Vaccine Safety: What’s in the Vial

Vaccine Safety: What’s in the Vial

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Adjuvants

Adjuvants help improve the immune response to vaccines. They allow the vaccine to work better by requiring lesser amounts of active ingredients or fewer doses.

Five adjuvants are currently used in vaccines in the United States:

Aluminum

  • Aluminum has been in vaccines in the United States since the 1930s.
  • The amount of aluminum in vaccines given during the first six months of life is very small (about 4 milligrams). This is less than what is consumed from breast milk (10 milligrams), regular infant formula (40 milligrams), or soy-based formula (120 milligrams) during the first six months of life.
  • Everyone is exposed to aluminum in the environment. For most of us, these exposures are not harmful.
  • Typically, aluminum is only harmful when two things happen. First, a person’s kidneys do not work properly or at all. AND, second, when large amounts of aluminum are given. For example, long-term intravenous use or consumption of large amounts of antacids.

Monophosphoryl lipid A

  • Monophosphoryl lipid A is a detoxified bacterial surface protein.
  • It was approved for use as an adjuvant in the United States in 2009.

QS21

  • QS21 is a soap-based molecule isolated from the bark of the Quillaja saponaria tree.

MF59

  • MF59 is an oil-in-water emulsion.
  • The oil is in the emulsion is called squalene. Squalene is found in people, animals and plants.

CpG

  • CpG is an adjuvant made from two nucleic acids found in repeat sequences in bacterial DNA but not vertebrate DNA. The two nucleic acids are cytosine and guanine.

Find out more about the adjuvants in vaccines.

Antibiotics

Antibiotics are used in some vaccines. They are added to prevent bacterial contamination.

Some examples of antibiotics used in vaccines include:

  • Neomycin
  • Streptomycin
  • Polymyxin B
  • Chlortetracycline
  • Amphotericin B

People are not usually allergic to these antibiotics. So, they are not likely to have an allergic reaction to vaccines because of them.

Find out more about which vaccines contain antibiotics.

Formaldehyde

Formaldehyde is used during vaccine manufacture. It is used to inactivate viruses and bacterial toxins. Most of the formaldehyde is purified away at the end of the process. This means that only small amounts remain in the final vaccine. In fact, vaccines have 10 times less formaldehyde than what is typically found in the bloodstream. Formaldehyde is found in the bloodstream because of everyday functions, like protein and DNA synthesis.

Find out more about which vaccines contain formaldehyde and how much.

Egg proteins

People with egg allergies sometimes wonder which vaccines they can or cannot get. Two vaccines are made in eggs:

  • Yellow fever vaccine
  • Influenza vaccine

Yellow fever vaccine

People with egg allergies should speak to their doctor before getting this vaccine.

Influenza vaccine

Most people can safely get influenza vaccine even if they have an egg allergy. This is because the amount of egg proteins that remain in the final vaccine are very low. To be cautious, they should stay in the doctor’s office for 15 minutes after getting influenza vaccine in case they have any reaction.

Find out more about what we know related to egg allergies.

Fetal cells

Bacteria can grow on surfaces of the nose and throat, but viruses need cells in which to grow. Different viruses grow in different types of cells, so part of making a viral vaccine is to figure out in which type of cell the vaccine virus will grow the best. In the case of a few vaccines, fetal cells were found to be suitable. The routinely recommended vaccines that contain viruses grown in fetal cells include: 

  • Chickenpox
  • Hepatitis A
  • Rubella

A few less commonly used vaccines are also made using fetal cells, including one version of the rabies vaccine, chikungunya vaccine, and the J&J/Janssen COVID-19 vaccine, which is no longer available in the U.S.

Why fetal cells?

Scientists tried fetal cells for a few reasons:

  • Fetal cells can reproduce more times than older cells.
  • Viruses that infect people tend to grow better in human cells than in animal cells.
  • Animal cell lines and older human cells can contain viruses. Fetal cells would not be likely to contain other viruses because the womb is a sterile environment. Unborn babies are protected from most viruses and bacteria because these pathogens cannot get into the womb.

Fetal cells used to make vaccines were obtained from elective pregnancy terminations several years ago. These cells have been cared for in labs, so that additional abortions are not needed to continue making these vaccines.

Religious concerns

The use of vaccines grown in fetal cells was questioned because of the Catholic Church’s position on abortion. So, the Pontifical Academy for Life, the major policymaking body of the Catholic Church, reviewed the matter. The Academy concluded that while it was unfortunate those cells were used to make vaccines, it was still acceptable for a Catholic person to use the vaccines. 

Find out more about the use of fetal cells in vaccines, including additional resources.

Preservatives

Preservatives are used in vaccines with more than one dose in the vial. When multiple doses of vaccine are in one vial, a needle is inserted through a rubber stopper to withdraw each dose. Sometimes, bacteria can get into the vial from this action and contaminate remaining doses of the vaccine. Preservatives lower the chance for of this happening.

The most commonly used preservative was thimerosal. Thimerosal contains ethylmercury. It was removed from most vaccines because of concerns about the amount of mercury in vaccines. Since then, scientists have done many studies that proved the amount of thimerosal in vaccines did not cause harm.

Thimerosal is no longer allowed to be used in U.S. vaccines, but it is still used in vaccines in some other countries where they can’t afford single dose vials of vaccines. Since its presence does not cause harm, it is an important way to protect people from vaccine-preventable diseases in resource-limited parts of the world.

Find out more about the science behind thimerosal.

Stabilizers

Stabilizers are used in vaccines to prevent active ingredients from breaking down during vaccine manufacture, shipment and storage. The most common stabilizer used in vaccines is gelatin. It is made from the skin and hooves of pigs. People who are allergic to gelatin may have an allergic reaction to vaccines with gelatin. 

Polysorbate 80 is another stabilizer used in vaccines.

Find out more about each of these stabilizers and which vaccines contain them.

Yeast proteins

Some people who are allergic to bread and other bread products wonder whether they need to be concerned about vaccines that are made using yeast cells, such as hepatitis B and HPV vaccines, among others.

Yeast cells are used to make recombinant vaccines. As the yeast cells reproduce, the protein for the vaccine is also made. Then the vaccine protein is purified. So, the final vaccine contains only very small quantities (about 1 to 5 millionths of a gram) of yeast proteins. No evidence suggests that yeast proteins can generate a large enough immune response to cause an allergic reaction. Therefore, the risk of developing a severe allergic reaction after receiving these vaccines is theoretical.

Find out more about yeast and vaccines.

Resources

Reviewed by Paul A. Offit, MD, on Feb. 2, 2026