Understanding Vaccine Ingredients

Vaccines work by safely teaching your immune system how to recognize and respond to a germ, without the risk that comes with getting sick. But how exactly do vaccines work? What are the different types of vaccines? And what’s in a vaccine anyway?

Here, we address some of the most common questions about vaccine ingredients.

This page has been verified for accuracy by a member of VYF’s Committee of Scientific and Medical Advisors, last updated February 2024.


A Vaccine's Journey Through the Body

Vaccines don’t cause long term side effects because all the ingredients of the vaccine are harmlessly flushed from the body within a week or so. All that remains is the immune system’s memory of how to fight infection.

Vaccine Basics

When germs invade the body, they attack and multiply. This invasion is called an infection, and the infection is what causes illness. The first time the body encounters a germ, it can take several days to make and use all the tools it needs to fight the infection. After the infection is over, the body’s immune system keeps memory cells that remember how to protect against that disease. If your body encounters the same virus or bacteria again, it will produce antibodies to attack the germ quicker and more efficiently.

Vaccines work by introducing a weakened, killed, or sub-part of a germ into the body that will mimic infection so that the body can make immunity without getting the disease. Vaccines safely imitate natural infection so that you will never have to risk getting sick with a serious, sometimes deadly disease.

Some people think that it’s better to acquire immunity naturally (being infected by the germ), however, many vaccine-preventable diseases can have dangerous or life-threatening consequences. Vaccines are much safer than getting the diseases they protect against.


There are several different kinds of vaccines, which include:

  • Live, attenuated vaccines contain a weakened form of the virus
  • Inactivated vaccines use the killed version of the germ that causes the disease
  • Toxoid vaccines prevent diseases caused by bacteria that produce toxins in the body
  • Subunit vaccines contain only a part of the germ that is best suited to stimulate the immune system
  • Conjugate vaccines use a harmless protein attached to the bacteria’s sugar coating that the immune system can recognize
  • mRNA vaccines contain germ mRNA to teach the cell how to make a protein the body will recognize as foreign
  • Viral vector vaccines use a modified version of a harmless virus to deliver genetic code to the body’s cells, which make proteins that the immune system can recognize

Scroll down for a full explainer on each of these types of vaccines!

All vaccines are monitored for safety, which includes the safety of the ingredients in vaccines. The main ingredients in vaccines are antigens, which is the substance that the immune system sees as foreign. Since the immune system knows this is not part of the body, it fights it off and then remembers it should the body encounter it again. Vaccine antigens are either altered, weakened, or killed so that they don’t cause disease.

Ingredients in vaccines have specific and necessary functions. They’re added in very small amounts and when the vaccines reach the body, the trace amounts of ingredients that are left are safe for humans. Most of the additional ingredients in vaccines are in our environment, things we eat and drink every day, and other medications that are safe for humans.

If someone has an allergy to a component in a vaccine, it’s important to let the person administering the vaccine so that they can be watched for any allergic reaction after getting the vaccine. Severe allergic reactions to vaccines are very rare.


Vaccine Ingredients And Their Purposes

Thimerosal is a preservative that is no longer found in childhood vaccines.

Vaccines that are drawn from multi-dose vials are required to include a preservative. A multi-dose vial is one that contains more than one dose of the vaccine. Every time a new needle is used to draw up a vaccine, the vial is susceptible to contamination with bacteria or fungi. Since 1968, the United States Code of Federal Regulations has required the addition of a preservative to multi-dose vials of vaccines so that they do not become contaminated. Tragic consequences have followed the use of multi-dose vials that did not contain a preservative (including deaths) and are the driving force for this requirement.

Thimerosal is a preservative that is used in multi-dose vials to prevent contamination with bacteria or fungus, which could be deadly. Thimerosal is an ethyl- mercury-containing compound that is one of the most widely used preservatives in vaccines since the 1930s. When the preservative is metabolized by the body, it breaks down into thiosalicylate and ethylmercury. This type of mercury is then broken down by the body and excreted quickly; it does not accumulate and cause harm.

