What different types of vaccines are there?
There are several types of vaccine that use different approaches to train your immune system to fight off germs, such as viruses, bacteria or parasites that cause disease. They use three basic approaches – using a weakened version of the virus, bacteria or parasite (microbe), a dead or inactive version of it or fragments of it that your immune system can recognise. During vaccine development, scientists carefully weigh up what type to use for a given germ based on our understanding of the germ and the nature of immunity, what is found to work best, and the most cost-efficient way to provide protection.
The different types of vaccine currently used are listed below:
Live attenuated vaccines
These vaccines contain a low dose of a weakened form of the virus or bacteria. These have the benefit of producing a strong immune response as the virus or bacteria will replicate inside your body and behave in much the same way as a real – but very mild – infection. This means high levels of antibodies and immune memory cells are produced throughout your body. As the virus or bacteria is weakened, it will cause you only mild symptoms such as a temperature, tiredness or a headache.
But live vaccines tend to be unsuitable for people with weakened immune systems, such as those undergoing certain cancer treatments, as the pathogen used can still cause disease in these people. The combined measles, mumps and rubella (MMR) vaccine, BCG (the TB vaccine), yellow fever, chickenpox and the rotavirus vaccines are all examples of live vaccines.
Inactivated vaccines
Inactivated vaccines also contain the whole virus, bacteria or parasite, but in this case it has been killed so it cannot cause an infection. These are safe for people with weakened immune systems but tend to be not as effective at training your immune system as live vaccines. This means you can often require additional doses and boosters to help your immune system to learn to recognise the germ. The polio vaccine given to children in the UK and some vaccines offered to travellers, such as the one for Japanese Encephalitis Virus JEV, are examples of inactivated vaccines.
Sub-unit vaccines
Vaccines that use only part of a virus, bacteria or parasite are known as sub-unit vaccines. These contain a fragment or fragments from the germ known as an antigen. In some cases, these can be a protein or a sugar that is often found on the outside of the germ, which means it is easy for your immune system to spot.
Sub-unit vaccines can be manufactured and designed either by growing the virus or bacterium and then removing the relevant part for use, or by synthesising that part in a laboratory. The whooping cough vaccine used in the UK is an example where a sub-unit is purified from a bacterium that has been killed (another version of the whooping cough vaccine uses a whole killed bacterium that is used widely in low and middle-income countries primarily due to cheaper production and current evidence suggests also longer protection, but it can cause more common mild side-effects).
Some vaccines, known as recombinant vaccines, use genetically engineered yeast, bacteria or mammalian cells to produce large quantities of a sub-unit before it is purified. The hepatitis B component in the 6-in-1 vaccine given to infants in the UK and the HPV vaccine are both examples of recombinant vaccines.
Others combine two sub-units together – such as a sugar and a protein – to produce a stronger and longer lasting immune response. These are known as conjugate vaccines. The MenACWY vaccine, which contains sugars from the surface of these bacteria which have been chemically connected to a protein (usually toxoid from diphtheria or tetanus) protects against meningococcal disease in the UK is an example of a conjugate vaccine. Other examples are the pneumococcal vaccine and Haemophilus influenzae type b vaccine given to babies.
Sub-unit vaccines cannot cause infections and do not run the risk of causing the disease. But they may require multiple doses and sometimes need adjuvants to ensure robust and long-term immunity.
Toxoid vaccines
Toxoid vaccines are a type of sub-unit vaccine that focus on the harmful toxin produced by a pathogen that causes disease. The bacterium that causes tetanus, for example, produces a powerful toxin that damages cells in your body.
Toxoid vaccines use a version of the toxin that has been deactivated. This means your immune system will be able to recognise it as a potential threat, but it cannot cause you harm. In this way, your immune system can be safely trained to target the real toxin if it encounters it again in the future. After vaccination you will have more antibodies in your blood that can recognise and neutralise the toxin.
