Vaccines work by making us produce antibodies to fight disease without actually infecting us with the disease.
If the vaccinated person then comes into contact with the disease itself, their immune system will recognise it and immediately produce the antibodies they need to fight it.
Newborn babies are already protected against several diseases, such as measles, mumps and rubella, because antibodies have passed to them from their mothers via the placenta. This is called passive immunity.
Passive immunity usually only lasts for a few weeks or months. In the case of measles, mumps and rubella, it may last up to one year, which is why the MMR jab is given to children just after their first birthday.
How are vaccines made?
The first step is to make the organism that produces the disease, which is called the pathogen. The pathogen is a virus or a bacterium. Viruses and bacteria can be mass-produced in the laboratory by infecting cells grown in tissue culture.
The pathogen must then be altered to ensure it doesn't trigger the disease itself. The treated pathogen is then combined with other vaccine ingredients, such as stabilisers and preservatives, to produce a vaccine dose.
Can you overload a child's immune system?
You might be concerned that giving too many vaccines at the same time will "overload" your child's immune system, especially at one year of age, when four injections are given in a single session.
But this isn't the case. Studies have shown there are no harmful effects from giving multiple injections or vaccines in one session.
As soon as a baby is born they come into contact with a huge number of different bacteria and viruses every day, and their immune system copes with them and becomes stronger as a result.
The bacteria and viruses used in vaccines are weakened or killed, and there are far fewer of them than the natural bugs that babies and children come into contact with.
Immunisation helps to improve protection against life-threatening diseases at the very earliest opportunity.
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How long does a vaccination last?
Some vaccines provide very high levels of protection - for example, MMR provides 90% protection against measles and rubella after one dose. Others are not as effective - typhoid vaccine, a travel vaccine, provides around 70% protection over three years.
How a vaccination programme works
When a vaccination programme is introduced, everyone in the population of a certain age or risk group is offered a specific vaccine to try to reduce the number of cases of the disease.
Vaccination programmes aim to protect people for life. They often concentrate on young children, as they're particularly vulnerable to many potentially dangerous infections.
When a vaccination programme against a disease begins, the number of people catching the disease goes down. As the threat decreases, it's important to keep vaccinating, otherwise the disease can start to spread again.
If enough people in a community are vaccinated, it's harder for a disease to pass between people who have not been vaccinated. This is called herd immunity.
Herd immunity is particularly important for protecting people who can't get vaccinated because they're too ill or because they're having treatment that damages their immune system.
Public Health England (PHE) records the vaccinations that adults and children receive. PHE also records the number of cases of each disease each year.
This way, PHE can work out the impact that each vaccination has on a particular disease. This data helps the Joint Committee on Vaccination and Immunisation (JCVI) consider whether the routine vaccination programme needs to be changed.
As more of the population is vaccinated, the disease can sometimes disappear completely and the vaccination programme can be stopped, as has happened with smallpox.
The more infectious the disease, the greater the number of people who have to be vaccinated to keep the disease under control. Measles, for instance, is highly infectious. If vaccination rates go down, measles will quickly spread again.
We know that at least 90% of children have to be immune to stop the disease from spreading. If 95% of children are protected by MMR, it's possible to eliminate not just measles, but mumps and rubella as well.
Article provided by NHS Choices