The Safety and Science of Vaccine Ingredients

When it comes to myths about vaccines online, misinformation about vaccine ingredients is often at the forefront. Contrary to what you might read on blogs or social media, vaccines don’t contain toxins. In fact, many of the chemicals and substances found in vaccines are not only safe, they’re crucial to boosting your immune system against diseases, protect vaccines from contamination, and ensure they stay potent during storage and handling.

Parents holding baby girl while pediatrician prepares vaccination
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What Ingredients Are In Vaccines

Vaccines contain a combination of ingredients to help them do their jobs, stay potent, and prevent contamination. These include:

  • Antigens: The part of the vaccine that prompts the body to make antibodies and develop immunity against a particular germ. Sometimes, this component is a whole virus or bacteria that has been weakened or inactivated (“killed”) in the laboratory, while other vaccines are made using small pieces of the germ or something it makes (like a protein).
  • Suspending Fluid: Fluids like sterile water or saline used to suspend other vaccine components. 
  • Adjuvant: Ingredients that help the body make a stronger immune response to the vaccine, allowing the vaccines to be given in fewer or smaller doses.
  • Preservative or Stabilizers: Ingredients that protect the vaccine from temperature changes, sunlight, contaminants, or other environmental factors that could make the vaccine less safe or effective.
  • Culture Material: Materials left over from the manufacturing process.  

What Science Says About Vaccine Ingredients

For those concerned about the various ingredients found in vaccines, it can be helpful to dive into what these substances actually are, why they’re there, and — most importantly — how the human body responds to them. 

Here are some examples of things found in some vaccines and what research says about their safety.


When people think of mercury exposure, they often think of the kind found in tuna and other big fish that can build up in the body and cause serious health concerns, including brain damage. This type is called methylmercury, and it has never been included in vaccines.

The vaccine ingredient thimerosal, however, uses ethylmercury, a different type of mercury that is processed much more quickly by the body than methylmercury. It doesn’t accumulate, and it doesn’t cause harm. The difference between the two is a lot like the difference between ethyl alcohol (or ethanol) and methyl alcohol (or methanol). Ethanol you can safely drink in a cocktail, while methanol is used in gasoline and antifreeze.

Thimerosal was used for decades to protect vaccines from contamination. Many vaccines used to be sold in multi-dose vials, and every time a needle was inserted into the vaccines, it risked introducing microbes like bacteria or fungi into the vaccine and causing serious infections in those subsequently receiving the vaccine. Thimerosal protected against these microbes and, as a result, made some vaccines safer to use.

The ingredient was removed from childhood vaccines in the early 2000s out of an abundance of caution and is now only in a small number of flu vaccines. Even so, studies looking at the safety of thimerosal-containing vaccines show them to be safe and don’t affect a child’s development or risk of autism spectrum disorder.


Aluminum salts are sometimes used in vaccines as an adjuvant — a substance added to a vaccine to make it more effective. Adjuvants help the body have a stronger, more effective immune response, which allows the vaccines to be given in fewer or smaller doses or contain fewer antigens (the parts of a germ that the body reacts to). In short, adjuvants make vaccines safer and more effective.

Aluminum salt is by far the most common adjuvant used in vaccines. It has been included in vaccines for more than 70 years, and more than half a century of research shows it to be safe. We have more years of safety data on aluminum in vaccines than we have for Tylenol.

As one of the most common elements on the planet, aluminum is everywhere, including in the air we breathe, the food we eat, and the water we drink. That’s perhaps why the human body is able to process aluminum very quickly. A person (even a small child) would need to be exposed to very large quantities of aluminum — far more than what’s found in vaccines — in a short period of time before they are likely to experience any harmful effects from it.


Antibiotics are sometimes used in the manufacturing or storage process to protect vaccines from contamination. As a result, trace amounts of antibiotics can be found in some vaccines. While some people are allergic to antimicrobial medications like penicillin or cephalosporins, these antibiotics are not in vaccines, and the tiny amounts of the medications that are used don’t appear to cause severe allergic reactions.

