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How do vaccines work?

How do vaccines work and why are they important? In this section we introduce vaccination, herd immunity and other key concepts. ​​Scroll down for more information or download the full guide here.

Section 1: How vaccines work


  • How do vaccines work?

  • What is vaccination?

  • How does vaccination work?

  • Is it better for my child to get the disease naturally?

  • How effective is vaccination?

  • Have vaccines made a difference?

  • If these diseases are so rare, why does my child need to be vaccinated?

  • What is ‘herd immunity’?

  • How do I know vaccines are safe?

  • What are vaccines made of?


How do vaccines work

What is vaccination?

Vaccination is the safest way to protect your child against an infectious disease. Once your child has been vaccinated, they should have the ability to fight off the disease if they come into contact with it. They will have a level of protection, or immunity, against the disease.​

How does vaccination work?

The immune system is a network of cells, tissues and organs that work together to help fight off infection from harmful bacteria or viruses. When a disease-causing agent, such as virus or bacteria, invades your body, your immune system recognises it as harmful and will trigger a response to destroy it.

One of the ways your immune system fights off infection is by creating large proteins known as antibodies. These antibodies act as scouts, hunting down the infectious agent, and marking it for destruction by the immune system. Each antibody is specific to the bacteria or virus that it has detected and will trigger a specific immune response. These specific antibodies will remain in the immune system after the infection has gone. This means that if the same disease is encountered again, your immune system has a ‘memory’ of the disease and is ready to quickly destroy it before you get sick and any symptoms can develop.

Sometimes, however, the immune system doesn’t always win this initial battle against the harmful bacteria or virus and you can become very ill or – in extreme cases – die. Vaccination is the safest and most common way to gain immunity against a bacteria or virus that your body has yet to encounter. Vaccines contain a harmless form of the bacteria or virus that causes the disease you are being immunised against. The bacteria or virus will be killed, greatly weakened, or broken down into small parts before use in the vaccine so that they can trigger an immune response without making you sick. Your immune system will still attack the harmless form of bacteria or virus from the vaccine and will produce antibodies to fight it off. The immune system then keeps a memory of the disease, so if a vaccinated person encounters the disease years later, their immune system is ready to fight it off and prevent an infection from developing.

Is is better for my child to get the disease naturally?

No. The only way to get the disease naturally would be through infection with the bacteria or virus that causes the disease. This would pose a serious risk to your child’s health, potentially making them very ill and causing long-term effects. Some diseases, such as measles and meningitis, can also be fatal. Natural infection also enables the disease to spread from your child to those around them, increasing the risk of others getting ill. Vaccination allows your child to build up immunity in a safe and controlled environment without becoming ill with the disease and passing it to others.

How effective is vaccination?

Vaccination is extremely effective with most childhood vaccines effective in 85% to 95% of children who receive them.1 It is considered one of our greatest global health achievements and is estimated to save 2–3 million lives a year.2 Thanks to vaccines, life-threatening diseases that used to be common in young children in the UK, such as diphtheria, whooping cough and polio, are now relatively rare. Looking at the history of vaccine-preventable disease, there is a huge drop in the number of cases of a disease following the introduction of a vaccine against it. If smallpox had not been eradicated, it would cause 5 million deaths worldwide a year!3 Through vaccination, some diseases have even been eradicated completely, for example smallpox.

Have vaccines made a difference?

How effective is vaccinations


If these diseases are so rare, why does my child need to be vaccinated?

All of the diseases that we vaccinate against exist in the world today. Therefore, if your child has not been vaccinated, there is still a risk that they could get the disease and become very sick. We know that decreases in vaccination uptake can result in outbreaks of diseases such as measles.5 Regular vaccination is needed to keep our children healthy, prevent outbreaks from occurring and to eventually eradicate these diseases altogether. Infectious diseases are easily passed from person to person and entire communities can rapidly become infected. If a high enough proportion of a community is protected by vaccination, it makes it difficult for the disease to spread because the number of people who can be infected is so small.

Your immune system is there to protect you; by vaccinating your child, you give his/her immune system all the tools it needs to keep them safe from many severe diseases Meike Heurich-Sevcenco, BSI Vaccine Champion

This type of protection is known as ‘herd immunity’ and is particularly crucial for some individuals who are unable to receive some vaccines. This may include those that are too young, undergoing certain medical treatment (such as for cancer) or have a health condition that impairs the function of their immune system (such as HIV). Declines in herd immunity caused by decreasing vaccination rates have recently caused outbreaks of measles and whooping cough in the UK.6,7

For herd immunity to work, a high percentage of the community needs to be vaccinated. Although average vaccination rates in the UK are relatively high, there are still pockets of the UK where rates fall significantly below what is required for herd immunity.8 If the vaccination rates in your community are not high enough, it will leave the most vulnerable in your neighbourhood at a much greater risk of catching the disease. By vaccinating your child, you’re not only protecting them, but you are also protecting the most vulnerable in your community.

What is herd immunity?

What is herd immunity

How do I know vaccines are safe?

Before a vaccine can be given to the population it must go through rigorous testing. Like all medicines, vaccines go through many clinical trials, where they are administered and monitored in groups of volunteers. In the UK, the results of trials are then assessed by the Medicines and Healthcare products Regulatory Agency (MHRA). Once licensed, the vaccine must then be further approved by the MHRA before it is added to the routine vaccination programme. Even once a vaccine becomes part of the vaccination programme, it is continually monitored for safety and effectiveness by the MHRA. Any suspected side effects are reported by medical providers or patients to the MHRA using the yellow card scheme. No medicine can ever be completely risk free or 100% effective. However, strong licensing processes and safety tests ensure that the health benefits of medicines being given through the NHS greatly outweigh any risks. As vaccines are given to healthy people, these regulatory measures are even stricter, meaning that the level of ‘acceptable risk’ for vaccines is much lower than it would be for other medicines.9

What are vaccines made of?

Each vaccine will be made up of slightly different ingredients depending on the disease it is targeting. The active ingredient in a vaccine is a very small amount of the killed, greatly weakened or broken-down parts of the bacteria or virus you are vaccinating against. Vaccines also contain small amounts of preservatives and stabilisers, such as sorbitol and citric acid. These can already be found in the body or in food – usually in much larger quantities than the amount used in a vaccine. However, the most abundant ingredient in a vaccine is water.

Some vaccines also contain aluminium – usually in the form of aluminium hydroxide. Aluminium is found naturally in nearly all food and drinking water and is used in vaccines to strengthen and prolong the immune response they generate.10 The amount of aluminium in vaccines is extremely small and a recent study found that, in an infant’s first year of life, the total amount of aluminium in both vaccines and food is less than the weekly safe intake level.11

Aluminium is also found in many other medicines, such as heartburn medication.12 Formaldehyde is used in the manufacture of vaccines. It is an organic compound which is found in many living things and humans produce formaldehyde naturally as part of the metabolic process. While it is true that high levels of formaldehyde can be harmful to humans, the amount of formaldehyde present in any vaccine is fifty times smaller than that found in a pear.10 For a complete list of ingredients in each individual vaccine, you can refer to the Patient Information Leaflet (PIL) or Summary of Product Characteristics (SPC) sheet that comes with each vaccine. Both can be found online. Helpful information can also be found on the University of Oxford’s Vaccine Knowledge Project webpages.

Immunisation is a proven tool for controlling and eliminating life-threatening infectious diseases and is estimated to avert between 2 and 3 million deaths each year - World Health Organization