Immunization Glossary: Vaccine Terms and Abbreviations A-Z

Last updated November 1, 2026

Immunization has its own language. Health agencies, news reports and research papers often use technical terms and short abbreviations, such as DTP3, MCV1 or zero-dose.

This glossary explains common immunization terms and abbreviations in plain English. Terms are listed from A to Z. Where a term has its own page on this site, we link to it.

A

  • Acellular vaccine: A vaccine that contains only parts of the cells of a germ, not whole cells.
  • Active immunity: Protection that a person’s own immune system produces against a disease. A person can gain active immunity by having the disease or by vaccination. It usually lasts a long time.
  • Adjuvant: An ingredient in some vaccines, separate from the antigen, that makes the immune response to the antigen stronger.
  • Adverse event: An unwanted medical problem that happens after vaccination. It might be caused by the vaccine, or it might happen by chance.
  • Anaphylaxis: A sudden and severe allergic reaction. Signs include trouble breathing, loss of consciousness and a drop in blood pressure. It can be fatal and needs emergency medical care.
  • Antibody: A protein in the blood that the immune system makes in response to germs, such as bacteria or viruses. Antibodies help protect the body by binding to germs.
  • Antigen: A foreign substance, such as part of a bacterium or virus, that triggers an immune response. The response is usually the production of antibodies.
  • ART (antiretroviral therapy): Medicines that treat HIV. They stop the virus from copying itself in the body, but do not cure the infection.
  • Asymptomatic infection: An infection that causes no symptoms. Also called an inapparent or subclinical infection.
  • Attenuated vaccine: See Live vaccine.

B

  • B cells: Small white blood cells that help the body fight infection. B cells are made in the bone marrow and can develop into cells that produce antibodies.
  • Booster dose: An extra dose of a vaccine, given after the first series, to “boost” the immune system and renew protection. For example, a tetanus and diphtheria (Td) booster is recommended for adults every 10 years.
  • Breakthrough infection: A case of a disease in a person who has been vaccinated against it.

C

  • Cold chain: The temperature-controlled supply chain for vaccines, including all the equipment and procedures involved. It starts with cold storage at the factory, continues through transport and storage at the clinic, and ends when the vaccine is given.
  • Combination vaccine: A single product that protects against more than one disease. Combination vaccines, such as DTP, reduce the number of injections a child needs.
  • Community immunity: When enough people in a population are immune to a disease, through vaccination or past illness, to make its spread from person to person unlikely. This also gives some protection to people who are not vaccinated, such as newborns. Also called herd immunity.
  • Conjugate vaccine: A vaccine in which two compounds, usually a protein and a sugar chain (polysaccharide), are joined to make the vaccine work better.
  • Contraindication: A condition in a person that increases the risk of a serious reaction to a vaccine.

D

  • Drop-out: When a child starts the vaccination schedule but does not complete it. A common measure is the number of children who get their first DTP dose but not their first measles dose.
  • DTP: A combination vaccine against diphtheria, tetanus and pertussis (whooping cough).
  • DTP1: The share of infants who receive their first dose of DTP vaccine. Missing this dose points to a lack of access to immunization and other health services.
  • DTP3: The share of infants who receive all 3 doses of DTP vaccine. It is the global marker for immunization coverage.

E

  • Efficacy rate: A measure of how well a vaccine prevents disease.
  • Endemic: Present in a given area, usually at a low or baseline level.
  • EPI: The Expanded Programme on Immunization, founded by the World Health Assembly in 1974. It is now called the Essential Programme on Immunization.
  • Epidemic: More cases of a disease in an area or population than would normally be expected.

H

  • Herd immunity: See Community immunity.
  • Hib: Haemophilus influenzae type b, a bacterium that can cause meningitis and pneumonia. A vaccine protects against it.
  • HPV: Human papillomavirus, the most common viral infection of the reproductive tract. It can cause cervical cancer and other cancers. A vaccine protects against it.

