What Goes Into a Vaccine?

Aside from antigens, ingredient components of a vaccine include adjuvants, added to enhance the immune system response; antibiotics, to prevent contamination during the manufacturing process; and preservatives and stabilizers.

These additional ingredients are often a source of concern for wary parents and patients. Below is a list of common ingredients in many vaccines, along with information on their purpose and safety.

Thimerosal (ethylmercury)

  • Why is it used? This mercury-containing ingredient has been used as a preservative in vaccines since the 1930s. Today, it is only found in vaccines for influenza. Preservatives are necessary for preventing dangerous bacterial or fungal contamination, but thimerosal has since become a major source of vaccine safety concerns.
  • Health concerns? While mercury is a naturally-occurring element found in soil, water, and food, large amounts of it can be harmful, especially for children. Back in 1997, children were receiving three vaccines that together contained more mercury than the EPA recommended limit (though not more than the FDA limit). This led to speculation that thimerosal in vaccines could be linked to autism or other conditions.
  • Is it safe? Years of research have reduced concerns here. The type of mercury found in thimerosal, ethylmercury, differs from methylmercury, the type commonly found in fish and known to be harmful in large amounts. Ethylmercury is broken down and excreted from the body much more quickly than methylmercury, and no scientific study has found a link between ethylmercury and autism or any other harmful effects.
  • Amount in vaccines? Nonetheless, several public health agencies and vaccine manufacturers agreed in 1999 to cease using thimerosal as a precautionary measure. Today, no vaccine contains Thimerosal except the influenza vaccine, and Thimerosal-free alternatives are available.


  • Why is it used? Formaldehyde has been used for decades in vaccines to inactivate viruses and detoxify bacterial toxins, ensuring they don’t result in sickness when injected.
  • Health concerns? The U.S. EPA classifies formaldehyde as a carcinogen, as does the International Agency for Cancer Research and the National Toxicology Program. Additionally, several studies have since linked strong, long-term formaldehyde exposure to certain types of cancer.
  • Is it safe? The potential for harm depends on the amount. Formaldehyde is always present in the human body as part of our natural metabolic process, but long-term exposure to high amounts can overwhelm our system and be harmful. Fortunately, the amount of formaldehyde found in vaccines is very small, most of it being diluted down to residual amounts during the manufacturing process. In fact, the FDA reports there is 50 to 70 times more formaldehyde present in an average newborn’s body than in a single dose of vaccine. In brief, current science shows formaldehyde in vaccines to be harmless.
  • Amount in vaccines? The highest amount of formaldehyde present in any vaccine is .02 mg per dose. An average two-month-old baby would have around 1.1 mg of formaldehyde circulating in their body, with higher naturally-occurring amounts for older children.


  • Why is it used? Aluminum is used as an adjuvant in vaccines. That is, it makes them more effective by strengthening the immune system response. Thanks to adjuvants, people need fewer doses of vaccine to build immunity.
  • Health concerns? Sometimes the mention of aluminum in vaccines makes parents uneasy; that’s because there has been some evidence that long-term exposure to high amounts of aluminum can contribute to brain and bone disease. However, aluminum is naturally present in water, foods, even breast milk. Aluminum has only been shown to harm people if absorbed in very high amounts and when a person’s kidneys aren’t working properly. In contrast, the amount of aluminum in vaccines is negligible.
  • Is it safe? Aluminum is the third most common naturally-occurring element, after oxygen and silicon. It is found in plants, soil, air, and water. A breast-fed infant will naturally ingest around 7 milligrams of aluminum in her diet throughout the first six months of her life. In contrast, the standard vaccines administered over the first six months of an infant’s life contain an average of just 4.4 milligrams of aluminum. Aluminum has been used safely for over six decades in vaccines, with no scientific evidence indicating otherwise.
  • Amount in vaccines? The amount of aluminum in vaccines is tiny. In fact, babies always have a small naturally occurring amount of aluminum in their bloodstreams, about 5 nanograms. The quantity of aluminum in a vaccine is so small it doesn’t cause any noticeable raise in this base amount found in the blood, even immediately after an injection.


  • Why are they used? During the production process of some vaccines, antibiotics may be used to counter the risk of dangerous bacterial infection.
  • Health concerns? Concern occasionally arises about antibiotics in vaccines because of the risk of allergic reactions in some children.
  • Is it safe? These fears are greatly exaggerated. Vaccine manufacturers only use antibiotics that are far less likely to provoke a reaction, and because antibiotics are only used during production, they are reduced to trace or undetectable amounts in the final product. In fact, no allergic reaction to a vaccine has ever been traced back to antibiotics. The overall odds a child will suffer from a severe allergic reaction from an MMR or Hepatitis B vaccine, from any ingredient, is 1 in 1,000,000, one hundred times less than the 1 in 10,000 chance a child will be struck by lightning.
  • Amount in vaccines? During the purification steps of the production process, antibiotics are removed, resulting in miniscule or undetectable amounts in the final vaccine.


  • Why is it used? Gelatin is used as a preservative and stabilizer, keeping vaccines effective under heat or cold and for the duration of their shelf life.
  • Health concerns? For a very small number of children, gelatin can cause an allergic reaction.
  • Is it safe? While gelatin is the single largest identifiable source of severe allergic reactions from vaccines, the incidence rate is still incredibly small. There is about one case of anaphylaxis caused by gelatin in vaccines for every two million injections.
  • Amount in vaccines? The amount of gelatin varies by vaccine, with the MMR vaccine on the high end, containing 14.5 mg per dose, and the DTaP on the low end, with only 0.0015 mg. Children with a history of gelatin allergies can seek alternatives or exemptions.

Monosodium Glutamate (MSG)

  • Why is it used? Like gelatin, MSG is used as a preservative and stabilizer in some vaccines, keeping them effective through heat, cold, and shelf life.
  • Health concerns? MSG gained a bad reputation starting in the 1960s after anecdotal reports surfaced of nausea, headaches, flushing, or sweating due to food with MSG. As a result, concern has spread about its use in vaccines. However, these concerns are not supported by scientific research.
  • Is it safe? While the scientific community acknowledges that a very small minority of people may suffer from short-term reactions to MSG, decades of research have not found the element to be harmful. As a result, the Food and Drug Administration, the World Health Organization, and the United Nations all declare MSG to be safe. It has been used for decades and continues to be used in foods as a flavor-enhancer.
  • Amount in vaccines? While some websites have trumped up MSG-based alarm, it is only present in two scheduled vaccines, adenovirus and influenza.