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What is a vaccine?

An explanation on what vaccines are and how they work.
To understand how vaccines work, it helps to look first at how the immune system works because vaccines harness the natural activity of your immune system. There are about 100 trillion bacteria and viruses on your body, not all of them cause disease but some are able to get inside our bodies to multiply and this can make us ill. There are barriers to stop this happening but suppose some disease causing bacteria do get through, your immune system is quick to recognise them as invaders. This is because the proteins or sugars on the bacteria surface have different shapes to any of the ones in the human body.
They trigger a complex chain of events involving many different types of white blood cells working together. One type of white blood cell is able to make antibodies to fight the invaders, antibodies can stick to the proteins or sugars on the bacteria surface and this kills the bacteria or disables them. However not all antibodies will work against these bacteria. They have to be exactly the right shape, a bit like a key fitting a lock. Our bodies have a library of billions of white blood cells, each of which can make just one shape of antibody. Only a few of these antibodies will match the invading bacteria. Producing antibodies at the right shape can take several days.
By this time there could be billions of disease causing bacteria in your body, once the right cells are activated, they quickly divide and turn into a production line, making masses of antibodies that stick to the bacteria. Eventually your body gets rid of all the bacteria and you recover. Antibodies remain in the blood and some white blood cells may also become memory cells. If those specific bacteria invade the body again, the immune system will respond so quickly that you won’t get ill. Vaccines work in the same way, they contain weakened or dead bacteria or viruses or even just a few proteins or sugars from the surface, this is enough to convince the immune system that a real invader has got in.
The same process takes place as when real bacteria or viruses invade our bodies, except you don’t get ill. Afterwards if your body ever meets the real thing, your immune system will remember it and get rid of it before you even know it’s there.

This video explains how vaccines help our bodies to recognise and fight diseases. You can learn more at the Vaccine Knowledge Project website:

How do vaccines work?

A vaccine is a preparation that is used to stimulate the body’s immune response against diseases.

Live vaccines are active vaccines such as attenuated strains that replicate in a host. The pathogen, such as virus or bacterium, has been weakened to reduce virulence to cause none or very mild disease in healthy people, which acts like a natural infection. Live vaccines stimulate the closest response to actual infection and therefore elicit good, strong, long-lasting immune responses.

A non-live vaccine does not have any live biological material and can’t cause the disease it is protecting. An adjuvant (substance used to increase the immune response to the vaccine) is required, and several doses of inactivated vaccines may be required to evoke a sufficient immune response.

Some examples of these are highlighted below:

  • Inactivated vaccines: suspensions of whole intact killed pathogens e.g. whole-cell pertussis (bacteria), inactivated poliovirus
  • Sub-unit vaccines: contain one or a few components (sub-units) of the pathogen which is important in protection e.g. hepatitis B vaccine which uses only surface proteins of the virus
  • Conjugate vaccines: These conjugate a poorly immunogenic antigen and a protein carrier that is highly immunogenic, providing a lasting immune response e.g. Haemophilus influenzae B (Hib) vaccine uses bacterial polysaccharides combined to its capsule proteins
  • Toxoid vaccines: based on the toxin produced by bacteria that evades the bloodstream and causes most of the disease symptoms e.g. Tetanus and diphtheria vaccines
  • mRNA vaccine: Messenger RNA (mRNA) vaccines give information to cells on how to make a protein that will trigger an immune response e.g. SARS-CoV-2 vaccine

Share other examples of vaccines in the comments section

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