Glossary and FAQ
Short answers that point to the right page of this guide, then a plain-English glossary. Definitions are quoted from official sources (CDC, WHO, MHRA, HRSA, GOV.UK) and each one links to its source. Where no official definition was found, the entry says so.
Common questions
Are vaccines compulsory in the UK?
The NHS says Vaccines are not mandatory in the UK and they cannot be given without your consent.
NHS The Scottish Government says Vaccinations are not compulsory
gov.scot
More: Your rights and consent (UK).
Are vaccines compulsory in the US?
School entry requirements come from state laws. NCSL says All 50 states and Washington D.C. have laws requiring certain vaccines for students to attend school.
and State laws vary with respect to non-medical exemptions, which may be based on religious or personal beliefs.
NCSL
What should I ask before I or my child has a vaccine?
Which brand, what is in it, the known side effects, whether it suits you now, which batch or lot it is, and whether you can take time to decide. The full checklist, with the official wording, is on Questions to ask before a vaccine.
How do I find out what is in a vaccine?
Read the official leaflet for the exact brand. Links for each vaccine on the schedules are on Official leaflets. The guide's own table of ingredients, with the reason each is used and a source, is on Ingredients.
Which vaccines does a child get, and when?
The UK and US schedules, with the source for each, are on Schedules. How they changed over the decades is on How the schedule has changed.
Can I miss or delay a vaccine and catch up later?
For the UK, the NHS says It's best to have vaccines on time, but you can still catch up on most vaccines if you miss them.
NHS This guide does not give medical advice: ask your GP surgery or pharmacist about your own situation.
Can my 16-year-old decide for themselves?
In England, the Green Book says young people aged 16 or 17 years are entitled to consent to their own medical treatment.
Green Book ch. 2 For under-16s, see the table on how consent works.
How do I report a suspected side effect?
Does a Yellow Card or VAERS report prove a vaccine caused a problem?
No. VAERS says The report of an adverse event to VAERS is not documentation that a vaccine caused the event.
VAERS The MHRA says You do not have to prove causality to report a suspected ADR, only a suspicion is needed.
GOV.UK More: what a report can and cannot show and Reported vaccine injuries over time.
Can I get compensation if I think a vaccine harmed me or my child?
There are separate schemes: the UK Vaccine Damage Payment and the US Vaccine Injury Compensation Program (and the CICP for COVID-19). Eligibility, amounts and deadlines are on Report a reaction: compensation. Official figures are on Safety and risk and the legal record is on Accountability.
Where are the disease case and death counts?
Official counts of cases and deaths are on Diseases and Deaths, each with its source. Benefit and risk per vaccine side by side: Benefit vs risk per vaccine.
What does "excess deaths" mean?
See the glossary entry. The guide's figures are on Deaths: excess deaths.
What is a placebo, and what is an "inert" placebo?
See Placebo and Inert placebo in the glossary. Which trials used which kind of control is on Studies: randomised trials.
Is there a study comparing vaccinated and unvaccinated children?
The guide lists the studies it found, what each compared and its limits, on Studies: vaccinated vs unvaccinated.
I heard a study was withdrawn. Where can I see which?
What is the difference between efficacy and effectiveness?
See Efficacy and effectiveness in the glossary.
Where do the numbers on this site come from?
Every figure links to its source. Definitions, the guide's counting method and its limits are on About: definitions and About this guide.
Does this guide tell me whether to vaccinate?
No. It gives sources and figures side by side and takes no position. For decisions about your own family, read the linked documents and speak to a clinician you trust. See About this guide.
Glossary
In alphabetical order. A quoted definition is the source's own wording. CDC terms are from the CDC vaccine glossary CDC glossary.
- ACIP (Advisory Committee on Immunization Practices)
A group of medical and public health experts who develop recommendations on the use of vaccines in the U.S. civilian population.
CDC- Adjuvant
A vaccine component distinct from the antigen that enhances the immune response to the antigen.
CDC See: Ingredients: aluminium salts- Adverse event
An undesirable medical condition that occurs following vaccination, which might be associated to the vaccine or its components, or which might be pure coincidence.
CDC See: Report a reaction- Adverse reaction and side effect
- MHRA:
A side effect is an unexpected effect that happens in addition to the main intended effect (of the vaccine). An adverse reaction is when the resulting effect causes harm (this can be both physical and mental).
MHRA CDC: side effect isUndesirable reaction resulting from a vaccine or medicine.
CDC - Antigen
Foreign substances (e.g., bacteria or viruses) in the body that are capable of causing disease.
CDC- Association, causal association, temporal association
an association between two variables does not necessarily imply a cause-and-effect relationship.
Causal association:A change in exposure leads to a change in the outcome of interest.
Temporal association:Two or more events that occur around the same time whether causally related or not, chance occurrences.
CDC See: What a report can and cannot show- Batch number (UK) and lot number (US)
- No official definition was found. In this guide's words: a code that identifies one production run of a vaccine. The MHRA says vaccines
may vary from batch-to-batch
MHRA and US providers must recordthe vaccine manufacturer and lot number of the vaccine used
CDC See: Question 6 - Booster
Additional doses of a vaccine needed periodically to “boost” the immune system and ”reactivate” a immune response already developed by an individual.
