vaccines take a gene from the target virus and insert it into yeast cells. The yeast then manufactures a single viral protein as it grows. That purified protein becomes the vaccine’s active ingredient, with no actual virus involved at any stage.
mRNA. COVID-19 vaccines from Pfizer and Moderna deliver a strand of genetic instructions wrapped in a tiny fat bubble called a lipid nanoparticle. Your cells read the instructions, build a harmless viral protein, and your immune system responds to it. The mRNA breaks down within days.
Adjuvants: Amplifying the Response
Some vaccines include an adjuvant, a substance that strengthens your immune reaction to the active ingredient. Without it, certain vaccines wouldn’t produce strong or lasting protection. Aluminum salts are the most common adjuvant and have been used in vaccines for decades. The specific forms include aluminum hydroxide, aluminum phosphate, and potassium aluminum sulfate.
Newer adjuvants appear in more specialized vaccines. The shingles vaccine (Shingrix) contains an adjuvant made from a purified fat-like substance combined with a compound extracted from the bark of the South American soap bark tree. A hepatitis B vaccine for adults uses an adjuvant based on a synthetic DNA sequence. An influenza vaccine designed for people 65 and older uses an oil-in-water mixture of squalene, a natural oil also found in your own skin. Each of these was developed because aluminum salts alone weren’t enough to produce the needed immune response for that particular vaccine.
Lipid Nanoparticles in mRNA Vaccines
mRNA is fragile. Left on its own, it would be destroyed by enzymes in your body before it could do anything useful. To protect it, mRNA vaccines wrap the genetic instructions inside a microscopic fat bubble made of four types of lipids. Ionizable lipids bind to the mRNA and help it enter your cells. Helper lipids and cholesterol give the bubble structural stability. A lipid coated with polyethylene glycol (PEG) sits on the surface, acting like a shield that prevents your immune system from clearing the particle before it reaches cells. These lipid nanoparticles are the reason mRNA vaccines need cold storage: the fat bubbles are sensitive to heat.
Preservatives and Contamination Prevention
Thimerosal is the preservative most people have heard of. It’s an organic mercury compound used in some multidose flu vaccine vials to prevent bacterial or fungal contamination after the vial is opened and punctured multiple times. A standard 0.5 mL dose from a multidose vial contains about 25 micrograms of mercury.
Thimerosal is not present in single-dose vials or prefilled syringes, which is how the vast majority of vaccines are now packaged. All vaccines routinely given to children 6 and younger in the U.S. are available in thimerosal-free versions, and the same is true for adolescent and adult vaccines.
Trace amounts of antibiotics like neomycin or streptomycin are sometimes used during manufacturing to keep bacterial contamination out of the cell cultures where vaccine components are grown. These antibiotics appear in the final product only as residual traces. Antibiotics known to cause common allergic reactions, like penicillin, are never used in vaccine production.
Residual Formaldehyde
Formaldehyde is used during production to inactivate viruses or detoxify bacterial toxins. After purification, only trace amounts remain. A single vaccine dose contains somewhere between 0.005 mg and 0.1 mg of residual formaldehyde. For context, a 2-month-old infant’s bloodstream naturally contains about 1.1 mg of formaldehyde at any given moment, produced by the body’s own metabolism. That’s 10 to 220 times more than what’s in any single vaccine dose.
Stabilizers That Protect the Vaccine
Vaccines can lose potency when exposed to temperature swings, light, or changes in acidity. Stabilizers prevent this. Common stabilizers include sugars like sorbitol and lactose, amino acids like glutamic acid, and gelatin. These substances protect the active ingredient during freeze-drying, shipping, and storage. The measles vaccine, for instance, uses a sorbitol-gelatin combination to preserve the weakened virus through the freeze-drying process that allows it to be stored and transported.
Gelatin is the stabilizer most likely to cause an allergic reaction, though this is rare. If you’ve had a reaction to gelatin in food, it’s worth mentioning before vaccination.
Buffers and Salts
A small amount of salt and phosphate compounds keep the vaccine at a pH close to 7.4, which matches the natural pH of your blood and tissue. Without these buffers, the solution could become too acidic or too alkaline, which would both damage the active ingredient and cause more pain at the injection site. The specific salts used are the same sodium phosphate and sodium chloride (table salt) compounds found in saline solutions throughout medicine.
Surfactants and Emulsifiers
Polysorbate 80 appears in some vaccines as either a stabilizer or a residual from manufacturing. It’s an emulsifier, meaning it keeps ingredients from separating, the same way it keeps ice cream smooth and slow to melt. The amounts in vaccines are extremely small, typically measured in micrograms.
Growth Medium Residuals
Viruses need living cells to grow in, so during manufacturing, vaccine viruses are often cultivated in chicken egg cells, yeast, or mammalian cell lines. After the virus or protein is harvested and purified through filtration and chemical treatment, tiny residual amounts of the growth medium can remain. This is why some vaccine labels mention egg protein or yeast protein. For egg-grown flu vaccines, the residual egg protein is present in quantities small enough that most people with egg allergies can still safely receive them, though cell-grown and recombinant alternatives exist for those who prefer to avoid egg-derived vaccines entirely. Manufacturing also includes steps to reduce residual DNA from cell cultures to less than 10 nanograms per dose.
What’s Not in a Vaccine
The total volume of a typical vaccine dose is 0.5 mL, about one-tenth of a teaspoon. Most of that volume is sterile water or saline. The active ingredient and all the supporting substances together amount to micrograms or milligrams, quantities far smaller than what you encounter daily in food, drinking water, and your body’s own metabolic processes.