Even so, those with a life-threatening allergy to antibiotics should talk with their doctors before receiving a new vaccine, just to be sure it is not included. 

Egg Protein

Vaccine manufacturers sometimes use eggs to grow the weakened or inactivated viruses used in vaccines, and that can lead some vaccines to have a small amount of egg protein in them. Individuals able to eat chicken eggs or egg-containing products safely shouldn’t have any problem with egg-containing vaccines.

Currently, egg protein is only found in the yellow fever vaccine (recommended only for travelers or those living in places where the virus is common), as well as most flu vaccines. Because of the risks posed by both yellow fever and flu, however, many individuals with egg allergies — even severe ones — can still be vaccinated. In addition, advances in technology have significantly reduced the amount of egg protein used for the influenza vaccine, making it safe for people with egg allergies.


Scientists use formaldehyde to inactivate (or “kill”) germs used in vaccines to make them safer and less harmful. Large amounts of formaldehyde can cause damage to DNA, but the amount found in vaccines is well within the safe range. Almost all of the formaldehyde is removed before the vaccine ever makes it into its packaging, leaving only trace amounts behind.

Like aluminum, formaldehyde is a naturally-occurring substance, and it’s essential for certain body processes like metabolism. As a result, formaldehyde is already present in the human body — and in quantities a lot greater than in vaccines. According to the Children’s Hospital of Philadelphia, a 2-month-old child likely has 1,500 times more formaldehyde already circulating in their body than they would receive from any one vaccine.

Monosodium glutamate (MSG)

Certain vaccine components can change if they’re exposed to environmental factors like too much heat, light, or humidity. So scientists add stabilizers like MSG or 2-phenoxy-ethanol to keep them safe and effective.

While some people report experiences like headaches or drowsiness after consuming MSG, there’s little scientific evidence to back up many of the claims. One report by the Federation of American Societies for Experimental Biology found that some sensitive individuals experienced mild, short-term symptoms — but only after taking in 3 grams of MSG without food. That’s more than 4,000 times greater than the amount found in any one vaccine.


Like MSG, gelatin is sometimes used as a stabilizer to protect vaccine components from damage due to light or humidity. Gelatin is the most common cause of severe allergic reactions to vaccines, but serious reactions like anaphylaxis are extraordinarily rare. Instances happen only in about one out of two million doses.

Aborted Human Fetal Tissue

The germs used to make vaccines are typically grown in a laboratory using animal cells (like those found in chicken eggs), but some are made using human cells — specifically, fetal embryo fibroblast cells, the cells responsible for holding skin and tissue together.

Viruses can be tricky to grow in a lab; they need cells to survive and replicate, and human cells tend to work better than animal cells. Fetal embryo cells can also divide many more times than other kinds of human cells, making them ideal candidates for growing vaccine viruses.

Back in the 1960s, scientists obtained fetal embryo cells from two pregnancies that were electively terminated, and they used them to grow weakened or inactivated forms of viruses to use in vaccines. The same cells have continued to grow and divide ever since, and they’re the same exact cell lines that are still used to make some modern vaccines — specifically vaccines against rubella, chickenpox, hepatitis A, shingles, and rabies. The original babies weren’t aborted to create the vaccines, and no new abortions or fetal tissue are necessary to make these vaccines today.

Some individuals who oppose abortion on religious grounds also oppose using these vaccines because of how they were first created. It should be noted, however, that many religious leaders have issued statements supporting the use of the vaccines. In its statement, the Catholic Church, for example, gave families the OK to vaccinate their children despite the vaccines’ history “in order to avoid a serious risk not only for one's own children but also, and perhaps more specifically, for the health conditions of the population as a whole — especially for pregnant women.” 

How Vaccine Ingredients Are Tested for Safety

It’s not easy to sell vaccines. In order to get approved for use in the United States and elsewhere, vaccine manufacturers have to show substantial evidence that their vaccines are safe and effective. The whole process often takes years and involves several phases of clinical trials in hundreds (if not thousands) of people. As a result, vaccines are one of the most highly tested medical products on the market, undergoing more safety testing than some medications and far more than nutritional supplements or vitamins.