Mercury is a naturally occurring element found in our environment. Some types of bacteria in the environment take mercury and change it into methylmercury, which is different from ethylmercury. Methylmercury works its way into the food chain in fish, animals, and humans, and at high levels can be toxic to people and cause damage to the nervous system. Over a lifetime, everyone is exposed to some level of methylmercury.

Even though there is no evidence that thimerosal is dangerous in vaccines, in 2001, thimerosal was taken out of childhood vaccines in the U.S. to reduce overall mercury exposure in infants. Vaccines containing thimerosal were either reformulated or put into single-dose vials.

Currently, there is an inactivated influenza vaccine that contains thimerosal, however, there are flu vaccines available that do not contain thimerosal. There’s no evidence that the thimerosal-containing flu vaccine is harmful, but it may cause minor reactions like swelling and redness at the injection site.

Measles, mumps, and rubella (MMR), chickenpox (varicella), inactivated polio (IPV), rotavirus, and pneumococcal conjugate vaccines (PCV) have never contained thimerosal as a preservative.

Watch Now: Is there mercury (thimerosal) in vaccines? Is that dangerous?

The Global Advisory Committee on Vaccine Safety, which provides independent, authoritative, scientific advice to the World Health Organization (WHO) on vaccine safety issues of global or regional concern, has concluded that there is no evidence of toxicity in infants, children, or adults exposed to thimerosal in vaccines.

No credible scientific studies have found an association between thimerosal in vaccines and autism. 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, visit our 

Visit the U.S. Food and Drug Administration (FDA) Thimerosal and Vaccines web page for more.

Formaldehyde is not only found in our environment, but it plays a part in our metabolism and is essential in the synthesis of DNA and amino acids (building blocks of proteins) that our bodies depend on every day. Humans have detectable levels of natural formaldehyde in their bloodstream all the time. In fact, many of our food and drink products contain formaldehyde, including some fruits, vegetables, dairy products, meats, seafood, and beverages like soda, tea, and alcohol.

Formaldehyde is used to inactivate (kill) viruses or detoxify bacterial toxins during the manufacturing process. At the end of the manufacturing process, the vaccine is purified, which removes all but a trace, safe amount of formaldehyde. In such trace amounts, formaldehyde is not dangerous. In fact, the amount of formaldehyde found naturally in an infant’s bloodstream is ten times higher than what is found in any vaccine. There is no evidence that the tiny amounts of formaldehyde found in some vaccines will cause harm.

Adjuvants are added to some vaccines to help the body make a stronger immune response to the germ in the vaccine. Vaccines made from weakened, killed, or components of germs may need adjuvants to help the vaccine work better. The adjuvant allows for less doses of the vaccine to be given for protection from the disease.

Vaccines with adjuvants may cause local reactions like redness, swelling, and pain at the injection site. Sometimes these vaccines create systemic reactions like fever, chills, and body aches. This happens more often than with vaccines that do not contain an adjuvant.

Studies show that vaccines containing adjuvants do not cause autoimmune disease. In fact, some studies found that patients who received large quantities of aluminum adjuvants in an immunotherapy treatment had lower incidence of autoimmune disease than in control patients.

Aluminum is one of the most abundant metals on earth, and found in plants, soil, water, air, household products, many of our foods and beverages, medicines, infant formula, breastmilk, building materials, and so much more. Aluminum salts have been used in vaccines since the 1930s and safety is continuing to be monitored.

Small amounts of aluminum salts are added to vaccines as adjuvants. Adjuvants help vaccines work better by aiding the body in producing a stronger immune response.

A study by the FDA shows risks due to aluminum exposure to the entire series of recommended childhood vaccines over the first year of life is very low. The same study showed that the benefits of aluminum-containing vaccines given during the first year of life outweigh any concerns of potential effects of aluminum on infants.

Video: Can you explain why adjuvants, like aluminum, are in some vaccines?

Antibiotics are sometimes used during the manufacturing process of vaccines to prevent bacterial contamination. This means that trace or undetectable amounts of antibiotics may still be present in some vaccines after purification of the vaccine takes place. The antibiotics that are most likely to cause severe reactions in people are not used in vaccine production. Antibiotics used may include neomycin, polymyxin B, streptomycin, and gentamicin.

