Vaccines either contain parts of germs, inactivated (dead) germs, or weakened versions of live germs that teach your body how to fight off those infections. And some vaccines don’t contain any germs at all.
Vaccines also have ingredients that keep them preserved and strengthen your body’s response to the vaccine.
For the most part, vaccines are very safe. Complications are possible but rare.
Vaccines are an important way to protect yourself and those around you from getting sick. They are medicines that contain either a living or dead germ — the active ingredient — that teaches your immune system to fight off an infection before it has a chance to make you sick. Getting vaccinated protects you from infection. And people around you are protected because you’re less likely to spread infection to them.
Vaccines also contain ingredients that strengthen the immune response, prevent the vaccine from getting contaminated, and help it last until you can get it into your body.
It’s understandable that people worry about vaccine ingredients. With names like “inactivated toxins” or “preservatives,” they sound like things you might not want to put in your body. But vaccine ingredients undergo thorough testing to make sure they’re safe and nontoxic. And vaccines are important to keep you protected from dangerous infections.
Here are the main vaccine types:
Inactivated (dead) vaccines
Subunit and conjugate vaccines
Let’s take a closer look at the active ingredients in these vaccines.
Inactivated vaccines are made from killed viruses or bacteria. When used in a vaccine, the dead germs teach your immune system how to defend the body against the living versions. Some examples include:
Most flu shots
Subunit and conjugate vaccines use only a specific piece of the germ — like a protein or part of the outer layer of the germ. They teach the immune system to identify an infection by recognizing that specific piece.
Examples of subunit and conjugate vaccines include:
Hepatitis B vaccine
HPV (human papillomavirus) vaccine
Pneumococcal conjugate vaccines
Novavax COVID vaccine
Flublok Quadrivalent flu vaccine
Live vaccines are made of living viruses or bacteria. These germs have been attenuated, meaning that they’re weaker than the normal germs. Because they’re similar to the full-strength germs, your body learns how to fight them off. But because the germs are weakened, a healthy immune system can easily stop them — so they never cause an infection.
Examples of live attenuated vaccines are:
Measles, mumps, and rubella (MMR)
Chicken pox (varicella)
Nasal spray flu vaccine
An mRNA vaccine gives the immune system a genetic blueprint for a specific part of a germ. That genetic blueprint is like an instruction manual for your immune cells to learn how to fight off the real thing if it comes along. The most common examples of mRNA vaccines are the Pfizer and Moderna COVID vaccines.
Even though they contain genetic material, mRNA vaccines don’t change your genes. They just show the body how to create an immune response if the time comes.
There are additional ingredients in vaccines, aside from the parts of the bacteria or virus. The names of some of these ingredients can be the source of alarm and confusion for people. Here’s what you need to know about their safety.
Adjuvants are vaccine ingredients that stimulate your immune system to make a stronger immune response. Sometimes, the main ingredient of a vaccine alone is not enough to make lasting protection, so adjuvants are added to make the vaccine more effective. Here are some examples of adjuvants:
Aluminum, in small amounts that scientists have found to be safe
Squalene, a substance naturally found in skin oil
Monophosphoryl lipid A, a substance found on the surface of bacteria
Chilean soap bark tree extract
Lab-made genetic material that resembles DNA from bacteria or viruses
Many vaccines contain preservatives. In vaccines, preservatives prevent contamination with bacteria — which keeps them safe until they’re injected into the body.
Studies have shown that thimerosal is safe when used in vaccines. It’s only used in certain versions of the influenza vaccine. And trace amounts may be found in one version of the tetanus vaccine from the manufacturing process.
Stabilizers are ingredients that protect vaccines while they’re stored and transported. These are common, nontoxic ingredients, like sugars and gelatin.
Vaccines have trace amounts of leftover ingredients from the process of making the vaccine. Examples of these include:
Ingredients to grow organisms in culture: Egg protein is an example.
Ingredients to kill or damage active ingredients: Formaldehyde, for example, can be toxic in large amounts, but it’s safe in small amounts. Your body even makes small amounts of formaldehyde naturally, which it breaks down. The amount of formaldehyde in vaccines is tiny compared with the natural amount of formaldehyde in your body, so it doesn’t put you at risk of health problems.
Antibiotics to kill bacteria and prevent contamination: The antibiotics in vaccines are not the same antibiotics that people take normally. Because of this, it’s rare for people to be allergic to these antibiotics.
The ingredients in the COVID vaccine vary slightly depending on the manufacturer. But, in general, they contain:
Either mRNA or specific proteins from SARS-CoV-2
Sugars (used as stabilizers)
Killed influenza virus or a small part of it
Live, weakened flu virus (live vaccines only)
Egg protein in some
Some versions contain small amounts of thimerosal and/or formaldehyde
Small amounts of antibiotics
Some vaccine ingredients can be toxic in large amounts. The same is true for any medications that you take. The amount of any potentially toxic ingredient in vaccines is very small. The CDC and FDA make sure vaccines go through safety testing to ensure they’re not toxic to humans.
Some vaccines are made using cells that once came from fetal tissue. Many years ago, scientists collected cells from fetal tissue, and since then they have grown replica cells. Some of these lab-grown cells are used to make certain vaccines. For example, the weakened viruses for the varicella vaccine and some COVID vaccines are grown or tested in these cells. The final product doesn’t have any of the fetal tissue in it. During the manufacturing process, the cells are removed from the vaccine.
Thimerosal is a preservative used in some vaccines which is made of mercury. This is a different type of mercury than that which is found in the environment, such as in fish. Your body is able to get rid of the mercury in thimerosal much faster than it can remove other types of mercury. Even so, it’s used in very small amounts and only in specific vaccines.
Vaccines go through rigorous testing to make sure they’re safe for people. Rarely, people can have serious allergic reactions to vaccines, but this is not the norm.
Certain people shouldn’t get certain vaccines, such as people who are immunocompromised. For most people, vaccines are a safe way to get protection against potentially serious infections.
All vaccines are designed to teach the immune system how to fight off a specific infection. Different types of vaccines do this in different ways. For many vaccines, the active ingredient is either a killed or weakened version of the virus or bacteria. And some vaccines — like mRNA vaccines — don’t even contain the germ at all. This makes vaccines safe, and they’re the best way to protect yourself from future infections.
Centers for Disease Control and Prevention. (2021). Epidemiology and Prevention of Vaccine-Preventable Diseases.
Centers for Disease Control and Prevention. (2021). Vaccine excipient summary.
Centers for Disease Control and Prevention. (2022). Adjuvants and vaccines.
Centers for Disease Control and Prevention. (2022). What's in vaccines?
Children's Hospital of Philadelphia. (2021). Vaccine ingredients — fetal cells.
Mitkus, R. J., et al. (2011). Updated aluminum pharmacokinetics following infant exposures through diet and vaccination. Vaccine.
U.S. Food and Drug Administration. (2018). Thimerosal and vaccines.
U.S. Food and Drug Administration. (2019). Common ingredients in U.S. licensed vaccines.