Every vaccine contains an active ingredient that teaches your immune system to recognize a specific germ, plus a short list of supporting ingredients that keep the vaccine stable, sterile, and effective. The exact formula varies by vaccine, but most contain fewer than a dozen total components, and nearly all of them are present in tiny quantities.
The Active Ingredient: Training Your Immune System
The core of any vaccine is an antigen, the substance that triggers your immune system to build defenses against a particular disease. What that antigen looks like depends on the type of vaccine.
Live, attenuated vaccines use a weakened version of the actual virus or bacteria. The germ has been altered in a lab, usually through repeated culturing, so it can still replicate just enough to provoke a strong immune response without causing illness. The measles, mumps, rubella (MMR) and chickenpox vaccines work this way.
Inactivated vaccines use germs that have been killed through heat or chemicals. They cannot replicate at all, which means they can’t cause disease even in someone with a weakened immune system. Polio, hepatitis A, and rabies vaccines fall into this category.
Subunit vaccines skip the whole germ entirely and use just a piece of it, typically a protein or sugar molecule from the germ’s surface that the immune system can learn to target. Influenza and pneumococcal vaccines are common examples. A related approach, toxoid vaccines, uses an inactivated version of the toxin a bacterium produces rather than the bacterium itself. The tetanus and diphtheria vaccines are toxoids.
Recombinant vaccines are made by inserting a gene from the target germ into yeast or another cell, which then manufactures a specific protein. The hepatitis B and HPV vaccines use this technique. The cell grows, produces the protein, and that purified protein becomes the vaccine’s active ingredient.
mRNA vaccines, the newest type, deliver a small piece of genetic instructions wrapped in a protective fat bubble called a lipid nanoparticle. Your cells read the instructions, build a harmless protein that resembles part of the virus, and your immune system learns to attack it. The mRNA breaks down within days. The lipid shell that protects it is made of four components: an ionizable lipid, cholesterol, a phospholipid, and a polyethylene glycol (PEG) lipid. These are all variations of fats your body already encounters naturally.
Adjuvants: Strengthening the Response
Some vaccines include an adjuvant, a substance that amplifies your immune system’s reaction to the antigen. Without an adjuvant, certain vaccines would produce a weaker or shorter-lasting response, potentially requiring more doses.
Aluminum salts are the most common adjuvant and have been used in vaccines for over 70 years. Small amounts of aluminum hydroxide, aluminum phosphate, or related compounds help your body build stronger immunity against the germ in the vaccine. For context, aluminum is one of the most abundant elements on Earth and is present in drinking water, many foods, and breast milk.
Newer adjuvants target specific parts of the immune response. Some examples currently in use: MF59, an oil-in-water mixture made from squalene (a naturally occurring oil found in plant and animal cells, including human skin cells), is used in one version of the flu vaccine for older adults. Matrix-M, derived from compounds in soapbark tree bark along with cholesterol and phospholipids, is used in the Novavax COVID-19 vaccine. CpG 1018, a synthetic form of DNA that mimics bacterial genetic material, is used in a hepatitis B vaccine to boost the immune response. Not every vaccine needs an adjuvant. Live vaccines and mRNA vaccines generally work well without one.
Stabilizers: Keeping the Vaccine Intact
Vaccines need to survive manufacturing, shipping, and storage without breaking down. Stabilizers protect the active ingredients from heat, light, and the freeze-drying process that some vaccines undergo. The stabilizers used are common, familiar substances: sugars like sucrose and lactose, amino acids like glycine, and proteins like gelatin or human serum albumin. These prevent the active proteins or weakened viruses from degrading before the vaccine reaches your arm.
Preservatives: Preventing Contamination
Multi-dose vaccine vials, where a healthcare provider draws several doses from a single container, need a preservative to prevent bacterial or fungal growth after the vial is first punctured. Thimerosal is the most well-known vaccine preservative. It contains a form of mercury called ethylmercury, which the body clears much faster than the methylmercury found in certain fish.
A vaccine dose preserved with thimerosal contains roughly 25 micrograms of mercury, about the same amount found in a 3-ounce can of tuna. Most vaccines on the current childhood schedule are available in single-dose vials or prefilled syringes that contain no thimerosal at all. The preservative is primarily found in some multi-dose flu vaccine vials.
Surfactants and Buffers
A few additional ingredients keep the vaccine mixed evenly and at the right pH. Polysorbate 80, a common emulsifier also found in ice cream and cosmetics, acts as a stabilizer in some vaccines or remains as a trace residual from manufacturing. Buffers like sodium chloride (salt) or sodium citrate maintain the vaccine’s acidity at a level that keeps the active ingredients functional and comfortable at the injection site.
Manufacturing Residuals
During production, vaccines may be grown in egg proteins, yeast cells, or cell cultures, and tiny amounts of antibiotics are sometimes used to prevent bacterial contamination during manufacturing. Traces of these substances can remain in the final product, but in extraordinarily small quantities, measured in parts per million or parts per billion. For example, flu vaccines grown in eggs may contain a minuscule amount of egg protein, which is why people with severe egg allergies are sometimes asked about this before vaccination. Antibiotics like neomycin may appear in trace amounts, but penicillin and related antibiotics that commonly cause allergic reactions are never used in vaccine production.
What a Vaccine Does Not Contain
The full ingredient list for every approved vaccine is published by the FDA and listed on the vaccine’s package insert. The lists are short. A typical vaccine contains an antigen, possibly an adjuvant, a stabilizer or two, a buffer, and water. The total volume of a standard dose is 0.5 milliliters, about one-tenth of a teaspoon. Every ingredient serves a specific, functional purpose: teach the immune system, boost the lesson, or keep the product stable long enough to do its job.