ADJUVANT
 An ingredient added to some vaccines to help your immune system respond more strongly. Think of it as a booster for the booster.

ADVERSE EVENT
 A health problem that happens after vaccination. It doesn't necessarily mean the vaccine caused it ‚ it just means it happened around the same time. Researchers investigate to find out if there's a real link.

ANTIBODIES
 Proteins your body makes to fight infections. Vaccines teach your body to make the right antibodies before you get ill, so you're ready if you ever meet the real thing.

ANTIGEN
 The part of a germ (or vaccine) that your immune system recognises and learns to fight.

BOOSTER
 An extra dose of a vaccine given after the initial course, to top up your protection.

CLINICAL TRIAL
  A carefully designed study to test whether a vaccine (or other treatment) is safe and works. Vaccines go through several stages of trials before they're approved.

COMMUNITY PROTECTION (sometimes called "herd immunity") When enough people in a community are vaccinated, it becomes harder for the disease to spread ‚ which helps protect people who can't be vaccinated, like very young babies or people with certain health conditions.

CONTRAINDICATION
  A reason why someone shouldn't have a particular vaccine, for example, a severe allergy to one of its ingredients. Your GP or pharmacist can advise on this.

EFFICACY
  How well a vaccine works in clinical trials. For example, "95% efficacy" means that in the trial, vaccinated people were 95% less likely to get the disease than unvaccinated people.

EFFECTIVENESS
  How well a vaccine works in the real world (as opposed to in a clinical trial). Real-world effectiveness can be slightly different from trial efficacy because real life is messier than a controlled study.

THE GREEN BOOK
  The UK’s official guide to vaccination. It's written for healthcare professionals but it's publicly available online. It contains detailed information about every vaccine used in the UK.

IMMUNE RESPONSE
  How your body reacts when it meets something it recognises as foreign ‚ like a virus, bacterium, or vaccine. Your immune system learns from each encounter, which is why vaccines work.

IMMUNISATION
 The process of becoming protected against a disease, usually through vaccination. Technically, "vaccination" is the act of getting the jab, and "immunisation" is what happens in your body afterwards ‚ but in everyday language, they mean the same thing.

LIVE VACCINE
 A vaccine made from a weakened (but still living) version of the virus or bacterium. The weakened germ can't cause the disease in healthy people, but it teaches your immune system to recognise it. Examples: MMR, chickenpox.

INACTIVATED VACCINE
 A vaccine made from germs that have been killed. They can't cause infection but still teach your immune system. Examples: flu, hepatitis A.

MHRA
 The Medicines and Healthcare products Regulatory Agency. The UK body responsible for making sure vaccines (and all medicines) are safe before they're approved for use.

SIDE EFFECTS
 Common reactions after vaccination, like a sore arm, tiredness, or a mild temperature. These usually go away within a day or two and are a sign your immune system is responding.

UKHSA
  UK Health Security Agency ‚ the government body responsible for protecting the public from infectious diseases. They run the national vaccination programmes.

VACCINATION SCHEDULE
  The recommended timetable for which vaccines you should have and when. In the UK, the NHS schedule starts from 8 weeks old and includes boosters throughout life.

YELLOW CARD SCHEME
  The UK system for reporting suspected side effects of vaccines and medicines. Anyone, patients, parents, healthcare workers, can submit a report. It's run by the MHRA and helps monitor safety.