What’s in a Vaccine?

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 bacterium again, it will produce antibodies to attack it more quickly and efficiently.

Vaccines work by introducing a weakened, killed, or part of a germ into the body, mimicking an infection so the body can build 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.

Resources:

• LetsGetRealAboutVaccines.org: The Real Science Behind Childhood Vaccines
• VaccineInformation.org: Vaccine Basics
• CHOP Vaccine Education Center: The Story of Chip and Dale

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, including the safety of their ingredients. The  main ingredients  in vaccines are  antigens , which are the substances 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 found in our environment, in the things we eat and drink every day, and in other medications that are safe for humans.

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

Resources:

• CHOP Vaccine Education Center: Q&A Vaccine Ingredients: What You Should Know (view in Spanish )
• VaccineInformation.org: Vaccine Basics

Thimerosal is a preservative that has not been in childhood vaccines since 2001, and was removed from all US vaccines as of mid-2025, despite no scientific evidence of harm.

Thimerosal, a preservative, was used in multi-dose vial vaccines that contained multiple doses in each vial. 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 to prevent contamination. 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 an ethylmercury-containing compound —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 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 bacteria in the environment take up mercury and convert it to methylmercury, which differs from ethylmercury. Methylmercury enters the food chain through fish and other animals and, at high levels, can be toxic to people and damage the nervous system. Over a lifetime, everyone is exposed to some level of methylmercury.

Although there is no evidence that thimerosal is dangerous in vaccines, in 2001 it was removed from childhood vaccines in the US to reduce overall mercury exposure in infants. Vaccines containing thimerosal were either reformulated or put into single-dose vials.

Measles, mumps, and rubella vaccine (MMR), chickenpox vaccine (varicella), polio vaccine (IPV), rotavirus vaccine , and pneumococcal conjugate vaccine (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 US 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 

Formaldehyde is found not only in our environment but also plays a role in our metabolism and is essential for the synthesis of DNA and amino acids (the building blocks of proteins) that our bodies depend on every day. Humans have detectable levels of natural formaldehyde in their bloodstream at all times. 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, removing 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 require adjuvants to help them work better. The adjuvant allows fewer doses of the vaccine to provide protection against the disease.

Vaccines with adjuvants may cause local reactions like redness, swelling, and pain at the injection site. Sometimes these vaccines cause systemic reactions such as 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 a lower incidence of autoimmune disease than 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 vaccines use chicken eggs or embryo during the manufacturing process, which means that trace amounts of egg proteins may be found in the final vaccine product. These vaccines include:  

• Measles, mumps, and rubella  (MMR) – grown in chick embryo  fibroblast cells , and contains no egg allergen because they are not grown in eggs.
• Rabies  –  some are grown in chick embryo. 
• Yellow fever  – grown in chick embryo.
• Flu shot, and the flu nasal spray  – most flu vaccines are  produced using eggs  and may contain trace amounts of egg proteins. There are flu vaccines that are not made using egg-based technology.  

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 monitored for rare symptoms of allergic reaction.

Gelatin is a protein derived from boiling skin or connective tissue (usually porcine) and is commonly found in foods like Jell-O, gummy candies, and marshmallows, among many others. The gelatin in vaccines protects the vaccine’s contents 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 perspectives on gelatin in vaccines vary. Some leaders of the Muslim faith have ruled that vaccines containing porcine gelatin are permissible due to medical necessity, the purification process, and/or because they are injected rather than eaten. However, other Islamic authorities consider porcine gelatin impermissible even in vaccines, and many Muslims prefer gelatin-free vaccine alternatives when available.

Similarly, according to the Jewish faith , holy laws can be broken to save a life. By vaccinating, we protect ourselves and others; Therefore, vaccines are life-saving tools.

This link goes to the same Muslim document. I couldn’t find a lot about this, but here is a video that is good: https://www.youtube.com/watch?v=EW5g_1NlyLw

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

Sometimes polyethylene glycol is mistaken for ethylene glycol, a very different compound 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, drinking eight glasses of water a day is good for your body. However, drinking eight liters of water a day would kill you. This is true for every chemical we encounter daily. The recommended dose may lead to good health, while an 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:

• Is there mercury (thimerosal) in vaccines? Is that dangerous?
• I’ve heard there are ingredients in vaccines that can harm children. Is this true?
• Is the Aluminum Added to Some Vaccines Safe?
• The Truth About Thimerosal in Vaccines

Fetal fibroblast cells collected from two elective pregnancy terminations in the 1960s are used to produce the chickenpox, rubella, hepatitis A, rabies, and Janssen (Johnson & Johnson) COVID-19 vaccines (no longer used in the US). The fetal cells obtained in the 1960s have continued to grow in laboratories and are still used to make vaccines today. These cells have divided so many times over decades that they are now very different 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 better in human cells (not animal cells). Fetal cells can also continue dividing and can be stored at very low temperatures for continued use. It is important to note that no fetal tissue is present in the final vaccine product.

