In short: Vaccines train your immune system to recognise an infection so you are less likely to become seriously ill if you are exposed. For licensed vaccines used in the UK, the benefits of protection far outweigh the risks for most people. Common side effects such as a sore arm, mild fever or tiredness are usually short-lived, and serious reactions such as anaphylaxis are very rare. Every UK vaccine is monitored for safety after licensing by the MHRA through the Yellow Card scheme.

This guide, written by a Ready Health prescribing pharmacist, explains how vaccines work, what the benefits and risks are, and how to make an informed decision.

How do vaccines work?

A vaccine contains a harmless form or part of a germ, or instructions for making a part of it. Your immune system responds by producing antibodies and memory cells. If you later meet the real infection, your body can recognise and fight it faster, preventing illness or making it milder. Some vaccines need more than one dose or a booster to build or maintain protection.

What are the benefits of vaccination?

Individual protection against infections that can cause serious illness, disability or death, such as measles, meningitis, pneumococcal disease, flu and hepatitis.
Community protection: when enough people are vaccinated, it is harder for infections to spread, which protects babies, older people and those who cannot be vaccinated for medical reasons.
Disease control: vaccination eradicated smallpox worldwide and has made diseases such as polio and diphtheria rare in the UK.
Cancer prevention: the HPV vaccine protects against the virus types that cause most cervical cancers and some other cancers.
Safer travel: travel vaccines reduce the risk of infections that are uncommon in the UK but present elsewhere.

When vaccine uptake falls, diseases can return. England has seen outbreaks of measles in recent years linked to lower MMR uptake.

What are the possible side effects of vaccines?

Type	Examples	How common
Common, mild	Sore, red or swollen arm; mild fever; tiredness; headache; aching	Common; usually settle within 1 to 2 days
Vaccine-specific	A mild rash or fever 7 to 12 days after MMR; diarrhoea after the oral rotavirus vaccine	Varies by vaccine
Serious allergic reaction (anaphylaxis)	Swelling of the face or throat, breathing difficulty, collapse	Very rare; staff giving vaccines are trained and equipped to treat it
Other rare serious events	Specific to particular vaccines and described in the patient information leaflet	Rare; monitored by the MHRA

You can reduce discomfort by keeping the arm moving and using paracetamol if needed (following the dose on the pack). Seek urgent help for any signs of a severe allergic reaction.

How are vaccines checked for safety in the UK?

Vaccines go through laboratory testing and clinical trials before they are licensed by the MHRA.
The Joint Committee on Vaccination and Immunisation (JCVI) advises which vaccines should be offered and to whom.
After licensing, the MHRA continues to monitor safety. Anyone, including patients and parents, can report a suspected side effect through the Yellow Card scheme.
If a safety concern emerges, the MHRA can update advice, restrict use or withdraw a product.

Who should not have certain vaccines?

Most people can be vaccinated safely. Some vaccines are not suitable for people who have had a severe allergic reaction to a previous dose or an ingredient, and live vaccines are usually avoided in pregnancy and in people with significantly weakened immune systems. A minor illness without fever is not usually a reason to delay. A clinician will check your history before vaccinating.

Common vaccine myths

"The MMR vaccine causes autism." Large studies involving millions of children have found no link. The original claim was discredited and retracted.
"Natural immunity is better." Catching the disease can mean serious complications; vaccines aim to give protection without that risk.
"Too many vaccines overload the immune system." The immune system deals with many germs every day; the UK schedule is designed to be given safely together.

Frequently asked questions

Can a vaccine give me the disease it protects against?

Inactivated and subunit vaccines cannot cause the infection. Live vaccines can occasionally cause a mild version of the illness, which is why they are not given to people with weakened immune systems.

Can I have more than one vaccine at the same appointment?

Usually yes. Many vaccines can be given together; a clinician will check timing, especially for live vaccines.

What should I do if I think I have had a side effect?

Speak to a pharmacist, GP or NHS 111, and report it through the MHRA Yellow Card scheme. Call 999 for signs of a severe allergic reaction.

Is there support if someone is seriously harmed by a vaccine?

In the UK, the Vaccine Damage Payment scheme can provide a one-off payment in rare cases of severe disability caused by certain vaccines. Details are on GOV.UK.

Key takeaways

Vaccines prevent serious illness for individuals and reduce the spread of infection in the community.
Most side effects are mild and short-lived; serious reactions are very rare.
UK vaccines are licensed and continuously monitored by the MHRA.
Tell the person vaccinating you about allergies, pregnancy and any conditions or treatments that affect immunity.

Sources and further reading

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician.

If you have questions about vaccines for travel or other needs, you can book a consultation at Ready Health in Standish, serving Wigan, Chorley and Bolton, or find out more about our vaccination clinic. Routine childhood and NHS-eligible adult vaccines are available free through your GP.

Vaccines: Benefits, Risks and Side Effects Explained