Some vaccine manufacturing processes use chicken eggs, which means that trace amounts of egg proteins may be found in the final vaccine product. The vaccines that are grown in egg include measles, mumps, and rubella (MMR), rabies, yellow fever, the flu shot, and the flu nasal spray.

Recent studies show that people with a severe egg allergy can safely receive egg-grown vaccines.

CDC recommends that everyone 6 months and older with an egg allergy receive the annual flu vaccine. The Joint Task Force on Practice Parameters of the American Academy of Allergy Asthma and Immunology (AAAAI) and the American College of Allergy Asthma and Immunology (ACAAI) as well as the American Academy of Pediatrics (AAP) back this recommendation by stating that no special precautions are required for the administration of influenza vaccine to egg-allergic patients no matter how severe the egg allergy.

Even though it’s considered safe to vaccinate people with egg allergies with egg-grown vaccines, it’s important to let the doctor know so the vaccine recipient can be watched for rare symptoms of allergic reaction.

Gelatin is a protein that comes from boiling the skin or connective (usually porcine), that’s found commonly in foods like Jell-o, gummy candies, and marshmallows, among many others. The gelatin in vaccines protects the virus in the vaccine from conditions such as freeze-drying or heat during transport and delivery.

The incidence of anaphylaxis due to a severe allergic reaction to gelatin is very low (about 1 case/2 million vaccines given), however, people with a known severe allergy to gelatin should ask their healthcare provider before getting vaccinated with certain vaccines.

Religious leaders of the Muslim faith have ruled that the use of gelatin in vaccines  does not break religious dietary laws because it is highly purified and injected or inhaled rather than eaten. Similarly, according to Jewish laws, there is no problem with gelatin or animal-derived ingredients in non-oral products, including vaccines.

Polyethylene glycol (PEG) is a medication that has many pharmaceutical uses, most often to treat constipation. It’s also found in some vaccines as a stabilizer. mRNA vaccines, like some COVID-19 vaccines, contain PEG to surround and protect the mRNA as it’s transported to the cell.

Sometimes polyethylene glycol is confused with ethylene glycol, which is a very different compound and not found in vaccines. While PEG is safe to introduce into the body, ethylene glycol is not. Ethylene glycol is very toxic and is best known for its use in antifreeze. There is no antifreeze in vaccines.

There are no ingredients in vaccines that are toxic in the amounts they are given. The toxicity of any chemical depends on the amount that enters the body. For example, if you drink eight glasses of water a day, that is good for your body. However, if you drink eight liters of water in a day that would kill you. This is true for every chemical we encounter daily. The recommended dose may lead to good health, where the excessive dose can lead to death. There is not enough of any ingredient in a vaccine to cause harm to the body.

Each vaccine ingredient has a specific and necessary function to make the vaccine effective and safe. Not getting vaccinated is much riskier than getting a vaccine with trace amounts of safe, well-tested ingredients. Without vaccines, people are exposed to dangerous vaccine-preventable diseases and can spread them to others.

Watch These Short Videos from Vaccinate Your Family:

Fetal fibroblast cells collected from two elective pregnancy terminations in the 1960s are used in the production of the chickenpox, rubella, hepatitis A, rabies, and Janssen (Johnson & Johnson) COVID-19 vaccine (no longer used in the U.S.). The fetal cells that were obtained in the 1960s have continued to grow in laboratories and are still being used to make vaccines today. These cells have divided so many times for decades that they are now very far removed from the original cells taken from the fetus. No further sources of fetal cells have been used to make vaccines.

Human fetal cells are used because viruses that infect humans grow best in human cells (not animal cells). The fetal cells are also able to keep dividing and can be stored at very low temperatures for continued use.


Vaccine Types

Live, attenuated vaccines, such as MMR (measles, mumps, and rubella), chickenpox, and flu nasal spray (LAIV) lead to a strong and prolonged immune response. This type of vaccine contains a virus that has been weakened so that it cannot replicate enough in the body to cause illness. Since these types of vaccine are the closest to natural infection, they’re very effective and require fewer doses.

