Vaccines consists of various components, each of which plays an important role in the efficacy and safety of the vaccine. The package leaflets for vaccines state all the components that the vaccine contains. When certain substances are present in a concentration that may provoke a reaction in sensitive or allergic individuals, this is explicitly stated (for example, a vaccine containing traces of chicken egg protein).
In addition, all vaccines are extensively evaluated for quality, efficacy and safety by the regulatory authorities before being placed on the market. The final composition of the vaccine undergoes extensive testing to ensure safety.
Main component or active substance
The main component or active substance is the most important part of a vaccine. The main component of a vaccine is one or more antigens, against which the body will produce an immune response. The antigen comes from the pathogen that the vaccine is intended to protect against. Other components may also be added to vaccines to ensure efficacy and stability. These components are harmless due to their small concentrations.
Adjuvants or excipients
Adjuvants improve the immune response to the vaccine by making the response stronger, faster and more durable. An example of adjuvants is aluminium salts. The amount of aluminium in vaccines is relatively low, at least ten times lower than the amount of aluminium we absorb daily through our environment and food. Adding such small amounts of aluminium is therefore perfectly safe, even for children and babies. In addition to aluminium, new adjuvants and adjuvant combinations have been developed that target specific components of the immune response.
Stabilisers
Stabilisers are important for the shelf life of vaccines. They help maintain the efficacy of the vaccine during storage and distribution. Factors that can affect stability include changes in temperature and acidity (pH), which cause the vaccine to lose efficacy. Adding potassium phosphate and sodium phosphate ensure the correct pH, which is important for the stability of the vaccine. Sorbitol, a naturally occurring sugar, is also often used as a stabiliser. Polysorbate is used in very small amounts to keep the vaccine components soluble and avoid sedimentation, ensuring that the correct vaccine dose is administered. All these components are completely harmless due to the small concentrations in which they are present in vaccines.
Preservatives
Multidose vaccines, vials containing more than one dose, sometimes contain a preservative to prevent the growth of bacteria and fungi. That way, the vaccine can be used for several hours after opening. In the past, very small amounts of mercury derivatives (thimerosal, thiomersal or timerfonate) were used for this purpose. The presence of mercury-based preservatives has prompted concerns. The European Medicines Agency (EMA) conducted extensive research in this regard and concluded that there is no evidence that the amount of thiomersal used in vaccines poses a health risk. Moreover, thiomersal is broken down and removed from the body, there is no danger of accumulation of mercury or mercury derivatives in the body.
Nevertheless, as a precaution, it is recommended that vaccines without mercury derivatives be used whenever possible. Vaccines for children no longer contain mercury in Belgium. Thiomersal is still rarely used in vaccines. When an antimicrobial preservative is still needed, new products such as phenoxyethanol are used, and in small amounts that are not harmful. All of these substances are systematically monitored to ensure that they are in concentrations that have been shown to be safe and that permissible levels are not exceeded.
Residuals
Some vaccines may contain traces of other substances used in the manufacturing process, such as ovalbumin (a protein found in eggs) or neomycin (an antibiotic). These residuals are removed as carefully as possible before the vaccine is packaged and sold. The small amounts of residuals left behind are not harmful to health. When substances are used that may cause a reaction in sensitive or allergic individuals, this is stated in the package insert.
Another example of a residual is formaldehyde. This substance is used to inactivate bacteria or viruses. The small amount of formaldehyde contained in the vaccine is not harmful to health. The daily exposure and uptake of formaldehyde through the environment, especially through plastic, is much higher than any residuals in certain vaccines.
Cells
Vaccines can sometimes contain cells to make antigens, for example, to grow the virus. These cells are destroyed and removed during purification and are no longer present in the final vaccine. They are therefore no longer present in the final product, except possibly in minimal amounts of residuals such as proteins. The quantities present are low, have been extensively tested and found to be completely safe. Some vaccines are made in bacteria, for example E. coli. The bacterial cells are destroyed and removed during purification. Vaccines are mandatorily tested for sterility. There cannot therefore be any residual bacteria present in vaccines.
Purifying agents
Finally, vaccines may also contain residues of purifying agents such as hexadecyltrimethyl-ammonium bromide. This substance helps to isolate desired antigens from impurities from the production process. These substances are also removed during purification and are present only in very low concentrations that are not harmful.