What is a vaccine?

a profile photo of Joanne Blyfeet
GP and Lead Medical Appraiser, Bupa Health & Dental Centre, Reading
22 August 2023
Next review due August 2026
Many of us will have childhood memories of leaving the nurse’s office with a plaster on one arm and a sticker on the other, after vaccinations. But what exactly is a vaccine? And how do they help to protect you from disease? Here, I’ll explain.
dad cuddling a baby

What is a vaccine?

A vaccine is a medicine given to prevent disease. Vaccines teach your body’s immune system to recognise and fight off harmful diseases that it hasn’t encountered before. This lowers your risk of getting that disease and also helps prevent the spread of that disease to other people. And while vaccines may not always stop you from getting a disease, they can reduce the severity of the symptoms.

Vaccines are usually given by an injection, or sometimes by mouth or a nasal spray. You might also sometimes hear injections referred to as ‘jabs’.

Vaccines are an effective way to protect yourself from disease. You may have heard in the media or online that vaccines are unsafe. For example, there have been claims that the measles, mumps, and rubella (MMR) vaccine is linked to autism in children. But research has shown that there’s no link between the MMR vaccine and autism.

Vaccines go through extensive clinical trials and thorough safety testing before they’re approved and made available to the public. Even then, they’re continuously monitored and reassessed by healthcare professionals to ensure they’re still safe to use. Monitoring is important because it helps ensure that the benefits of vaccination are greater than the risks, which are usually much smaller.

How do vaccines work?

Vaccines contain a weakened or dead version of the bacteria or virus that causes a disease. Because the bacteria or virus is weakened, it won’t cause disease – it will imitate it. Once the germ is in your body, your immune system will respond to it and produce antibodies against it. Antibodies are proteins that recognise germs, like viruses and bacteria, and destroy them. Your immune system will then remember how to fight that germ if you’re exposed to it again in the future.

If you’re vaccinated against a certain disease, you have immunity because your immune system knows how to fight that disease. Vaccination is often called immunisation – this is the process through which you become immune to disease because you’ve been vaccinated.

Some vaccines contain several different germs or viruses in a single injection, so you can be vaccinated against several diseases at once. This is known as a combined vaccination. It’s safe to receive more than one vaccine at a time - your body already encounters hundreds of germs every day.

You might need more than one dose of a vaccine to build immunity. For some illnesses, such as seasonal influenza (flu), the immunity provided by vaccines does not last that long. This is why annual flu jabs are recommended for people at risk of severe flu. Other vaccines may need a booster to bring immunity levels back up after several years.

You might experience some mild side-effects after having a vaccination. For example, your arm might feel sore if you’ve had an injection, or you might have a slight temperature. This is nothing to worry about, as your body is learning to fight the disease, and you should feel better within a few days.

Who should get a vaccine?

There are lots of different vaccines that can help protect you and those around you against many different diseases. In the UK, you’ll usually have a series of vaccinations for common diseases when you’re a baby and throughout childhood as part of the routine childhood immunisation programme.

Other vaccinations are offered to certain groups of at-risk people. For example, the flu vaccine is recommended each year for pregnant women and people with long-term health conditions. Anyone with a weakened immune system or aged 65 or older is also recommended to get the flu vaccine.

There are some circumstances where it might not be suitable for you to have a vaccine. For example, if you’ve had an allergic reaction to one in the past – although this is rare. Your doctor or nurse will be able to let you know which vaccines are recommended for you personally.

If you’ve missed a vaccination, it’s not too late to have one – contact your GP surgery to book an appointment.

If you’re planning a trip overseas, you might also need travel vaccinations before you go, to protect you from any diseases that may be common in that country. This will depend on where you’re travelling to. Contact your travel nurse for more advice at least eight weeks before you plan to leave. This is because your body needs time to build a response to the vaccine, so having it just before you travel means it won’t be effective.

Why are vaccinations important?

The World Health Organization estimates that more than four million lives are saved every year due to vaccines. If you don’t get vaccinated, you’re at a much higher risk of becoming ill with a disease – and passing it to others – if you come into contact with it. Some of these diseases can even be fatal.

It’s especially important for babies and young children to be vaccinated because they’re exposed to many different germs every day, but their immune systems aren’t fully developed yet.

a profile photo of Joanne Blyfeet
Jo Byfleet
GP and Lead Medical Appraiser, Bupa Health & Dental Centre, Reading



Sheila Pinion, Health Content Editor at Bupa UK

    • Vaccines and immunization: what is vaccination? World Health Organization., published 30 August 2021
    • 5 reasons it’s important for adults to get vaccinated. Centers for Disease Control and Prevention., last reviewed 12 September 2022
    • Immunization: the basics. Centers for Disease Control and Prevention., last reviewed 1 September 2021
    • People urged to get COVID-19 jab before offer ends. UK Government., published 5 May 2023
    • Autism and vaccines. Centers for Disease Control and Prevention., last reviewed 1 December 2021
    • How vaccines are developed and approved for use. Centers for Disease Control and Prevention., last reviewed 30 March 2023
    • Vaccine. Encyclopaedia Britannica., last updated 17 July 2023
    • Antibody. National Human Genome Research Institute., updated 1 August 2023
    • Antibody. Encyclopaedia Britannica., last updated 19 June 2023
    • Vaccines and immunological products. Patient., last updated 14 December 2022
    • Overview of immunization. MSD Manual., revised July 2023
    • Seasonal influenza vaccination strategies. European Centre for Disease Prevention and Control., last updated 8 March 2023
    • Explaining how vaccines work. Centers for Disease Control and Prevention., last reviewed 24 May 2023
    • Vaccine side effects. US Department of Health and Human Services., last reviewed 6 May 2022
    • Routine childhood immunisation schedule. UK Government., last updated 17 February 2022
    • Risk groups for severe influenza. European Centre for Disease Prevention and Control., last updated 9 March 2023
    • Common questions about vaccines. Centers for Disease Control and Prevention., last reviewed 7 July 2022
    • Travel vaccinations. Cancer Research UK., last reviewed 24 November 2022

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