Components of a vaccine

Vaccines include a variety of ingredients including antigens, stabilizers, adjuvants, antibiotics, and preservatives.

They may also contain residual by-products from the production process. Knowing precisely what is in each vaccine can be helpful when investigating adverse events following immunization (AEFIs) and for choosing alternative products for those who have allergies or have had an adverse event known or suspected to be related to a vaccine component.


Antigens are the components derived from the structure of disease-causing organisms, which are recognized as 'foreign' by the immune system and trigger a protective immune response to the vaccine.

You have already learned about antigens. Click here if you want to review the chapter on antigens.


Stabilizers are used to help the vaccine maintain its effectiveness during storage. Vaccine stability is essential, particularly where the cold chain is unreliable. Instability can cause loss of antigenicity and decreased infectivity of LAV. Factors affecting stability are temperature and acidity or alkalinity of the vaccine (pH). Bacterial vaccines can become unstable due to hydrolysis and aggregation of protein and carbohydrate molecules. Stabilizing agents include MgCl2 (for OPV), MgSO4 (for measles), lactose-sorbitol and sorbitol-gelatine.


Adjuvants are added to vaccines to stimulate the production of antibodies against the vaccine to make it more effective.

Adjuvants have been used for decades to improve the immune response to vaccine antigens, most often in inactivated (killed) vaccines. In conventional vaccines, adding adjuvants into vaccine formulations is aimed at enhancing, accelerating and prolonging the specific immune response to vaccine antigens. Newly developed purified subunit or synthetic vaccines using biosynthetic, recombinant, and other modern technology are poor vaccine antigens and require adjuvants to provoke the desired immune response.

Chemically, adjuvants are a highly heterogeneous group of compounds with only one thing in common: their ability to enhance the immune response. They are highly variable in terms of how they affect the immune system and how serious their adverse reactions are, due to the resulting hyperactivation of the immune system.

Today there are several hundred different types of adjuvants that are being used or studied in vaccine technology.


Antibiotics (in trace amounts) are used during the manufacturing phase to prevent bacterial contamination of the tissue culture cells in which the viruses are grown. Usually only trace amounts appear in vaccines, for example, MMR vaccine and IPV each contain less than 25 micrograms of neomycin per dose (less than 0.000025 g). Persons who are known to be allergic to neomycin should be closely observed after vaccination so that any allergic reaction can treated at once.

  • Used during the manufacturing phase to prevent bacterial contamination of tissue culture cells in which viruses are grown,
  • Usually only trace amounts appear in vaccines, for example, MMR and IPV vaccines each contain less that 25 micrograms of neomycin per dose,
  • Persons known to be allergic to neomycin should be closely observed after vaccination so any allergic reaction can be immediately treated.


Preservatives are added to multidose vaccines to prevent bacterial and fungal growth. They include a variety of substances, for example Thiomersal, Formaldehyde, or Phenol derivatives.


  • Very commomly used preservative. Thiomersal is an ethyl mercury-containing compound,
  • It has been in use since the 1930s and no harmful effects have been reported for doses used in vaccination except for minor reactions (e.g. redness, swelling at injection site),
  • It is used in multidose vials and for single dose vials in many countries as it helps reduce storage requirements/costs,
  • Thiomersal has been subjected to intense scrutiny, as it contains ethyl mercury. The Global Advisory Committee on Vaccine Safety continuously review the safety aspects of Thiomersal. So far, there is no evidence of toxicity when exposed to Thiomersal in vaccines. Even trace amounts of thiomersal seem to have no impact on the neurological development of infants.


  • Used to inactivate viruses (e.g. IPV) and to detoxify bacterial toxins, such as the toxins used to make diphtheria and tetanus vaccines,
  • During production, a purification process removes almost all formaldehyde in vaccines,
  • The amount of formaldehyde in vaccines is several hundred times lower than the amount known to do harm to humans, even infants. E.g., DTP-HepB + Hib "5-in-1" vaccine contains less than 0.02% formaldehyde per dose, or less than 200 parts per million.


Which of the following answers is incorrect?

A. Thiomersal prevents bacterial growth and therefore makes vaccines more durable, which is particularly helpful for storing and use of multi-dose vials.
B. Aluminium salts primarily serve to prevent bacterial contamination of tissue culture cells.
C. Adjuvants serve to enhance the immune response.
D. Stabilizers make a vaccine more stable towards temporary changes in temperature and pH.


Answer B is incorrect.

  • Aluminium salts primarily slow the escape of the antigen from the site of injection. As the exposure between the antigen and the immune system, they increase the effectiveness of the vaccine.