Communicating about Vaccine Ingredients

In October 2025, the U.S. Centers for Disease Control and Prevention (CDC) and the Advisory Committee on Immunization Practices (ACIP) established a new working group to assess the childhood vaccine schedule, with a specific focus on the safety of vaccine ingredients and aluminum adjuvants. Use these up-to-date messages to effectively communicate about vaccine ingredients, particularly aluminum and thimerosal, with your community.

About Vaccine Ingredients

What ingredients are typically in vaccines?

Vaccines contain antigens, a vaccine ingredient that instructs the body how to protect against infection. Antigens come in several forms, including:

Inactivated viruses: found in polio, hepatitis A, and some influenza vaccines. They prompt the body to create an immune response and build protection against disease, without causing illness.
Weakened live viruses: used in measles, mumps, and rubella (MMR), chickenpox (varicella), and one type of flu vaccine. The viruses are too weak to cause disease but can cause a strong immune response.
Virus protein subunit: fragments of a dead virus that may be used in hepatitis B and HPV vaccines to activate the body’s immune system.
Bacterial component: some vaccines contain weakened or killed bacteria, and others contain bacterial toxins that are treated to be non-toxic. Bacteria components in vaccines activate the body’s immune system without causing disease.

Aside from antigens, some vaccines may also contain ingredients intended to keep a vaccine usable for longer periods of time but have no impact on the body, like:

Preservatives, which keep vaccine containers from being contaminated.
Additives keep a vaccine effective while it is transported and stored. Common additives are gelatin, sucrose, lactose, and glycine.

Vaccine ingredients made from weakened, killed, or components of viruses may require adjuvants to enhance the vaccine’s effectiveness. Adjuvants are found in hepatitis A, hepatitis B, HPV, and other routine vaccines. Adjuvants help reduce the number of vaccine doses needed to strengthen the immune response.

A Closer Look: Aluminum Adjuvant

What is an aluminum adjuvant?

An aluminum adjuvant, or aluminum salts, is an ingredient that strengthens the immune response to a vaccine. Aluminum salts are found in soil, plants, water, air, and even some foods. Aluminum salts have been added to vaccines in the United States for over 100 years, helping billions of people prevent disease.

How much aluminum adjuvant is in a single vaccine?

Vaccines that use an aluminum adjuvant contain a microscopic amount of aluminum salts. Adults typically ingest seven to nine milligrams of aluminum salts daily through food and water intake without any harm to their health., A single vaccine dose typically contains less than one milligram of aluminum salts.

Do aluminum adjuvants cause autism?

No, there is no evidence that aluminum adjuvants cause autism. Billions of people have received vaccines containing aluminum salts so they can prevent disease and thrive. Aluminum is the third most abundant element, just behind oxygen and silicon. Adults are exposed to aluminum salts every day through water, air, and food. Infants are also exposed to aluminum salts through breast milk, regular formula, and soy-based formula. Ingesting aluminum salts in small amounts is common and has no effect on human health. Thus, vaccines containing tiny amounts of aluminum salts have no health risks for infants, children, and adults.

Can scientists create vaccines without aluminum?

Vaccines that contain aluminum salts are essential in protecting against diseases that can cause severe illness or death. Small amounts of aluminum salts in vaccines strengthen individual immunity and community resilience against disease.

Vaccines are currently developed, tested, and monitored as part of a phased and rigorous process. Developing vaccines without aluminum salts would require years of safety tests and hundreds of millions of dollars. Developing a new vaccine, when vaccines proven to prevent diseases and are safe already exist, could leave a critical gap in disease prevention and put infants, young children, and adults at risk.

A Closer Look Thimerosal

What is Thimerosal?

Thimerosal, also known as ethylmercury, is a preservative used in some types of influenza vaccines that the human body can process with minimal risk. Ethylmercury is sometimes confused with methylmercury, a higher-risk mercury compound that is not used in vaccines. Unlike methylmercury, ethylmercury does not accumulate in the body.

Understanding Thimerosal Use

Vaccines are contained in a small glass bottle called a vial, which can either hold a single dose or multiple doses of a vaccine. A single-dose vial is used for a single injection in one person, while a multiple-dose vial contains multiple vaccine doses and can be used for more than one dose administration. Preservatives, like thimerosal, may be used in multiple-dose vials to prevent the growth of bacteria or fungi once the vial is opened.

Flu and Pediatric Vaccines

Thimerosal is used in a small percentage of multiple-dose flu vaccines. Most flu vaccines are administered from a single-dose vial – only 4% of flu doses are packaged in multiple-dose vials. While used infrequently in vaccines, extensive research demonstrates that thimerosal is a safe additive. Thimerosal is not used in any vaccine on the AAP’s childhood vaccination schedule.

Thimerosal Safety

There is no scientific evidence that thimerosal is harmful or causes autism. In 1999, as a precaution, the American Academy of Pediatrics, the Public Health Service agencies, and vaccine manufacturers agreed to reduce or eliminate thimerosal from vaccines. As a result, thimerosal has not been used in any vaccine on the AAP’s childhood vaccination schedule since 2001.

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