Immunisation

Why do we immunise?

The UK immunisation programme aims to give children the best possible protection against the widest range of infectious diseases (see 'What is immunisation?' for more information).

There are 3 reasons why we immunise children: disease prevention, herd immunity, and disease eradication.

A paediatrician talks about why we immunise babies and young children.

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There are three reasons why we immunise children. First, immunisation prevents children from becoming ill with unpleasant and serious infectious diseases, which have a risk of complications and long-term side effects. Until these diseases are eradicated, each child that is not immunised is at risk of these complications if they catch the actual infectious disease itself, for instance in the case of children who actually catch measles itself complications such as ear infections, pneumonia and in rare cases, measles encephalitis (a rare brain disease which can occur in one in 5000 cases of actual measles infections) can occur.

Second, we immunise to try and help protect all children in the population. The more people who are immunised, the less of the infectious disease there is around so the less chance there is of anyone catching it. When levels of immunisation against an infectious disease is really, really high - then something happens called 'herd immunity' where the risk of the disease occurring is so low that even those who cannot be vaccinated are unlikely to be affected. When this happens it means that even children who have leukaemia or other forms of lowered resistance to infections who cannot be given live vaccines such as measles, mumps and rubella vaccine and the polio vaccine - and also babies under the age of one who are too young to have the measles, mumps and rubella vaccine will also not be exposed to the disease.

Third, we immunise to try and wipe out as many infectious diseases as we can everywhere in the world. Although improvements in living standards and cleaner water have been key factors in the decline of infectious diseases in the United Kingdom and elsewhere in the world, it is quite clear that the immunisation programmes have made a huge and significant impact in getting rid of infectious diseases as well. In the ten years before the measles vaccine was introduced in to the United Kingdom in 1968, there were around 380,000 cases a year of measles and 850 deaths (Bedford, Elliman, 2001). Measles, mumps and rubella vaccine was introduced in the United Kingdom in 1988. In the ten years since then - between 1989 and 1998, there were only four deaths associated with acute measles and 19 deaths resulting from the measles complications.

In 2008, there were outbreaks of measles in London, resulting in a total of 664 cases.

The recent increase in the numbers of cases of mumps and measles in the United Kingdom is a direct result of children not being immunised following unfounded and unnecessary scares about the measles, mumps and rubella vaccine.

Immunising for disease preventaion

Immunisation prevents children from becoming ill with unpleasant and serious infectious diseases, which have a risk of complications and long-term side effects.

Until these diseases are eradicated, every child that is not immunised is at risk of complications and fatal outcomes (death) if they catch the actual infectious disease itself. Serious complications such as ear infections, pneumonia, and in rare cases, measles encephalitis (swelling of the brain) can occur and can lead to damaging and fatal outcomes (University of Oxford Vaccine Knowledge, 2025).

Immunising for herd immunity

We immunise to try and help protect all children in the population. The more people who are immunised, the less of the infectious disease there is around, and so the less chance there is of anyone catching it.

When levels of immunisation against an infectious disease are really, really high, then something called 'herd immunity' happens, where the risk of the disease occurring is so low that even those who cannot be immunised are unlikely to be affected by the disease.

When herd immunity happens, it means that even children who have lowered or weakened resistance to infections (are immunocompromised) who cannot be given certain vaccines such as the measles, mumps and rubella (MMR) vaccine and the polio vaccine, as well as babies under the age of one who are too young to have the MMR vaccine, will also not be exposed to the disease. These groups especially rely on herd immunity to protect them as they are especially at risk of fatal outcomes from infectious diseases.

The proportion of immunised children (the vaccine coverage) required to produce herd immunity varies from disease to disease; with measles this number is estimated to be about 95%.

Immunising for disease eradication

We immunise to try and wipe out as many infectious diseases as we can everywhere in the world. Although improvements in living standards and cleaner water have been key factors in the decline of infectious diseases in the United Kingdom and elsewhere in the world, it is quite clear that the immunisation programmes have made a significant impact in getting rid of infectious diseases as well.

In the 10 years before the measles vaccine was introduced in the UK, the recorded rate of measles infections was between 160,000-800,000 per year and the recorded annual (yearly) rates of death resulting from measles ranged from 100-1,000 (NICE, December 2024; UK Security Agency, 12 September 2024).

After the MMR vaccine was introduced in the UK, rates of measles infections and deaths fell substantially. To compare, in the 10 years (1989-1998) after the MMR vaccine was introduced in the United Kingdom, the number of recorded deaths associated with measles was 23 total.

The UK Health Security Agency estimates that around 20 million measles cases and 4,500 deaths from measles have been avoided since the vaccine was introduced (University of Oxford Vaccine Knowledge, 24 January 2024).

The majority of measles cases have been in people who are not vaccinated, and this number is especially high in young people aged 15 and over who did not receive the MMR vaccination when they were younger. Further, since 2000, all reported deaths from measles have been in people who did not receive the MMR vaccine (NICE, December 2024).

The recent increase in the numbers of cases (the transmission rate) of mumps and measles in the United Kingdom is a direct result of children not being immunised following unfounded and unnecessary scares about the measles, mumps and rubella (MMR) vaccine. 

With more people not immunising against measles, infection and death rates have continued to increase since 2018 at a high rate, and the UK Health Security Agency (HSA) has raised a national incident urging people to vaccinate their children against measles (University of Oxford Vaccine Knowledge, 24 January 2024).

*For more information about the total recorded rates of measles infections and deaths per year, see the UK Health Security Agency, ‘Research and analysis: Measles notifications and deaths in England and Wales’, GOV.UK.

Last reviewed: April 2025.
Last updated: April 2025.