How safe are vaccines and their contents?

As mentioned previously, the active component in a vaccine is the ‘antigen’. This can be in the form of weakened (attenuated live vaccine) or inactivated microbes, parts of microbes, or modified microbial toxins (toxoid vaccine). Vaccines are made up not only of the antigen, but also multiple excipients that improve the effectiveness of vaccines and provide a longer expiry date. The following Figure shows a summary of the multiple vaccine components.

Examples of attenuated live vaccines that are available in the market include the tuberculosis (BCG) vaccine and oral polio vaccine. The attenuated microbes present in the vaccines are unable to reproduce to the quantities required to cause disease. This relatively small amount of attenuated microbes however, is sufficient to trigger our bodies to produce antibodies that give our bodies the ‘memory’ to prevent future infection.

Meanwhile, inactivated vaccines contain microbes killed using formaldehyde. These microbes are unable to reproduce once introduced to the human body but are still able to stimulate the body to produce immunity, albeit through multiple doses.

The hepatitis B and human papillomavirus (HPV) vaccines on the other hand, are made up only of the immune response-inducing segment of microbes without the rest of the illness-causing portions. The hepatitis B vaccine for example, contains the protein coat on the hepatitis B virus surface. Toxoid vaccines such as the diphtheria vaccine do not contain the bacteria itself but a toxoid modified through inactivation of the diphtheria exotoxin. Actual bacterial toxins cause illnesses but the inactivated toxoids do not result in illnesses, yet they afford immunity to future toxin exposure.

These are examples of the active ingredients of vaccines. However, in recent years many people have been expressing concern about the other ingredients in vaccines and their safety. Let’s have a look at these other possible ingredients, which include adjuvants, stabilizers, and preservatives, as well as trace components such as antibiotics and inactivating agents.

Adjuvants are added to enhance the body’s immune response towards the antigen. Examples of common adjuvants include aluminum hydroxide and aluminum phosphate. Despite some concerns regarding the side effects of aluminum, studies have shown that the exposure to aluminum through vaccination is much lesser than the amount consumed from food and water!

As most vaccines are produced through freeze-drying of multi-component mixtures, there is a risk of destabilization of the vaccine. Thus, stabilizers are primarily used to prevent separation of the vaccine components. The common stabilizers used are human serum albumin, gelatin, lactose, sucrose and amino acids. Human serum albumin is a natural blood component while the latter examples are commonly found in our daily diet.

In order to ensure the absence of microbial growth, preservatives are usually added –in a way similar to how preservatives are added to processed food such as jams. Examples of preservatives in vaccine include thiomersal, phenol and phenoxyethanol. Thiomersal has been safely used as a vaccine preservative for over 80 years, phenol is commonly found in mouthwash and lozenges while phenoxyethanol is commonly found in cosmetic products. In fact, most single-dose vaccines no longer contain preservatives.

Vaccines may also contain minute (sometimes undetectable) quantities of substances remaining from the manufacturing process known as ‘trace components’. These could be anything ranging from antibiotics, inactivating agents, cell culture fluid, egg proteins to yeast.

Antibiotics are added into vaccines during manufacturing to prevent microbial contamination but are mostly removed in subsequent purification. While there may be traces of these antibiotics left, it is usually in very small ‘micro-gram’ quantities that are mostly undetectable in the final vaccine. Furthermore, antibiotics known to cause allergies in selected individuals, such as penicillin and cephalosporin are not used in vaccine manufacture.

Small amounts of inactivating agents such as formaldehyde and glutaraldehyde, on the other hand, are incorporated during the manufacturing of inactivated and toxoid vaccines. As a matter of fact, the amount of formaldehyde present naturally in the body of a newborn is more than 50 times higher than that received from a dose of vaccine.

Contrary to the growing misconception about vaccine safety, most of the excipients in vaccines can in fact be found in our day to day household products. Furthermore, numerous studies have shown that concerns linking vaccination to autism and other chronic illnesses are largely unfounded. Do not hesitate to clarify any of your doubts regarding vaccination with your healthcare providers and together, we can improve immunization rates through proper understanding!



  1. Factsheet: Vaccine Components. National Centre for Immunisation Research and Surveillance. May 2013.
  2. Common Ingredients in U.S Licensed Vaccines. Food and Drug Administration (FDA). US Department of Health and Human Services.
  3. Understanding Thimerosal, Mercury and Vaccine Safety. Centers for Disease Control and Prevention. Feb 2013.
  4. Study Reports Aluminum in Vaccines Poses Extremely Low Risks to Infants. Food and Drug Administration (FDA). US Department of Health and Human Services.


Hui Ling Lau
Hui Ling Lau

PhD, MPharm, RPh

A Doctor of Philosophy (PhD) in Pharmacy currently lecturing in Taylor's University.

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