Anti-infective
Vaccines
Immunisations — Train the immune system to recognise and fight specific infections before you ever meet them.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
Quick answer
What is Vaccines?
Vaccines are preparations that safely expose the immune system to a harmless piece or weakened form of a germ so the body learns to recognise and defend against it. They prevent infections and their complications, and reduce spread to others.
- How it works: A vaccine shows the immune system a recognisable part of a virus or bacterium — a protein, a sugar coat, or a weakened or inactivated germ, or instructions for the body to make a harmless protein.
- In practice: In practice vaccines are one of the most effective preventive tools in medicine, given on a national schedule from infancy through childhood, with boosters and extra vaccines in pregnancy, older age and for at-risk groups, travel and certain jobs.
What it is
Vaccines are preparations that safely expose the immune system to a harmless piece or weakened form of a germ so the body learns to recognise and defend against it. They prevent infections and their complications, and reduce spread to others.
How it works
A vaccine shows the immune system a recognisable part of a virus or bacterium — a protein, a sugar coat, or a weakened or inactivated germ, or instructions for the body to make a harmless protein. The immune system responds by making antibodies and memory cells, so if you later meet the real infection it is recognised and cleared quickly, often before you become ill.
In practice
In practice vaccines are one of the most effective preventive tools in medicine, given on a national schedule from infancy through childhood, with boosters and extra vaccines in pregnancy, older age and for at-risk groups, travel and certain jobs. The everyday practical themes are the schedule and catch-up (completing courses on time, and catching up missed doses rather than restarting), the difference between live and non-live vaccines (live vaccines such as MMR are generally avoided in pregnancy and significant immunosuppression, where non-live vaccines are preferred and may work less well), and managing expectations about side effects — a sore arm, mild fever and feeling off for a day or two are common and reflect the immune system responding, not the infection itself. Serious allergic reactions are very rare, which is why a short observation period and trained staff with adrenaline are standard. Vaccination also protects others by reducing spread (herd protection), which matters for those who cannot be vaccinated. Timing around immunosuppressants and some treatments (such as before starting biologics or chemotherapy, or after a transplant) needs planning.
Examples
Practical use
How to take it & use it well
- Attend your appointment as booked and bring a record of any previous vaccines, as completing or catching up on a course is far better than starting again from scratch.
- Expect to wait nearby for a short observation period after the injection, which is a routine precaution so that the very rare serious allergic reaction can be treated immediately if it happens.
- It is normal to get a sore, red arm and to feel a little off colour, tired or feverish for a day or two, and simple paracetamol can help if you are uncomfortable.
- Tell the nurse if you are pregnant, have a weakened immune system, or take medicines that suppress immunity, as this affects whether a live vaccine is suitable for you.
- Keep going back for the further doses or boosters you are offered, as protection from many vaccines builds up over a course and can fade without top-ups.
- If you have ever had a severe allergic reaction to a vaccine or one of its ingredients, mention this beforehand so it can be checked and the right precautions taken.
Common uses
- Routine childhood and adult immunisation schedules
- Seasonal (flu) and at-risk group protection
- Travel, occupational and pregnancy vaccination
Monitoring
- Up-to-date status against the schedule and any catch-up needed
- Immune status/response in at-risk or immunosuppressed people
- Any significant reaction (rare) and post-vaccination observation
Weighing it up
Advantages & disadvantages
Advantages
- They train the immune system to recognise and fight serious infections before you ever encounter them.
- They have dramatically reduced, and in some cases nearly wiped out, diseases that once killed or disabled many people.
- By protecting enough people, they also shield those who cannot be vaccinated through herd protection in the community.
- Most cause only mild, short-lived effects and give long-lasting protection from a brief course.
- They are among the most studied and closely monitored of all medical treatments, with a strong safety record.
Disadvantages
- Mild local soreness and short-lived feeling unwell are common in the day or two afterwards.
- Live vaccines are not suitable for some people, including those who are pregnant or significantly immunosuppressed.
- Some vaccines need several doses or boosters, so a single appointment may not give full protection.
- Serious allergic reactions are very rare but are the reason for the short observation period.
- Protection from some vaccines fades over time, so top-up doses are needed to stay covered.
Key safety principles
What to watch for
- Live vaccines are generally avoided in pregnancy and significant immunosuppression — non-live vaccines are preferred there.
- Mild, short-lived effects (sore arm, low fever, tiredness) are common and expected; serious allergic reactions are very rare.
- Plan timing around immunosuppressants, biologics, chemotherapy and transplant; complete or catch up courses rather than restarting.
Key interactions
What to avoid or check alongside
- Medicines that weaken the immune system, such as steroids at higher levels, chemotherapy and biologic treatments, can reduce how well a vaccine works and make live vaccines unsafe.
- If you are due chemotherapy or strong immune-suppressing treatment, the timing of vaccines is planned carefully, often beforehand, with your specialist.
- Two live vaccines are generally either given on the same day or otherwise separated by a few weeks rather than just a few days apart, so plan multiple vaccines with the nurse.
- Recent treatment with certain blood products or antibody infusions can blunt the response to some live vaccines, so a waiting period may be advised.
- Pregnancy means live vaccines are usually avoided, while several non-live vaccines are actively recommended to protect mother and baby.
Patient & carer advice
- A sore arm, mild fever or feeling off for a day or two is normal and means your body is responding
- Tell us if you are pregnant or have a weakened immune system, as this affects which vaccines are suitable
- Completing the full course gives the best, longest protection — you do not need to start again if a dose was late
Answers
Vaccines: frequently asked questions
Why do I have to wait for a while after my vaccine?
The short wait is a routine safety precaution. Serious allergic reactions are very rare, but if one did happen it would usually occur soon after the injection, so staying nearby means it can be treated straight away. Once the time is up you are free to go.
Is it normal to feel unwell after a vaccine?
Yes. A sore arm, tiredness, mild fever or feeling generally off for a day or two is common and shows your immune system is responding. Paracetamol can help, and it usually settles quickly. Seek advice only if symptoms are severe or last longer than expected.
Can I have a live vaccine if my immune system is weak?
Often not. Live vaccines contain a weakened form of the germ, which can be risky if your immune system is significantly suppressed by illness or medicines. Tell the nurse about your condition and treatments so a safe, non-live alternative can be considered where possible.
I missed a dose in my vaccine course — do I have to start again?
Almost never. In nearly all cases a delayed course is simply continued from where you left off rather than restarted, even after a long gap. Bring your vaccine record so the nurse can work out exactly which doses you still need.
How do vaccines protect people who can't have them?
When enough people in a community are vaccinated, the infection struggles to spread, which indirectly protects those who cannot be vaccinated, such as newborns or people with weakened immunity. This is called herd protection, and it is one reason wide uptake matters.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
Related guides
Medicines in this class
Common vaccines by active ingredient
Individual, dose-free guides to specific medicines in this class:
Browse by body system
Part of the infections
See all the conditions, medicine classes and active-ingredient guides for this body system in one place:
Need a custom medicines or prescribing resource?
We build evidence-led clinical references, calculators and decision aids for teams.