Vaccines work by stimulating our immune system to produce antibodies (substances produced by the body to fight disease) without actually infecting us with the disease.
They trigger the immune system to produce its own antibodies, as though the body has been infected with a disease. This is called "active immunity". 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 into them from their mothers via the placenta. This is called "passive immunity". Passive immunity 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 (called the pathogen) that produces the disease. 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 that it doesn't trigger the disease itself. This can be done by:
- weakening, or "attenuating", it by growing it repeatedly to select a strain that's less dangerous – MMR vaccines are attenuated
- taking out the part of the pathogen that causes the immune response and using this in the vaccine – the Hib vaccine is made in this way
- using the toxin that the pathogen makes and inactivating it – this is how the tetanus vaccine is produced
The treated pathogen is then combined with other ingredients, such as stabilisers and preservatives, to produce a vaccine dose.
Can you overload a child's immune system?
You might be concerned that too many vaccines at a young age could "overload" your child's immune system, but this really isn't the case. Studies have shown that vaccines do not weaken a child's immune system.
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 well with them.
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. In fact, if a child was given 11 vaccines all at the same time, it would only use a thousandth of their immune system!
Watch this short animation to find out more.
How long does a vaccination last?
In many cases, vaccination provides lifelong protection against a disease, but this depends on which vaccination a person is given. How long a vaccination lasts depends on the disease that is being protected against, the vaccine and the person who has been vaccinated.
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. Some vaccination programmes are targeted at older people or certain risk groups.
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 in protecting people who can't get vaccinated because they're too ill, or they are 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 in order 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.
Read about how vaccines are fighting disease around the world.