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The concept of immunisation is far from a modern idea. Before Edward Jenner (an 18th century scientist considered to be the founder of vaccinology), the practice of immunisation was recorded in 17thcentury China, where Buddhist monks drank snake venom to create immunity to snake bites. The concept of variolation (deliberate infection) occurs even earlier in history, with smallpox survivors in 430 BC being called upon to nurse the afflicted.
To read more about the history of vaccines, please visit: https://historyofvaccines.org/
An antigen is the active ingredient of the vaccine that generates an immune response against a specific disease-causing organism. Vaccines are broadly classified by how the antigen(s) are prepared. Vaccines may be viral (live or inactivated), viral vector, subunit (protein or polysaccharide), or nucleic acid (DNA or RNA). Combination vaccines may include inactivated, protein-based and/or protein-conjugated polysaccharide vaccine components.
For further information on the classification of vaccines, please refer to the Immunisation Handbook section 1.4.
Live attenuated vaccines:
Live vaccines contain pathogens, usually viruses, which have been weakened (attenuated) so that they are able to replicate enough to trigger a immune response, but not cause disease. Immunity from live vaccines is usually very long-lived.
Vaccine examples: MMR, varicella, rotavirus
Inactivated or whole killed:
Killed vaccines contain whole bacteria that have been killed.
Vaccine example: whole-cell pertussis vaccine
Inactivated vaccines contain viruses that have been inactivated in some way, so they are unable to replicate or cause disease.
Vaccine examples: Influenza, hepatitis A and poliovaccines.
Subunit:
Subunit vaccines contain pieces of the pathogens they protect against. There are several different types of subunit vaccines:
- Toxoid
Toxoid vaccines are produced by harvesting a bacterial toxin and changing it chemically (usually with formaldehyde), to convert the toxin to a toxoid. Toxoid vaccines induce antibodies that neutralise the harmful toxins released from these bacteria.
Vaccine examples: Diphtheria, tetanus
- Polysaccharide & Conjugate vaccines
Polysaccharides are strings of sugars. Some bacteria such asStreptococcus pneumoniae have large amounts of polysaccharide on their surface, which encapsulate the bacteria. Polysaccharide vaccines are poorly immunogenic, and can only induce a primary immune response, so no immune memory is made for protection later on.
Polysaccharide conjugate vaccines contain carrier proteins that are chemically attached to the polysaccharide antigens. This addition results in the activation of a T-cell response, inducing both high-affinity antibodies against the polysaccharide antigens, and immune memory and can be used in infants.
Vaccine examples: Hib-PRP, PCV13 and MenACWY
- Recombinant
Recombinant vaccines are made using a gene from the disease-causing pathogen. The gene is inserted into a cell system capable of producing large amounts of the protein of interest. The protein produced can generate a protective immune response.
Vaccine examples: Hepatitis B vaccine and HPV vaccine
Nucleic acid
Recent developments in vaccine technology have allowed the use of messenger ribonucleic acid (mRNA) to deliver the genetic code to our dendritic cells to make specific viral proteins. Since mRNA is easily destroyed by ubiquitous ribonuclease enzymes, it is protected inside a lipid nanoparticle that also facilitates uptake by the dendritic cells. Inside the dendritic cell, ribosomes and vaccine mRNA generate the viral protein which is then presented to the T and B cells in the lymph nodes.
Vaccine example: Pfizer COVID-19 Vaccine (mRNA-CV)
Viral vector
Viral vector vaccines also use mRNA to code for a protein to be made in the body, however, the method of transport into cells is different. A viral vector will use a harmless adenovirus to introduce the protein to immune cells. The immune cell then creates the protein from the mRNA instructions and triggers an immune response.
Vaccine example: AstraZeneca COVID-19 vaccine (ChAd-CV)
In addition to an antigen, a vaccine may contain a range of other substances; for example, an immune enhancer (adjuvant) and/or a preservative.
Animal derived products
Some people have concerns about animal-derived products such as gelatin in vaccines. This may be for faith-based reasons or concerns about the safety of animal derived products. More information on animal derived products in vaccines can be found on the Written Resources page on the Immunisation Advisory Centre web site
Allergies to vaccine ingredients
Very rarely, vaccines provoke a serious allergic reaction called anaphylaxis. The risk of this happening varies from vaccine to vaccine. Over all the risk is between less than one to up to three times, out of every million doses of a vaccine. The components more likely to cause such a reaction are gelatin, egg proteins and antibiotics, although theoretically an allergic reaction can be triggered by almost anything.
A person’s allergy history should always be assessed prior to immunisation, however there are very few occasions when vaccines should not be given. A vaccine should not be given when there is a history of anaphylaxis to an ingredient in the vaccine, except for egg anaphylaxis and influenza vaccine, or to a previous dose of the same vaccine. A vaccine can be given when past reactions were not anaphylaxis, for example, reactions which have only involved the skin.
Vaccine additives:
- Adjuvant
- Excipients
