The Safety and Science of Vaccine Ingredients

How Vaccine Safety Is Ensured in the United States

There are a lot of myths and misconceptions about the safety of vaccines. Some are relatively common, like claims that the flu shot causes flu. Others—like the COVID-19 vaccines "change your DNA"—are absurd, yet they get shared repeatedly on social media.

Parents holding baby girl while pediatrician prepares vaccination
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Contrary to what you hear, vaccines (including those that contain the word "toxoid," like the tetanus toxoid vaccine) are not toxic. Both the active and inactive ingredients are rigorously tested and regulated to ensure that the benefits of vaccinations far outweigh any potential risks.

In fact, many of the substances used in vaccines are not only safe, they’re crucial to boosting your immune response, protecting the vaccine from contamination, or ensuring that the vaccine remains potent during storage and handling.

Vaccines protect not only the person who gets them, but their community as well. Learning the truth behind vaccine safety (from reliable sources like those incorporated here) can help you feel more confident about your choice to vaccinate.

Vaccine Components

Vaccines differ in their mechanism of action and the types of active and inactive ingredients they contain. Broadly speaking, a vaccine will contain some or all of the following:

  • Antigens: Antigens are any substance that the body recognizes as harmful. By introducing it into the body, the immune system develops defensive antibodies specific to that antigen. The vaccine may be a weakened live virus, a killed virus or bacteria, or a specific piece of a virus or bacteria.
  • Suspending fluid: This is the carrier fluid that delivers the vaccine into the body either through injection, intranasal spray (such as FluMist nasal flu vaccine), or oral drops (such as RotaTeq rotavirus vaccine). The suspending fluid may be water, saline, oil, or some other sterile liquid.
  • Adjuvant: Adjuvants are trace ingredients used in some vaccines that provoke a stronger immune response to the vaccine. This allows the vaccine to be given in fewer or smaller doses. Vaccines that use an adjuvant are called adjuvanted vaccines.
  • Preservative or stabilizers: To ensure that vaccines can be shipped and administered safely to large sectors of the population, they typically include preservatives or stabilizers that protect them from contamination, temperature changes, sunlight, and other environmental factors that can do them harm.

In addition to ingredients that serve a direct purpose, there may be substances—referred to as cell culture materials—that remain in trace amounts after vaccine production. An example is egg proteins from fertilized chicken eggs used to make most flu vaccines.

Specific Ingredients

When reading a vaccine label, you may be startled by ingredients that you assume to be harmful to the human body. Invariably, these substances are either altered so that they pose no risk to the body or delivered in trace amounts so that they neither accumulate nor cause toxicity.

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


Many vaccines used to be sold in multi-dose vials. Every time a needle was inserted into the vial, it risked introducing bacteria, fungi, or other harmful microbes into the vaccine.

Thimerosal—a type of mercury—was added to protect against these microbes and, as a result, make vaccines safer to use.

The idea of this has raised some eyebrows over the years, as many have worried that thimerosal would build up in the body and pose the same serious risks as mercury obtained from the consumption of ocean fish.

However, the type of mercury found in fish is called methylmercury, which has never been used in vaccines.

Thimerosal uses ethylmercury, a different type of mercury that is processed more quickly by the body than methylmercury. It neither accumulates nor causes harm.

Still, and despite thimerosal's decades-long use in vaccines, there has been a move away from multi-use vials in vaccinations. Out of an abundance of caution, thimerosal was removed from childhood vaccines in the early 2000s and is now only found in a small number of flu vaccines.


Antibiotics are sometimes used in the manufacturing and storage of vaccines to protect them from contamination.

Allergy to penicillin or cephalosporins is not a concern, as they are not used in any vaccine. The trace amounts of antibiotics that are used—typically neomycin, polymyxin B, streptomycin, and gentamicin—don’t appear to cause severe allergic reactions.

Even so, people with a life-threatening allergy to antibiotics should talk with their doctors before receiving a vaccine just to be doubly sure it does not contain an antibiotic.

Egg Protein

Vaccine manufacturers sometimes use chicken eggs to grow a live-attenuated (weakened) vaccine or an inactivated (killed) vaccine. This can lead some vaccines to have a trace amount of egg protein in them.

Although vaccines are purified after production to remove the culture material—in this case, egg proteins—some remnants of the culture may be left behind. This poses a potential risk to people with a severe egg allergy.

In addition to most flu vaccines, trace amounts of egg protein can be found in the yellow fever vaccine. Yet despite concerns about a possible allergic response, the risk of getting the flu or yellow fever almost invariably outweighs the risks of getting the corresponding vaccines—even in people with severe egg allergies.

The Advisory Committee on Immunization Practices (ACIP) currently advises flu vaccination for people with a history of egg allergy, irrespective of severity. Even so, vaccine administration should be supervised by a healthcare provider able to recognize and manage a severe allergic reaction should one occur.


Aluminum salt is by far the most common adjuvant used in vaccines today. It has been used safely for more than 70 years and can be found in a plethora of vaccines, including the DTaP, hepatitis A, hepatitis B, and pneumococcal vaccines.

As one of the most abundant elements on the planet, aluminum is everywhere, including in the air you breathe, the food you eat, and the water you drink. As the human body can process significant amounts of this metal very quickly, the trace amounts found in vaccines pose no harm to the human body.


Scientist use formaldehyde to kill live germs for use in inactivated vaccines. Although large amounts of formaldehyde can be extremely toxic, the trace values found in vaccines are well within the range of safety established by the U.S. Food and Drug Administration (FDA) and Occupational Safety and Health Administration (OSHA).

Like aluminum, formaldehyde is a naturally-occurring compound that is already present in the human body. In fact, the body produces roughly 1.5 ounces of formaldehyde per day as a part of normal metabolism.