I

  • IA2030: The Immunization Agenda 2030, the global strategy for vaccines and immunization for the years 2021 to 2030.
  • Immunity: Protection against a disease. There are two types: active and passive.
  • Immunization: The process of becoming immune or resistant to an infectious disease, usually by getting a vaccine.
  • Inactivated vaccine: A vaccine made from viruses or bacteria that have been killed, or from parts of them. The killed germs cannot cause disease.
  • Incubation period: The time between contact with a germ and the start of symptoms.
  • Investigational vaccine: A vaccine that is still being tested in clinical trials and is not yet licensed for use by the public.
  • IPV: Inactivated polio vaccine.
  • IRS: Indoor residual spraying, one of the 2 core ways to control the mosquitoes that spread malaria.
  • ITN: Insecticide-treated net, a bed net that protects sleepers from mosquitoes. It is one of the 2 core tools to control malaria mosquitoes.

L

  • Live vaccine: A vaccine that contains a live virus that has been weakened, so that it causes an immune response without the severe effects of the disease. Also called an attenuated vaccine. People with a weakened immune system should not receive live vaccines.

M

  • MCV1 and MCV2: The first and second doses of a measles-containing vaccine. These are the key measures of measles vaccination coverage.
  • Memory cells: Cells that “remember” a germ the body has met before, so that the immune system can respond quickly if the germ returns.
  • mRNA vaccine: A vaccine that uses mRNA to teach the body’s cells how to make a protein, or a piece of a protein, that triggers an immune response.

O

  • Outbreak: More cases of a disease than expected in a certain community, area or season, but fewer than in an epidemic.

P

  • Pandemic: A worldwide epidemic.
  • Passive immunity: Protection from antibodies made by another person or animal. It works, but it fades over time, usually within weeks or months. For example, a mother passes antibodies to her baby before birth.
  • Pathogen: A germ, such as a bacterium, virus, parasite or fungus, that causes disease.
  • Placebo: A substance with no effect, used for comparison with a vaccine or medicine in a clinical trial.
  • Polysaccharide vaccine: A vaccine made of long chains of sugar molecules that look like the surface of certain bacteria.
  • PrEP: Pre-exposure prophylaxis, medicine that people without HIV take to lower their risk of getting HIV.
  • Prequalification: A WHO service that checks whether vaccines are safe and effective, using international standards. United Nations agencies, such as UNICEF, use it when they buy vaccines.

R

  • Recombinant: Made by joining genetic material (DNA) from different sources.
  • Routine immunization: Vaccines given through regular health systems, such as clinics, community centres, outreach services and health worker visits. Read more about routine immunization.

S

  • SAGE: The Strategic Advisory Group of Experts on Immunization. It advises WHO on global vaccine and immunization policies and strategies.
  • Seroconversion: When a person who had no detectable antibodies against a germ develops them, for example after vaccination.
  • Serosurvey: A study that measures how many people in a community have antibodies against a certain germ, from vaccination or past infection.
  • Side effect: An unwanted reaction to a vaccine or medicine.
  • SMC: Seasonal malaria chemoprevention, a full course of antimalarial medicine given to at-risk groups during the season of greatest malaria risk.
  • Strain: A specific version of a germ. Many germs, including HIV, have several strains.

T

  • Titer: The amount of antibodies found in the blood by a laboratory test.

U

  • UNICEF: The United Nations agency for children. It is one of the largest buyers of vaccines in the world. Learn more about UNICEF.

V

  • Vaccination: The act of giving a vaccine.
  • Vaccine: A product that trains the immune system to protect against a disease. Many vaccines contain killed or weakened forms of the germ, which do not cause the disease.
  • VAERS: The Vaccine Adverse Event Reporting System, a United States program run by the CDC and the FDA that collects reports of adverse events after vaccination.
  • Viral vector vaccine: A vaccine that uses a changed, harmless virus (the vector) to deliver instructions to the body’s cells.
  • Virus: A tiny germ that multiplies inside cells and causes diseases such as measles, mumps and hepatitis. Antibiotics do not work against viruses.

W

  • Waning immunity: The loss of protective antibodies over time.
  • WHO: The World Health Organization, the United Nations agency for health. Learn more about WHO.
  • WUENIC: The WHO and UNICEF Estimates of National Immunization Coverage. Based on data that countries report, it is the world’s largest dataset on immunization trends.

Z

  • Zero-dose children: Children who have not received any vaccine. UNICEF defines them as children who have not received their first dose of DTP vaccine.

Sources