Medicinal Chemistry - Common Components of Vaccines Summary

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The recent measles outbreak in the US has once again provoked discussion over vaccinations, and why some parents choose not to vaccinate their children despite the benefits of doing so. Whilst not the only factor, part of the blame lies with misinformation about the chemical composition of vaccines and the effects these compounds can have. This graphic summarises some of the key components in vaccines, as well as clarifying their purpose and safety in the concentrations present.

Active Components

Generally, vaccines have several major components. The active component, or antigen, is the important part, responsible for inferring immunity to the disease or infection the vaccine is designed to guard against. It’s composed of a modified form of the virus, bacteria, or toxin that causes the disease; the precise nature can vary between vaccines.

Some vaccines use an inactivated form of a virus; this is accomplished by treating the virus with a chemical that kills portions of it, and renders it unable to replicate. Whilst it cannot replicate, injection will still initiate an immune response from the body, conferring immunity, though this can require several doses in some cases. The advantage of this method of immunisation is that it can even be given to those with weak immune systems.

Other active components will constitute only parts of the bacteria or virus which cause immunity to develop – other portions of the pathogen which cause disease symptoms are removed. Alternatively, in cases of some bacteria, toxins produced by the bacteria can also be used to induce an immune response. Live, but weakened viruses can be used, too. These cause immune response without causing the serious symptoms that the non-weakened form otherwise would, and have the advantage of usually inducing permanent immunity.


Adjuvants are chemical compounds added to vaccines to help enhance the body’s immune response. These aren’t present in all vaccines – in live type vaccines, such as the MMR vaccine, they are not present. Their discovery was largely accidental; back when vaccines were first being produced, differences in the efficacy of the same vaccines in different batches were noted. This was theorised to be due to a degree of contamination of the vaccines; however, subsequent cleanliness of the reaction vessels in which they were produced actually brought about a decrease in overall efficacy. As it turned out, the contamination in the reaction vessels actually helped to enhance the effect of the vaccine.

Subsequent experimentation confirmed that certain compounds, when added to vaccines in small amounts, enhanced the immune response to the vaccine. Aluminium salts, in particular, were found to have a notable effect, and are still the primary adjuvants used in vaccines today. The mechanism by which they enhance the immune response still isn’t fully understood, but it’s thought that they help keep the active component of the vaccine near the site of injection, leading to it being more easily accessible for immune cells.

Whilst the aluminium compounds used as adjuvants have been associated with some minor local reactions at the site of injection, there is no evidence that they cause any long term serious health effects. When following recommended vaccine schedules, levels of aluminium remain within safe levels; vaccines only contain around a milligram of aluminium, and there is no evidence of any risk to children or infants.


In the manufacture of the vaccine, antibiotics will commonly be used to prevent bacterial contamination. Whilst these are removed after manufacture, trace amounts can still remain in the final vaccine. The antibiotics that commonly cause adverse allergic reactions, such as penicillins, are avoided, in favour of antibiotics such as gentamycin and neomycin. The very small amounts of antibiotics present in vaccines have not been conclusively linked with any allergic reactions, though it’s recommended that people with known antibiotic allergies seek medical advice before receiving vaccines with antibiotic traces.


Stabilisers are added to the vaccine to protect it from adverse conditions which could impact its efficacy, allowing it to be stored for longer periods of time. A range of different possible stabilisers can be used; sugars, amino acids and proteins can all be utilised for this purpose. They also prevent the vaccine components from adhering to any storage vessel. Many of the compounds used as stabilisers are found naturally in the body anyway, and so do not pose any risk.


Preservatives are used to prevent bacterial and fungal contamination of the vaccine after its manufacture. This is particularly important for so-called ‘multi-dose’ vaccines, where multiple injection doses are drawn from the same rubber-capped vessel.

The mercury-containing compound thiomersal is commonly used as a preservative in these multi-dose vaccines. It’s also probably the vaccine component dogged by the most (unjustified) controversy. In the 1998, the infamous Dr Andrew Wakefield published a paper purporting to link the childhood MMR vaccines (for measles, mumps and rubella) with increasing levels of child autism. The publication of this study led to a drop in child vaccination rates in the UK, and an increase in outbreaks of preventable diseases.

Regular attempts to reproduce Wakefield’s research failed. It also emerged that he had falsified data in his study, and had in fact been hired and paid by a legal firm specifically to find evidence that the MMR vaccine was harmful – an undeniable and unethical conflict of interests. The British Medical Journal, investigating the fraudulent paper, found that of the children examined as part of Wakefield’s study, “no case was free of misreporting or alteration”.

Whilst the MMR vaccine did not contain thiomersal, many other vaccines in use at the time did. Thiomersal was phased out of childhood vaccines, partly as a consequence of the panic Wakefield’s study generated, but autism rates continued to rise. To date, there has been no evidence linking the quantities of thiomersal found in vaccines with any harmful effects; whilst mercury is undoubtedly a threat to human health in sufficient quantity, blood levels of mercury after vaccinations containing thiomersal are still well within accepted safe levels.

Other compounds used as preservatives in vaccines include phenol and phenoxyethanol. Again, the levels of these compounds present in doses of vaccines are not associated with any harmful effects.

Trace Components

A number of trace components are left behind from the manufacturing process of the vaccine. The concentration of these components in the final vaccine is very low. Compounds such as formaldehyde, one of the agents that can be used to inactivate viruses, can be detected, but at levels far below that known to cause harm in humans. For comparison, the amount of formaldehyde found in these vaccines is less than 1% of the amount found naturally in a 200g pear. Formaldehyde is also generated in our bodies naturally as part of our metabolism, again in quantities greater than those contained in vaccines.


Vaccines need to be diluted to their required concentration. Most often, this will be accomplished using either sterile water, or a saline solution.

Hopefully, this graphic and article have made clear the various components contained within vaccines, and their purpose therein. For more information, a range of further reading is provided below. Additionally, this graphic does an excellent job of highlighting the impact that vaccines have had on the rates of a number of diseases.



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References & Further Reading