Vaccines contain a mix of ingredients, each serving a specific purpose. The core component is an active ingredient that teaches your immune system to recognize a threat, but vaccines also include smaller amounts of adjuvants, stabilizers, preservatives, and trace residual materials from manufacturing. Here’s what each of those categories includes and why it’s there.
The Active Ingredient: What Trains Your Immune System
The active ingredient is the whole point of a vaccine. It gives your immune system something to practice on so it can mount a faster, stronger defense if you encounter the real pathogen later. The form this takes varies depending on the vaccine type.
Inactivated vaccines use a killed version of the germ that causes a disease. Because the germ is dead, it can’t make you sick, but your immune system still learns to recognize it. The polio vaccine and many flu shots work this way.
mRNA vaccines take a different approach. Instead of delivering any part of the germ itself, they carry a set of genetic instructions that tell your cells to make a specific protein found on the germ’s surface. Your immune system then responds to that protein. The COVID-19 vaccines from Pfizer and Moderna are the most well-known examples. The mRNA breaks down in your body within days.
Subunit and recombinant vaccines use specific pieces of the germ, such as a surface protein, a sugar molecule, or its outer casing. Because they contain only isolated fragments rather than the whole organism, they tend to produce fewer side effects. The hepatitis B and HPV vaccines fall into this category.
Live attenuated vaccines use a weakened form of the living germ. These tend to produce a strong, long-lasting immune response, sometimes with just one or two doses. The MMR (measles, mumps, rubella) vaccine is a common example.
Adjuvants: Boosting the Immune Response
Some vaccines include an adjuvant, a substance that strengthens your body’s immune response to the active ingredient. Without an adjuvant, certain vaccines wouldn’t work well enough on their own, or would require more doses to achieve the same protection.
Aluminum salts are the most common adjuvant and have been used in vaccines for decades. They come in several forms, including aluminum hydroxide and aluminum phosphate. Small amounts are added to help your body build stronger immunity against the target germ.
Newer adjuvants have expanded the toolkit. MF59 is an oil-in-water mixture based on squalene, a naturally occurring oil found in plant and animal cells as well as in humans. It’s used in certain flu vaccines designed for older adults. Matrix-M, used in some COVID-19 vaccines, contains compounds derived from the bark of the Chilean soapbark tree, combined with cholesterol and fats. Another adjuvant called AS01B, used in the shingles vaccine Shingrix, pairs a bacteria-derived immune-boosting substance with a natural compound also extracted from the soapbark tree.
How Much Aluminum Is Actually in Vaccines
Aluminum in vaccines is one of the most common concerns people search for, so the numbers are worth spelling out. Researchers at the Vaccine Education Center at Children’s Hospital of Philadelphia calculated that if a person received every aluminum-containing vaccine on the recommended U.S. schedule, their cumulative lifetime exposure from vaccines would total roughly 12 milligrams. In contrast, aluminum absorbed from food and water over 100 years ranges from 468 to 2,785 milligrams.
In the first two years of life, when the vaccine schedule is most concentrated, maximum aluminum exposure from vaccines is about 4.4 milligrams. Over that same period, a child absorbs 3 to 18 milligrams of aluminum just from their diet. The average U.S. adult takes in 7 to 9 milligrams of aluminum daily through food and water. By age 18, cumulative dietary aluminum absorption reaches 73 to 438 milligrams, compared with less than 8 milligrams from all vaccines combined.
Stabilizers and Preservatives
Stabilizers protect the vaccine’s active ingredients from degrading during storage and transportation. Common stabilizers include sugars like sucrose and lactose, amino acids (the building blocks of proteins), and gelatin. Without them, temperature changes or time on a shelf could make the vaccine less effective before it ever reaches your arm.
Preservatives serve a different purpose: they prevent bacterial or fungal contamination in multi-dose vials. When a needle enters a vial to draw out one dose, there’s a small risk of introducing germs that could multiply in the remaining liquid. Thimerosal, a mercury-based preservative, has been used for decades in multi-dose vials to prevent this. It’s worth noting that most childhood vaccines in the U.S. now come in single-dose vials or prefilled syringes and do not contain thimerosal at all.
Trace Materials Left Over From Manufacturing
Some ingredients in vaccines aren’t there by design. They’re residual traces from the manufacturing process, present in extremely small or sometimes undetectable amounts after purification.
Formaldehyde is one example. It’s used during production to inactivate viruses so they can’t cause disease (as in the polio vaccine) and to detoxify bacterial toxins (as in the diphtheria vaccine). The formaldehyde is diluted during manufacturing, though residual quantities may remain in the final product. Your body naturally produces and processes formaldehyde as part of normal metabolism, in quantities far greater than what any vaccine contains.
Antibiotics like neomycin, polymyxin B, streptomycin, and gentamicin are sometimes used during production to keep bacteria from growing in the cell cultures or eggs where viruses are cultivated. Eggs, for instance, are not sterile, so antibiotics help keep the process clean. These antibiotics are reduced to very small or undetectable levels during purification. If you have a known allergy to specific antibiotics, this is worth mentioning to your provider before vaccination.
Some vaccines are grown in cells that need nutrition during the culturing process. Fetal bovine serum is occasionally used as that nutrient source. Like other manufacturing materials, it’s separated out during purification, leaving only trace amounts in the final product.
Surfactants and Diluents
A few additional ingredients help with the physical structure of the vaccine. Surfactants like polysorbate 80 keep ingredients evenly distributed in the liquid so they don’t separate or clump together. You’ll find polysorbate 80 in many foods and cosmetics as well. Sterile water or saline solution serves as the base liquid that carries all the other components and makes the vaccine injectable.
Allergens to Be Aware Of
A small number of vaccine ingredients are known allergens for some people. Gelatin, used as a stabilizer, can trigger reactions in people with gelatin allergies. Egg proteins may be present in trace amounts in vaccines grown in eggs, such as some flu vaccines and the yellow fever vaccine. The antibiotics neomycin and polymyxin B, present in nanogram-level traces in certain vaccines, are relevant for people with known antibiotic allergies. Yeast proteins can appear in vaccines produced using yeast cells, such as the hepatitis B vaccine.
Severe allergic reactions to vaccines are rare, occurring in roughly one to two cases per million doses. If you’ve had an allergic reaction to a previous vaccine dose or know you’re allergic to a specific ingredient, that information helps your provider choose the safest option for you.