You may need booster shots of toxoid vaccines to top up your protection over time. Examples of toxoid vaccines include those against diphtheria and tetanus, for which six doses are recommended through childhood for life-long protection.
Viral vector vaccines
Viral vector vaccines are made by placing a small piece of genetic material from a disease-causing virus into another harmless virus. This genetic material contains the instructions to make antigens – the key parts of the virus that the immune system recognises. The harmless virus – or vector – delivers these genetic instructions into your body, but it is not capable of causing disease. Instead, it uses the natural machinery of your cells to translate the genetic instructions to produce antigens that your immune system will recognise as a threat. This trains your immune system to spot a real germ carrying these antigens should you encounter it for real and so mount a more effective defence against it. Examples of viral vector vaccines include a type of Ebola vaccine and the Oxford/AstraZeneca Covid-19 vaccine.
mRNA and DNA vaccines
mRNA and DNA vaccines are fragments of genetic material from a virus. These provide the blueprints that your own cells can use to make sub-units of the virus in the same way as with the viral vector vaccines mentioned above. The genetic material piggybacks on the natural processes in your cells, turning them into the factories that can make their own vaccine antigens. The genetic material is short-lived and it is not incorporated into your own DNA found inside your cells. These vaccines do not carry the risk of causing disease and can be manufactured relatively quickly in response to a new variant of a pathogen. The mRNA vaccines against Covid-19 are examples of these.
Why are some vaccines given as injections and others are not?
Some vaccines are administered via an injection, others are given orally as drops, while some can be a nasal spray. The method of administering a vaccine is determined based on who it is being given to and where in your body the disease attacks. The routes for administering a vaccine are not interchangeable and a vaccine can only be given by the method it has been approved for by regulators.
How are vaccines manufactured?
There are several approaches to manufacturing vaccines that depend on the type of vaccine technology being used.
- Live attenuated virus vaccines are produced using cells grown in a laboratory that are then infected with the virus. This allows the virus to replicate to generate copies of itself before it is purified. Some viruses used to make live attenuated vaccines, such as some vaccines against influenza, are grown inside hen’s eggs (for other flu vaccines the virus is grown in cells in the laboratory).
- Inactivated vaccines are manufactured by first growing viruses or bacteria in the laboratory before then killing them with chemicals, heat or radiation to make them unable to grow.
Sub-unit vaccines can be produced by growing viruses or bacteria in the laboratory and then purifying antigens – the key parts of the virus or bacteria that the immune system recognises. The antigen can also be made in yeast, bacteria or mammalian cells by giving them the right genetic instructions. These cells then produce antigens in large quantities so they can then be purified. In some cases, two components – usually a sugar and a protein – are chemically linked together to create a conjugate vaccine. This can increase the quality and magnitude of the immune response. - Toxoid vaccines are manufactured by growing bacteria in the laboratory to obtain the toxin proteins they produce. The toxins are purified and then inactivated in ways that converts them into a safe form that will not harm your body. The resulting "toxoids" are then further purified before being mixed with other ingredients to stabilise and increase the immune reaction.
- Viral vector vaccines are made by first selecting a suitable harmless virus that can find its way into human cells without causing a disease. This is known as the vector. Genetic material from a virus that causes disease is then inserted into the vector before it is then grown in cells in controlled laboratory conditions and purified. The genetic material carried by the vector provides your cells with instructions to produce antigens from the disease-causing virus, which then train your immune system.
- mRNA vaccines are synthesised using snippets of genetic code from viruses or bacteria. mRNA is a single strand of genetic material that is found in all living cells on the planet. Its job is to carry information from the DNA inside your cells to the machinery that makes proteins your body needs to function. In a vaccine, the mRNA carries information about antigen proteins belonging to the virus or bacteria being targeted. This allows your cells to make many copies of the antigen. To create the vaccine, the mRNA molecule is synthesised in a laboratory and then encased inside a fatty droplet, which protects it and helps to deliver it to the cells in your body.