Resource :

• CHOP Vaccine Education Center: Fetal Cells and Vaccines: Common Questions Answered

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 weakened virus that cannot replicate enough in the body to cause illness. Since these vaccines are 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. Live, attenuated vaccines are also not recommended during pregnancy.

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 they cannot cause illness. DTaP and Tdap vaccines contain diphtheria and tetanus toxoids, as well as 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 cannot cause illness. The pertussis (whooping cough) component of the DTaP vaccine is a subunit vaccine, as is the Novavax COVID-19 vaccine.

Some bacteria have a sugar coating called polysaccharides. This coating benefits the bacteria, allowing them to infect the body without being detected 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 bacterium to a harmless protein. The body treats the protein as a foreign invader and responds to it as it would to polysaccharide-coated bacteria. If the body recognizes the polysaccharide as a natural infection, it will fight it off, reducing 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 molecules that instruct 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, triggering a response that causes 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 viruses 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 activate the immune system to protect itself against that disease.

Examples of viral vector vaccines include an Ebola vaccine and the Janssen (Johnson & Johnson) COVID-19 vaccine. As of May 2023, the Janssen (Johnson & Johnson) COVID-19 vaccine is no longer available in the US

There has been a rise in vaccine hesitancy in faith communities. In 2018–19, a large meals outbreak occurred in New York City after an unvaccinated child returned from Israel. A total of 649 cases were confirmed, with 93% of the cases being among the Orthodox Jewish community, with pockets of unvaccinated people.

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

• Orthodox Union: Statement on Vaccinations from the OU and Rabbinical Council of America (11/14/18) – strongly urges parents to vaccinate their children based on the recommended vaccination schedule
• The Yeshiva World: Rav Moshe Sternbuch Writes Letter To Rav Malkiel Kotler about The Halachic Requirement To Vaccinate (11/27/18) – Statement that vaccines are effective to prevent spread of disease and it’s an obligation to vaccinate children
• Orthodox Jewish Nurses Fighting Measles Outbreak One Living Room at a Time (3:30 audio)
• 2005 Rabbinical Assembly (RA), the international association of Conservative rabbis, ruling: Compulsory Immunization in Jewish Day Schools (12/7/05) – allows Jewish day schools to make immunizations compulsory in accordance with halacha (Jewish law)
• The Yeshiva World: Hatzolah in NYC Joins Fight Against The Measles Outbreak – Get Vaccinated Now (4/15/2019)
• The Union for Reform Judaism’s (URJ ) Resolution on Mandatory Immunization Laws – states Jewish religious tenets prioritize protecting the health of all individuals, the most medically vulnerable members of the community, and the community as a whole, and do not provide a basis for a religious exemption from mandatory immunizations
• Yeshiva: Ask the Rabbi: Are Vaccinations Kosher? – According to  Rabbi David Samson , “there is no prohibition in using medicines which contain forbidden ingredients if they are administered by injection, suppository, enema, medicated bandage, and the like, since they are not eaten.”
• New York Times : Religious Objections to the Measles Vaccine? Get the Shots, Faith Leaders Say (4/26/19)
• New York Times : My Fellow Hasidic Jews Are Making a Terrible Mistake About Vaccinations (4/23/19)
• Kveller : These Amazing Orthodox Nurses Are Fighting Measles in Their Community (4/2/19)
• Judaism and Science: A Nice Jewish Shot: Why Vaccinations are Kosher and Required (6/19/14)

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 is the best way to protect people from serious vaccine-preventable diseases.

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

Christian Scientists generally discourage the use of 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.

Resources:

• Catholic Church Bishops’ Conference of England and Wales: The Catholic Position on Vaccination
• My Catholic Doctor: The Catholic Answer to Vaccines
• Catholic Answers: Can Catholics Receive the COVID-19 Vaccine? (video)
• CHOP Vaccine Education Center: Fetal Cells and Vaccines: Common Questions Answered
• CHOP Vaccine Education Center: Are Fetal Cells Used to Make Vaccines? (Video)