You may have heard that vaccines contain all types of crazy ingredients that sound as though they don’t belong in a medical product. The truth is that a very small group of very vocal, but misinformed, individuals have made false accusations regarding the safety of vaccines and their ingredients. In most instances these allegations are just incorrect. In other cases, the claims are misinformed or taken out of context.

The main ingredients in vaccines are antigens, which are small amounts of the bacteria or virus against which the person is being vaccinated. Antigens are the parts of the vaccine that encourage your immune system to create antibodies to fight against future infections. To make sure that the vaccines cannot cause the disease you are trying to protect against, the antigens are altered or weakened. Learn more about how vaccines are made and how they work.

Like many of the foods we eat and beverages we drink, vaccines also contain a small amount of additional ingredients, and each has a specific, necessary function. These ingredients may be added to the vaccine to make it more effective, sterile and/or safe. These additional ingredients have been studied and are safe for humans in the amount used in vaccines.

In fact, the amount of these additional ingredients in vaccines is much less than children encounter in their environment, food and water. As the saying goes, “the dose makes the poison.” In other words, any chemical – even water or oxygen – can be toxic or even deadly in large enough quantities.

Sometimes a child may be sensitive to one of the components of a vaccine, and an allergic reaction may result. For this reason, you should discuss any allergies your child may have with your healthcare provider. Below are the ingredients that may be found in certain vaccines and their purpose:

Preservatives

When an individual vaccine dose is drawn from a multi-dose vaccine vial with a fresh needle, it is possible for bacteria or fungus to get into the vial, which could be very dangerous. To prevent this contamination, a preservative is added to the vaccine.

Thimerosal

Thimerosal is an ethylmercury-based preservative used in multi-dose vials of vaccines to prevent contamination. If someone were to receive a vaccine contaminated with bacteria, it could be deadly.

Ethylmercury is very different than methylmercury, which is found naturally in the environment, and what people commonly think of when they hear the word “mercury”. The low levels of ethylmercury in vaccines are broken down by the body differently and clear out of the blood more quickly than methylmercury. A buildup of methylmercury in the body is usually due to eating certain types of fish or other food, and high amounts can harm the nervous system. Over a lifetime, everyone is exposed to some methylmercury.

Even though there was no evidence that thimerosal in vaccines was dangerous, in 1999, it was removed from vaccines in the U.S. in an effort to reduce overall mercury exposure among infants. To keep vaccines safe from contamination without the use of thimerosal or other preservatives, they were either reformulated or put into single-dose vials. Now, the only vaccines in the U.S. that use thimerosal as a preservative are flu vaccines in multi-dose vials. (Thimerosal-free, single-dose vials of flu vaccine are also available in the U.S..) Thimerosal in multi-dose vials is necessary because each time an individual dose is drawn from a multi-dose vial with a new needle and syringe, there is the potential for contamination. There is no evidence that the small amounts of thimerosal in flu vaccines cause any harm, except for minor reactions like redness and swelling at the injection site.

To learn more about the thimerosal content in FDA-approved seasonal flu vaccines, visit the Thimerosal and Vaccines page on the the Food and Drug Administration (FDA) website. MMR (measles, mumps, and rubella), chickenpox (varicella), inactivated polio (IPV), and pneumococcal conjugate vaccines (PCV) NEVER contained thimerosal as a preservative.

No credible scientific studies have found an association between thimerosal in vaccines and autism. The studies used different methods to find out if there was any connection between thimerosal and autism. Some examined rates of autism a state or a country, comparing autism rates before and after thimerosal was removed as a preservative from vaccines. It is important to note that in the U.S. and other countries, the number of children diagnosed with autism has not gone down since thimerosal was removed from childhood vaccines. To read the studies yourself, visit our Vaccine Research page.

Formaldehyde

Formaldehyde is a colorless gas that is a byproduct of metabolism so it is already present in the human body. People also encounter formaldehyde every day in the environment. It is used in making building materials and many household products, and formaldehyde also gets into the air through car tailpipe emissions.

Formaldehyde is used is used to inactivate bacterial products for toxoid vaccines (DTaP and Tdap), and to kill unwanted viruses and bacteria that might contaminate the vaccine during the manufacturing process. Most formaldehyde is removed from the vaccine before it is packaged. In trace amounts, formaldehyde is not dangerous. In fact, there is more formaldehyde in a pear than in any one vaccine.

Aluminum

For over 75 years, aluminum gels or salts have been added as adjuvants to some vaccines to improve the vaccine’s abilities to help stimulate the body’s immune system. Without the use of an adjuvant, healthcare providers would need to give more shots in a vaccine series or face lower immunity from the vaccine, and therefore less protection from the disease. Aluminum is also commonly found in food, water, infant formula and even breast milk.

Adjuvants

Adjuvants, such as aluminum, are added to certain vaccines to help trigger a better immune response. Without the adjuvant, we would need to administer more shots in the vaccine series or face lower immunity from the vaccine and therefore, less protection from the disease. Studies have shown there is no connection between adjuvants and the development of autoimmune diseases.

Residual Antibiotics

Antibiotics are added to some vaccines to prevent the growth of bacteria during the vaccine manufacturing process and storage of the vaccine. No vaccine produced in the United States contains penicillin.

Egg Protein

Some vaccines, like MMR and some flu vaccines, are prepared in eggs which means that some egg proteins are present in the final vaccine product. The egg proteins help manufacturers to grow enough of the virus or bacteria needed to make the vaccine. Based on scientific evidence, the American Academy of Pediatrics (AAP) states that the MMR vaccine can be safely given to all patients with egg allergies, including patients with a history of severe, generalize anaphylactic reactions to eggs. AAP also states that “children with egg allergy can receive influenza vaccine with no additional precautions than those considered for any vaccine.” Additionally, the American Academy of Allergy, Asthma & Immunology (AAAAI) states “Studies show that flu vaccines can be safely administered to egg allergic individuals.” If you or your child are allergic to eggs, make sure to tell your doctor or healthcare provider before getting vaccinated.

Gelatin

Some vaccines contain gelatin to protect them against freeze-drying or heat during the transportation and storage process. People with severe allergies to gelatin should talk to their doctor or healthcare provider before getting vaccinated.

Antifreeze

You may have heard that vaccines contain products such as antifreeze. This is not true. Antifreeze typically contains ethylene glycol, an unsafe and highly toxic (poisonous) component, or propylene glycol, a safer and less toxic option to ethylene glycol. Neither of these members of the glycol family of compounds is used in vaccines. In vaccines, polyethylene glycol is used to inactivate the virus in some influenza vaccines and is also used to purify other vaccines. Polyethylene glycol is approved by the FDA and considered non-toxic for medical and other uses.* It is used in a variety of products including skin cream, toothpaste, lubricating eye drops, laxatives, and as an anti-foaming agent in food. It is also used as an irrigating solution in surgical procedures.

* Victor O. Sheftel (2000). Indirect Food Additives and Polymers: Migration and Toxicology. CRC Press, 1114-1116.

Read more about ingredients in vaccines and why they are in there.

Get more of your questions about vaccines answered by visiting our Questions About Vaccines section.