H1N1 vaccine

Vaccines: what’s inside?

Rob Reddick finds out what goes into vaccines, and why

A vaccine contains the antigens, or ‘non-self’ markers, of a pathogen, to provoke an immune response and stimulate the production of antibodies. This process, in essence, mimics real-life infection and allows the body to ‘learn’ how to identify the same pathogen in the future.

These antigens may be delivered by putting a whole, weakened pathogen into the body (eg whole bacteria or viruses), or by taking the antigens from a pathogen and delivering these alone.

Live attenuated vaccines

Different vaccines deliver antigens in different ways. Some vaccines – such as the BCG (Bacille Calmette–Guérin), which protects against tuberculosis – contain a live version of the whole pathogen. However, in such cases the pathogen’s strength (virulence) is weakened (attenuated) before it is given, to reduce the chance of infection.

Inactivated vaccines

Some vaccines contain an inactive version of a pathogen, one that has been killed – for instance by using heat or a chemical called formaldehyde. The immune system can still recognise and respond to the pathogen, but as the pathogen cannot reproduce, it poses no risk of infection. Cholera, hepatitis A and rabies vaccines all contain inactivated pathogens.

Toxoid vaccines

The symptoms of some diseases are caused by harmful products of bacteria, known as toxins. Vaccines for these diseases, which include tetanus, use inactivated versions of toxins (called toxoids) to stimulate an immune response.

Subunit vaccines

Other vaccines contain only the antigens of a pathogen that best stimulate a response – these are known as subunit vaccines. By including only the essential antigens, and not the whole pathogen itself, these vaccines are much less likely to cause an adverse reaction, and pose no risk of infection. However, subunit vaccines tend to induce a weaker immune response than live attenuated vaccines.

Conjugate vaccines

Finally, some pathogens can be hard for the immune system to identify, and so require a special type of vaccine.

Bacteria coated with sugar molecules known as polysaccharides are able to mask the antigenic material on their surface, making it difficult for the immune system to recognise them and mount a response (especially in children). To fight against this, conjugate vaccines are created: the bacteria’s sugar coatings are isolated as subunits and then chemically joined to larger, more readily identifiable carrier proteins. These new constructions, being recognisable, stimulate an immune response that creates antibodies that can also recognise and fight against the coated bacteria in the future.

Other ingredients

As well as these active ingredients, vaccines also contain:

  • fluid, such as sterile water, saline or a protein-containing fluid, to suspend the contents
  • stabilisers, such as albumin, gelatine or sugars, to help the active ingredients remain unchanged when exposed to adverse conditions, such as extreme temperatures or changes in light, humidity or acidity
  • preservatives, such as phenols or antibiotics, to prevent the growth of dangerous bacteria and fungi in the vaccine.

Some vaccines also contain adjuvants: substances such as aluminium compounds that strengthen the immune response to the vaccine’s antigens. These are commonly used in subunit vaccines.

Vaccines may also contain trace elements of substances used during their manufacture: for example, some vaccines are grown using fertilised hens’ eggs and so may contain traces of egg protein, and inactivated vaccines may contain extremely small traces of formaldehyde.

See our other stories on vaccination: ‘Vaccines: how well do they work, and are they safe?’, ‘Vaccines: how and when are they given?’, our ethics case study on vaccination and the history of vaccination.

Lead image:

Greg Hinson/Flickr CC BY NC


Questions for discussion

  • Why do you think subunit vaccines can be less effective than live vaccines?
  • Why might some people not be able or not want to receive certain vaccines?

Further reading

Downloadable resources

About this resource

This resource was first published in ‘Immune System’ in January 2015.

Immunology, Medicine, Health, infection and disease
Immune System
Education levels:
16–19, Continuing professional development