CDC WHO:Booster doses help strengthen and extend the protection, keeping the immune system ready to fight off infections.
WHO- Breakthrough infection
Development of a disease despite a person’s having been vaccinated.
CDC- CICP (Countermeasures Injury Compensation Program, US)
The Countermeasures Injury Compensation Program (CICP) provides compensation for covered serious injuries or deaths that occur as the result of the administration or use of certain countermeasures.
HRSA- Community immunity (herd immunity)
A situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely.
CDC- Conjugate vaccine
A vaccine in which two compounds (usually a protein and polysaccharide) are joined to each other to increase the vaccine’s effectiveness.
CDC- Contraindication
A condition in a recipient that increases their risk for a serious adverse reaction.
CDC See: Question 5- Efficacy and effectiveness
- WHO:
Vaccine efficacy is measured in controlled clinical trials.
andeffectiveness measures how well the vaccines work in the real world.
WHO CDC defines an efficacy rate asA measure used to describe how effective a vaccine is at preventing disease.
CDC - Excess deaths
Excess deaths are typically defined as the difference between the observed numbers of deaths in specific time periods and expected numbers of deaths in the same time periods.
CDC NCHS See: Deaths: excess deaths- Gillick competent
Children under the age of 16 can consent to their own treatment if they're believed to have enough intelligence, competence and understanding to fully appreciate what's involved in their treatment. This is known as being Gillick competent.
NHS See: how consent works- Green Book
The Green Book has the latest information on vaccines and vaccination procedures, for vaccine preventable infectious diseases in the UK.
GOV.UK- Immunisation (immunization) and vaccination
- CDC:
The process of being made immune or resistant to an infectious disease, typically by the administration of a vaccine.
andThe physical act of administering any vaccine.
CDC - Inactivated vaccine
A vaccine made from viruses and bacteria that have been killed through physical or chemical processes, or contain parts proteins of the infectious agents.
CDC- Inert placebo
- A WHO expert panel wrote that in trials, participants are randomized to receive either the vaccine or
a placebo (i.e. an inert substance, such as a saline injection)
. It also noted thatRather than using a true placebo control (i.e. an inert substance), it may be appropriate to use a vaccine against a disease that is not the focus of the trial
Rid et al., Vaccine 2014 (WHO expert panel) See: Studies: randomised trials - JCVI (Joint Committee on Vaccination and Immunisation)
is an expert scientific advisory committee that advises the UK government on vaccination and immunisation matters.
GOV.UK- Live (attenuated) vaccine
A vaccine in which live virus is weakened (attenuated) through chemical or physical processes in order to produce an immune response without causing the severe effects of the disease.
CDC- mRNA vaccine
A vaccine that uses mRNA to teach our cells how to make a protein - or even just a piece of a protein - that produces an immune response to a disease.
CDC See: Ingredients: mRNA vaccines- Placebo
A substance or treatment that has no effect on living beings, usually used as a comparison to vaccine or medicine in clinical trials.
CDC See: Inert placebo- Primary series
others need multiple doses spaced apart – called the primary series – to develop full protection.
WHO- Schedule (immunisation or vaccination schedule)
- No single official definition was found. In this guide's words: the list of vaccines recommended at each age. CDC describes its child schedule as a way to
Guide health care providers in determining recommended vaccines for each age group.
CDC: immunization schedules The NHS schedule is at NHS: vaccinations and when to have them See: Schedules - Thimerosal (thiomersal)
A mercury-containing preservative used in some vaccines and other products since the 1930s.
There is no convincing evidence of harm caused by the low concentrations of thimerosal in vaccines, except for minor reactions like redness and swelling at the injection site.
CDC See: Ingredients: thimerosal- Vaccine Damage Payment (UK)
If you’re severely disabled as a result of a vaccination against certain diseases, you could get a one-off tax-free payment of £120,000.
Vaccine Damage Payments are not a compensation scheme.
GOV.UK See: Compensation- VAERS (Vaccine Adverse Event Reporting System)
A national program managed by the CDC and the US Food and Drug Administration (FDA) to monitor the safety of all vaccines licensed in the United States.
CDC The VAERS FAQ saysVAERS is not designed to detect if a vaccine caused an adverse event
VAERS See: About: VAERS definition, how to report- Valid consent
For consent to be valid, it must be voluntary and informed, and the person consenting must have the capacity to make the decision.
NHS See: Your rights and consent- VICP (National Vaccine Injury Compensation Program, US)
The National Vaccine Injury Compensation Program is a no-fault alternative to the traditional legal system for resolving vaccine injury petitions.
HRSA See: Compensation- VIS (Vaccine Information Statement, US)
A VIS or Vaccine Information Statement is a document, produced by CDC, that informs vaccine recipients – or their parents or legal representatives – about the benefits and risks of a vaccine they are receiving.
CDC See: Official leaflets (US)- Yellow Card scheme (UK)
Anyone can report an issue with a medicine
.The Yellow Card scheme is run by the Medicines and Healthcare products Regulatory Agency (MHRA)
MHRA See: how to report
More terms (disease doses, injury, SIDS, SCCS) are defined for this guide's own counting on About: definitions.
Terms and definitions checked against the source pages on 31 October 2026. Quotation marks show exact source wording.