People often have questions about what ingredients are in vaccines and why. The ingredients can be described in five groups: antigens, adjuvants, stabilizers, preservatives and manufacturing byproducts. However, when thinking about vaccine ingredients, it can also be useful to think about the relative amounts of ingredients because as stated by Paracelsus, a 16^th century physician and alchemist, “the dose makes the poison.” 

Amounts of ingredients

The quote from Paracelsus is a good reminder that the quantity of an ingredient is equally important to what the ingredient is. For example, while we all know we need water to survive, too much water can be harmful. It may be tempting to dismiss this idea and instead try to avoid any exposure to an ingredient that is considered potentially harmful, but that approach is difficult to achieve because we live in a world made of chemicals. 

If we only think about exposure from a vaccine while excluding other sources of exposure, it can skew our sense of risk. Opting out of vaccines because of concerns about the quantity of a particular ingredient may not change a person’s overall exposure to the ingredient, but it would leave them vulnerable to the diseases that those vaccine protect against. In short, vaccines — and the ingredients in them — are part of a much larger picture that we need to consider.

On some of the pages that follow in this section, we provide the quantities of ingredients in different vaccines, so we wanted to offer a brief refresher on some of the math and chemistry terminology that you may not have thought about since high school.

How much is 1 gram?

One gram is equal to:

• One-fifth (1/5) of a teaspoon of water
• One raisin
• One thousand (1,000) milligrams (Milligrams are often abbreviated as “mg.”)
• One million (1,000,000) micrograms (Micrograms are often abbreviated as “mcg” or “μg.”)
• One billion (1,000,000,000) nanograms (Nanograms are often abbreviated as “ng.”)

What does this mean when we talk about vaccines?

Vaccines do not contain grams of any ingredient. Most often they contain milligrams, micrograms or nanograms, so when reviewing quantities of ingredients, a useful tip for picturing these amounts is to think about a raisin:

• Milligram: A raisin would need to be cut into one thousand (1,000) equally sized pieces, and one of those pieces would be a milligram.
• Microgram: A raisin would need to be cut into one million (1,000,000) equally sized pieces, and one of those pieces would be a microgram.
• Nanogram: A raisin would need to be cut into one billion (1,000,000,000) equally sized pieces, and one of those pieces would be a nanogram.

If this still seems like “a lot,” try cutting a raisin into even 10 equally sized pieces, and you will quickly realize that we are talking about very small quantities.

What do “parts per million” and “parts per billion” mean?

Sometimes, you may see a quantity described as “parts per million” or “parts per billion.” These are abbreviated as “ppm” and “ppb,” respectively.

This labeling tells us how the quantities in a solution were measured. Rather than measuring the weight of the ingredient, its ratio in the solution was measured:

• 1 ppm means if the solution was divided into 1 million equal parts, one of those parts would be the ingredient, and the other 999,999 would be other types of ingredients. 
• 1 ppb means if the solution was divided into 1 billion equal parts, one of those parts would be the ingredient, and the other 999,999,999 would be other types of ingredients.

A good rule of thumb is to remember that 1 ppm means about 1 mg of the chemical is present in 1 liter of water, so if a vaccine dose is 0.5 milliliters (ml):

• 1 ppm would be about 0.0005 milligrams in the dose.
• 1 ppb would be about 0.0000005 milligrams in the dose.

Antigens

Our immune systems respond to the part of the vaccine called the antigen. To be protected against potential disease-causing agents, the immune system needs to recognize specific parts of it. For this reason, vaccines will always contain something specific to the virus or bacteria they are designed to protect against.

The antigens in vaccines can be: 

• Whole viruses or bacteria
• Parts of viruses or bacteria
• Harmful proteins, called toxins, made by bacteria
• Nucleic acids (e.g., mRNA or DNA) that cause our cells to make part of the virus or bacteria

The antigens in vaccines are designed so that they cannot cause the severe illness that can occur during an infection. Even if whole viruses or bacteria are used, they cannot cause severe illness like an infection would. When whole viruses or bacteria are used, one of two approaches may be taken:

1. The viruses or bacteria are killed with chemicals so they can’t reproduce in the person
2. The viruses or bacteria are changed in some way so that they only replicate a few times compared to the natural pathogen.

Learn more about how vaccines are made.