The trace amounts found in vaccines represent a mere fraction of normal everyday values found in humans. Once consumed, the body quickly breaks formaldehyde down into its basic elements, preventing accumulation.

Monosodium Glutamate (MSG)

Certain vaccine components can change if they’re exposed to too much heat, light, or humidity. To avoid this, manufacturers will sometimes add stabilizers like monosodium glutamate (MSG) to keep the vaccine safe and effective after production.

While people often report having headaches, drowsiness, palpitations, and flushing after consuming MSG, there is little scientific evidence that the amounts used in food can cause these effects.

With that said, a report issued by the FDA found that some sensitive individuals experienced mild, short-term symptoms after taking in 3 grams of MSG without food. That is an amount that is more than 4,000 times greater than that found in any one vaccine.


Gelatin is also sometimes used as a stabilizer to protect vaccines from damage due to light or humidity. Overall, gelatin is the most common cause of allergic reactions to vaccines, but serious reactions like anaphylaxis are extraordinarily rare.

A 2017 study published in the Journal of Allergy and Clinical Immunology reported that, of the more than 25 million doses of childhood vaccines given between 2009 and 2011, only 33 cases of anaphylaxis were reported.

Human Fetal Tissue

Many vaccines are grown in a lab using animal cells (such as those found in chicken eggs), but there are some made from human cells—specifically, fetal embryo fibroblast cells responsible for holding skin and tissues together.

Viruses need cells to replicate, and human cells tend to work better than animal cells in growing the viruses needed to make vaccines. Fetal embryo cells divide faster than other human cells, making them ideal candidates for this purpose.

Back in the 1960s, scientists obtained fetal embryo cells from two pregnancies that were electively aborted and used them to grow viruses for the manufacturing of inactivated vaccines. These very same cell lines have been used ever since to make the vaccines for rubella, chickenpox, hepatitis A, shingles, and rabies.

People who oppose abortion often oppose the use of vaccines made from fetal cell lines. It should be noted, however, that many religious leaders—including the Catholic Church—have approved the use of these vaccines despite their historical origin.

Vaccine Safety Testing

It is not easy to get vaccines licensed by the FDA. In order to gain approval, manufacturers must show substantial evidence that their vaccines are safe and effective. This involves years of clinical trials, typically with thousands of study participants.

The phases of research can be broadly categorized as the exploratory/pre-clinical phase (during which vaccine candidates are identified and tested on animals or human tissues) and clinical trials (involving humans).

The phases of clinical trials can be broken down as follows:

  • Phase I: This phase tests the vaccine on a small group of adults (typically 20 to 80) to see if it prompts a significant immune response. The trial also records the types and severity of any side effects it may cause.
  • Phase II: This phase expands testing to hundreds of people who are randomly divided into groups to see how those who receive the vaccine respond compared to those who received a placebo. This includes an evaluation of the rate of side effects between the two groups as well as the number of discontinuations due to intolerable side effects.
  • Phase III: By the time a vaccine reaches this phase, it will have undergone safety testing for years. Larger groups of people of different ages, ethnicities, and chronic health conditions will be recruited to see if the vaccine affects them differently.

The safety of a vaccine is evaluated in each of these clinical trial phases. If an adverse event occurs at any point, the trial may be temporarily stopped or permanently discontinued.

If, however, these studies demonstrate that the vaccine is safe and effective, it can then go through the process of gaining FDA approval.

Post-Licensure Monitoring

Even after a vaccine has been license by the FDA, public health officials continuously monitor for adverse events. In the United States, there are five primary mechanisms to do so:

  • Phase IV clinical trials: This is a type of clinical trial that studies the side effects caused by a new treatment over time (after it has been approved and is on the market).
  • FDA inspections: Health officials routinely inspect plants where vaccines are manufactured and review batches to verify that they are potent, pure, and safe.
  • Vaccine Adverse Event Reporting System (VAERS): VAERS is a reporting tool for anyone to report any adverse event believed to be caused by a vaccine. The system helps health officials act promptly when a pattern of adverse events occurs.
  • Vaccine Safety Datalink (VSD): This collection of databases is used primarily for research and collects data from across the country to better analyze the effects of a new vaccine.
  • Clinical Immunization Safety Assessment (CISA): This is a collaborative effort between the Centers for Disease Control and Prevention (CDC) and a network of academic and managed care institutions that monitors vaccine safely and directly interacts with healthcare providers.

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. Despite what some people may tell you, vaccines cannot "overwhelm" the immune system of infants and small children or cause diseases of any sort. Any such assertion is simply not true.

Although side effects are possible, most are mild and transient. Even if these side effects occur, the benefits of vaccination—in preventing potentially serious infections and the rapid spread of disease throughout communities—outweigh the risks in almost all cases.

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14 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  8. American Cancer Society. Formaldehyde.

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  10. U.S. Food and Drug Administration. Questions and answers on monosodium glutamate (MSG).

  11. McNeil MM, Weintraub ES, Duffy J, et al. Risk of anaphylaxis after vaccination in children and adults. J Allergy Clin Immunol. 2016 Mar;137(3):868-78. doi:10.1016/j.jaci.2015.07.048

  12. Olshansky SJ, Hayflick L. The role of the WI-38 cell strain in saving lives and reducing morbidity. AIMS Public Health. 2017;4(2):127-38. doi:10.3934/publichealth.2017.2.127

  13. Zimmerman RK. Helping patients with ethical concerns about COVID-19 vaccines in light of fetal cell lines used in some COVID-19 vaccines. Vaccine. 2021 Jul 13;39(31):4242-4244. doi: 10.1016/j.vaccine.2021.06.027

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By Robyn Correll, MPH
Robyn Correll, MPH holds a master of public health degree and has over a decade of experience working in the prevention of infectious diseases.