Stages of Vaccine Testing 

There’s a certain process all vaccines have to go through before they can go on the market, and safety is a deal-breaker. If at any point during the process the vaccine doesn’t appear to be safe, it doesn’t move on to the next phase.

Exploratory Stage

Long before a vaccine can be tested in people, researchers first have to figure out what ingredients to include and in what quantities. Finding an effective antigen is one of the toughest parts of developing a vaccine, and the process can often take years before a successful candidate is identified.

Pre-Clinical Studies

Once a vaccine looks promising, it’s then tested in cell or tissue cultures or animal hosts to verify that it’s safe and can activate the body’s defenses. This stage allows researchers a chance to see how the human body might react to the vaccine before it’s tested in humans and tweak the formulation if need be. It can also give researchers an idea of what a safe dose might be in humans and the best and safest way to administer it (ex. injected in the muscle versus under the skin).  

This stage can also last years, and many vaccines don’t make it past this point.

Clinical Trials

Once vaccines look to be safe and effective in the lab, they’re tested on people. This stage takes place in at least three phases.

  • Phase I: The first phase tests the vaccine in a small group of adults (typically between 20-80 people) to see whether it provokes any side effects and determine how well it prompts an immune response. If the vaccine is meant for kids, researchers will progressively test the vaccine in younger and younger individuals until they reach the intended age group. Only vaccines that do well in phase I can progress to phase II.
  • Phase II: The next phase of clinical trials tests the vaccine in hundreds of people. These studies randomly assign some people to get the vaccine, while others get a placebo. The primary purpose of these studies is to evaluate the safety and effectiveness of the vaccine, as well as the best dosage, schedule of doses, and route of administration. 
  • Phase III: By the time vaccines reach phase III clinical trials, the vaccine has been undergoing safety testing for years. Researchers already have a pretty good idea of how safe and effective the vaccine is, including what side effects might occur, but they still need to see how a wide variety of people respond to the vaccine and how it compares to the status quo — that is, other vaccines typically given in that group of people or a placebo (if no vaccine is available). These studies test the vaccine in thousands —sometimes tens of thousands — of people and typically take place in areas or groups at higher risk for the disease or condition.

If (and only if) these studies can demonstrate the vaccine is safe and effective, it can then go through the process of getting approved by the U.S. Food and Drug Administration (FDA) or other countries’ governing bodies.

Post-Licensure Safety Monitoring

Safety testing doesn’t stop once a vaccine is approved for use. Researchers continuously monitor vaccines to ensure the benefits of the vaccine outweigh any risks.

In the United States, health officials rely on four primary methods to keep tabs on vaccine safety: inspections, phase IV clinical trials, the Vaccine Adverse Event Reporting System (VAERS), and the Vaccine Safety Data Link.

  • Inspections: Health officials routinely inspect factories where vaccines are manufactured and review or conduct tests on batches to verify that they are potent, pure, and safe.
  • Phase IV Clinical Trials: These studies use many of the same processes as phase III clinical trials to evaluate any safety concerns, effectiveness, or alternative uses for the vaccine.
  • Vaccine Adverse Event Reporting System (VAERS): VAERS is a reporting tool for anyone to report any adverse (or unwanted) event that happens after vaccination, even if they aren’t sure the vaccine caused it. This system is then used by researchers to spot any risks from a vaccine that might have been too rare to catch during the pre-licensure clinical trials.
  • Vaccine Safety Datalink (VSD): A collection of databases used to study adverse events after vaccination. The information is collected in real-time from patients across the country, making the VSD particularly valuable when studying the effects of new vaccines.

These aren’t the only systems used to monitor vaccine safety. The FDA, Centers for Disease Control and Prevention, and collaborating researchers utilize a collection of systems to spot potential safety issues.

A Word from Verywell

Vaccine ingredients are extensively tested for safety during all stages of development, and they continue to be tested for as long as they’re in use. While some things found in vaccines might sound scary, a closer look at the research shows them not only to be safe, but also to help make vaccines safer or more effective.

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