Adjuvants

Adjuvants are substances that make vaccines work better. They do this in one of three ways: 

1. Enhance the immune response to the vaccine
2. Decrease the quantity of antigen needed
3. Lessen the number of doses needed 

Five types of adjuvants are approved for use in the U.S.:

• Aluminum salts — Salts that contain aluminum have been used in vaccines for about 100 years. Find out which vaccines contain aluminum, how much is present, and how we know aluminum in vaccines is safe.
• Monophosphoryl lipid A — This substance was isolated from the surface of bacteria. It has been changed so that it is not harmful when added to vaccines. The vaccines in the U.S. that currently use this adjuvant are the shingles vaccine (called Shingrix), one of the RSV vaccines (called Arexvy), and the protein-based COVID-19 vaccine (Nuvaxovid).
• QS21— This soap-based substance comes from the bark of Quillaja saponaria trees. It is used in the same vaccines that use monophosphoryl A. Those include the shingles vaccine (called Shingrix), one of the RSV vaccines (called Arexvy), and the protein-based COVID-19 vaccine (Nuvaxovid).
• MF59 — A substance composed of squalene oil mixed with water. Squalene is found in people, animals and plants. It is used in the U.S. in a version of the influenza vaccine (called FLUAD). Because squalene is an oil that can be found in plants, some people wonder whether they can get vaccines that contain MF59 if they have allergies to peanut or corn oils. The answer is yes because the squalene oil used in vaccines does not contain these oils.
• CpG — The C in this name stands for Cytosine, and the G stands for Guanine (G). They are connected by phosphate, which is represented by the lowercase p. Cytosine and Guanine are building blocks of DNA. When used as an adjuvant these two building blocks are linked together and added to the vaccine. One hepatitis B vaccine (called Heplisav-B) uses this adjuvant.

Stabilizers

Stabilizers protect vaccines during manufacture, storage and transport so that they work as expected when administered. Stabilizers can include salts, sugars or fats.

We offer webpages about two stabilizers that people sometimes have questions about:

• Gelatin – This stabilizer can be associated with allergic responses.
• Polysorbate 80 – This stabilizer can also be present in very low quantities as a manufacturing byproduct. 

You can find other examples of stabilizers in the “ingredients by vaccine” section of our website. 

Preservatives

Preservatives prevent bacterial or fungal contamination of vaccines. The need for preservatives in vaccines arose in the early 20th century. After getting vaccines contained in multi-dose vials, some children developed severe and occasionally deadly infections. Healthcare providers realized that sometimes when putting a needle in the top of the vial to draw up a dose of vaccine, contaminants would accidentally be introduced into the vial. When this happened, the person who got the current dose was fine, but the person who got the next dose would also be inoculated with the contaminant. If sufficient quantities of the contaminant were present to cause harm, the person could get sick. 

In 1916, a typhoid vaccine contaminated with a bacteria, called Staphylococcus aureus, caused:

• 4 children to die 
• 26 to develop local sores, called abscesses 
• 68 to develop severe infections that spread throughout their bodies 

Preservatives prevent these inadvertent contaminants. As a consequence of these types of incidents in the early 1900s, preservatives have been required for most vaccines contained in multi-dose vials since the 1930s.

In the U.S. today, many vaccines are sold in single dose vials, making preservatives less necessary. However, single dose vials increase the cost of vaccines, so in many developing countries multi-dose vials continue to be used, making preservatives important in those settings.

Thimerosal

Thimerosal is a mercury-containing preservative used for decades in vaccines. Because it contains mercury, thimerosal has been the focus of intense scrutiny. Thimerosal  was removed from most routinely used childhood vaccines in the U.S. in the late 1990s. This is one of the reasons that the U.S. relies heavily on single-dose vaccine vials. However, thimerosal continues to be used in vaccines distributed in multi-dose vials in some other countries. 

Because of fears that thimerosal contained in vaccines could harm children, several studies have evaluated its safety when used in childhood vaccines. Several separate, large, controlled studies found no evidence of harm. To find out more about thimerosal, its history and some of the safety studies, check our thimerosal webpage.

Manufacturing byproducts

Vaccines are different from medicines because they contain part of the virus or bacteria. This means vaccines are made differently than medicines. To grow viruses, cells must be used. For bacteria to replicate, nutrients must be present. 

Once the antigen for a vaccine is produced, it is purified to remove the cells or nutrients used to make it. Purification processes have improved over time, but that doesn’t mean every last bit is gone. Think of the sand you bring back from the beach — no matter how much you try to get rid of it, some sand still ends up in your car or house. In the same way, small amounts of the nutrients and cell byproducts may remain in a vaccine. These amounts are so small that they cannot cause harm, but they are often still included on the vaccine ingredients list so that people are aware of their presence. 

We offer details about several manufacturing byproducts, including: 

• Antibiotics
• DNA
• Egg proteins
• Fetal tissues
• Formaldehyde
• Human proteins
• Yeast

Concerns have also been voiced about SV40, which is a virus found in cells used to grow the poliovirus vaccine during the 1950s and 1960s. These concerns were proven to be unfounded because children who received those vaccines were not at greater risk of cancer than those who didn’t receive them, and SV40 is not present in any of today’s vaccines. More information about SV40 is also contained on a separate webpage.