12/02/2021
Types of vaccine:
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Vaccines can be divided into a number of different types,but ultimately work on the same principle. This is to stimulate the immune response to recognise a pathogen (a disease-causing organism) or part of a pathogen.
Once the immune system has been trained to recognise this,if the body is later exposed to the pathogen, it will be removed from the body. Specifically, the immune system recognises foreign ‘antigens’, parts of the pathogen on the surface or inside the pathogen, that are not normally found in the body.
The oldest and most well-known method of vaccination is to use the whole disease-causing pathogen in a vaccine to produce an immune response similar to that seen during natural infection.
Using the pathogen in its natural state would cause active disease and could potentially be dangerous to the individual receiving it and risk the disease spreading to others. To avoid this, modern vaccines use pathogens that have been altered.
Live attenuated vaccines contain whole bacteria or viruses which have been “weakened”(attenuated) so that they create a protective immune response but do not cause disease in healthy people.
For most modern vaccines this “weakening” is achieved through genetic modification of the pathogen either as a naturally occurring phenomenon or as a modification specifically introduced by scientists.
Live vaccines tend to create a strong and lasting immune response and include some of our best vaccines. However, live vaccines may not suitable for people whose immune system doesn’t work, either due to drug treatment or underlying illness. This is because the weakened viruses or bacteria could in some cases multiply too much and might cause disease in these people.
Example of Live attenuated vaccines
Rotavirus vaccine
MMR vaccine
Nasal flu vaccine
Shingles vaccine
Chickenpox vaccine (special groups only)
BCG vaccine against TB (special groups only)
Yellow fever vaccine
Oral typhoid vaccine (not the injected vaccine)
Inactivated Vaccines
Inactivated vaccines contain whole bacteria or viruses which have been killed or have been altered, so that they cannot replicate. Because inactivated vaccines do not contain any live bacteria or viruses, they cannot cause the diseases against which they protect, even in people with severely weakened immune systems.
However, inactivated vaccines do not always create such a strong or long-lasting immune response as live attenuated vaccines.
Example of ‘Whole killed’ vaccines
Inactivated polio vaccine or IPV (in the 6-in-1 vaccine, pre-school booster, teenage booster and pertussis vaccine in pregnancy)
Some inactivated flu vaccines which are described as ‘split virion’
Hepatitis A vaccine (special groups only)
Rabies vaccine
Japanese encephalitis vaccine
Most of the vaccines in the UK schedule are subunit vaccines which do not contain any whole bacteria or viruses at all. Instead, these vaccines typically contain one or more specific antigens (or “flags”) from the surface of the pathogen.
The advantage of subunit vaccines over whole pathogen vaccines is that the immune response can focus on recognising a small number of antigen targets (“flags”).
Subunit vaccines do not always create such a strong or long-lasting immune response as live attenuated vaccines. They usually require repeated doses initially and subsequent booster doses in subsequent years. Adjuvants are often added to subunit vaccines.
These are substances which help to strengthen and lengthen the immune response to the vaccine. As a result, common local reactions (such as a sore arm) may be more noticeable and frequent with these types of vaccines.
Recombinant vaccines are made using bacterial or yeast cells to manufacture the vaccine. A small piece of DNA is taken from the virus or bacterium against which we want to protect and inserted into the manufacturing cells.
For example, to make the hepatitis B vaccine, part of the DNA from the hepatitis B virus is inserted into the DNA of yeast cells. These yeast cells are then able to produce one of the surface proteins from the hepatitis B virus, and this is purified and used as the active ingredient in the vaccine.
Most of the vaccines in the UK schedule are subunit vaccines which do not contain any whole bacteria or viruses at all. (‘Acellular’ means ‘not containing any whole cells’.) Instead these kind of vaccines contain polysaccharides (sugars) or proteins from the surface of bacteria or viruses.
These polysaccharides or proteins are the parts that our immune system recognises as ‘foreign’, and they are referred to as antigens. Even though the vaccine might only contain a few out of the thousands of proteins in a bacterium, they are enough in themselves to trigger an immune response which can protect against the disease.
Example of Recombinant vaccines
Hepatitis B vaccine (in the 6-in-1 vaccine and as the separate hepatitis B vaccine)
HPV vaccine
MenB vaccine. This contains proteins from the surface of meningococcal bacteria. Three of the proteins are made using recombinant technology.
Some bacteria release toxins (poisonous proteins) when they attack the body, and it is the toxins rather than the bacteria itself that we want to be protected against.
The immune system recognises these toxins in the same way that it recognises other antigens on the surface of the bacteria, and is able to mount an immune response to them.
Some vaccines are made with inactivated versions of these toxins. They are called ‘toxoids’ because they look like toxins but are not poisonous. They trigger a strong immune response.
‘Conjugate’ means ‘connected’ or ‘joined’. With some bacteria, to get protection from a vaccine you need to train the immune system to respond to polysaccharides (complex sugars on the surface of bacteria) rather than proteins. But in the early days of polysaccharide vaccines it was found that they did not work well in babies and young children.
Researchers discovered that they worked much better if the polysaccharide was attached (conjugated) to something else that creates a strong immune response. In most conjugate vaccines, the polysaccharide is attached to diphtheria or tetanus toxoid protein (see ‘Toxoid vaccines’ above).
The immune system recognises these proteins very easily and this helps to generate a stronger immune response to the polysaccharide.
Nucleic acid vaccines work in a different way to other vaccines in that they do not supply the protein antigen to the body.
Instead they provide the genetic instructions of the antigen to cells in the body and in turn the cells produce the antigen, which stimulates an immune response. Nucleic acid vaccines are quick and easy to develop, and provide significant promise for the development of vaccines in the future.
RNA vaccines use mRNA (messenger RNA) inside a lipid (fat) membrane. This fatty cover both protects the mRNA when it first enters the body, and also helps it to get inside cells by fusing with the cell membrane.
Once the mRNA is inside the cell, machinery inside the cell translates it into the antigen protein. This mRNA typically lasts a few days, but in that time sufficient antigen is made to stimulate an immune response.
It is then naturally broken down and removed by the body. RNA vaccines are not capable of combining with the human genetic code (DNA).
DNA is more stable than mRNA so doesn’t require the same initial protection. DNA vaccines are typically administered along with a technique called electroporation.
This uses low level electronic waves to allow the bodies’ cells to take up the DNA vaccine. DNA must be translated to mRNA within the cell nucleus before it can subsequently be translated to protein antigens which stimulate an immune response.
There are currently no licenced DNA vaccines, but there are many in development.