Unfortunately, not everyone can get a live, attenuated vaccine. Some people with compromised immune systems due to chemotherapy, HIV infection, or immune system disorders should not be given live, attenuated vaccines.

Inactivated vaccines use the killed version of the germ that causes the disease. This vaccine type doesn’t usually provide protection from the disease that’s as strong as live vaccines, so several doses may be needed over time for ongoing protection. Examples of inactivated vaccines include hepatitis A, influenza (shot only), polio (shot only) and rabies.

Toxoid vaccines prevent diseases caused by bacteria that produce toxins in the body. When making the vaccine, the toxoids are weakened so that they are not able to cause illness. DTaP and Tdap vaccines contains diphtheria and tetanus toxoids, in addition to protection against pertussis (whooping cough).

Subunit vaccines contain only a part of the germ that is best suited to stimulate the immune system. This type of vaccine can not cause illness. The pertussis (whooping cough) component of the DTaP vaccine is an example of a subunit vaccine, as is the Novavax COVID-19 vaccine.

Some bacteria have a sugar coating called polysaccharides. This coating is advantageous to the bacteria so that when it infects the body it allows for the bacteria to go unseen by the immune system. Because the body never sees the bacteria, it can cause disease.

A conjugate vaccine is made by attaching a portion of this polysaccharide-coated bacteria to a harmless protein. The body sees the protein as a foreign invader and will respond to that as well as the polysaccharide-coated bacteria. Should the body see the polysaccharide again as a natural infection, it will fight it off and reduce the likelihood of sickness.

Examples of conjugate vaccines are Haemophilus Influenza type b (Hib), meningococcal (MenACWY), and pneumococcal conjugate vaccines.

mRNA vaccines are new to the market but have been studied for decades. This type of vaccine contains messenger RNA (mRNA), which are regulatory molecules that show the cell how to make proteins. In the case of the vaccine, the mRNA teaches the cell how to make a protein from the germ. That protein is introduced to the immune system, which triggers a response, causing the body to develop immunity to the germ.

mRNA vaccines cannot affect or interact with your genetic material (DNA). The mRNA from the vaccine does not enter the nucleus of the cell where the DNA is located. Following the introduction of the germ’s protein to the immune system, the mRNA quickly breaks down and is eliminated from the body.

COVID-19 vaccines created by Pfizer-BioNTech and Moderna are mRNA vaccines. These vaccines do not contain any live virus and cannot give you COVID-19.

Video from American Academy of Pediatrics (AAP): https://youtu.be/YOlrNlvEiMw

Viral vector vaccines use a modified version of a harmless virus (the vector) to deliver genetic code to the body’s cells, which make proteins that the immune system can recognize as foreign invaders and trigger the immune system to protect itself against that disease.

Examples of a viral vector vaccine are an Ebola vaccine and Janssen (Johnson & Johnson) COVID-19 vaccine. As of May 2023, the Janssen (Johnson & Johnson) COVID-19 vaccine is no longer available in the U.S.


There has been a rise in vaccine hesitancy in faith communities. In 2019, the United States had large measles outbreaks with 75% of the cases in New York. The New York outbreaks in Rockland County, Brooklyn and Queens began after Orthodox Jews who were visiting Israel became infected with measles and brought the disease with them to the U.S. They then spread it to Orthodox communities that had pockets of unvaccinated people.

The following are statements and resources from the Jewish faith community about the importance of vaccination:

Due to the Catholic Church’s view on abortion, some people have been concerned about the use of cell lines that originated from terminated fetuses. Reviews by both the Vatican’s Pontifical Academy for Life, the National Catholic Bioethics Center, and the Catholic Church Bishops’ Conference of England and Wales have determined that vaccines grown in these cell lines do not go against the religion’s doctrine and using these vaccines are the best way to protect people from serious vaccine-preventable diseases.

Video: https://www.youtube.com/watch?v=MTKZYQ6BI5s

Christian Scientists do not believe in using any vaccines; however, when outbreaks have occurred in communities with Christian Scientists, some have agreed to be vaccinated. Their concerns are not related to the use of fetal tissue, but rather to the use of modern medical interventions.