Vaccine Dictionary

With vaccines in the news so much lately, you may be hearing words that are new or acronyms that are not always defined, so we wanted to start a vaccine dictionary. This tool will enable you to find brief definitions for words commonly associated with vaccines. In some cases, we also offer links to resources with more detailed information.

If you have ideas for additional words to include, please contact us so that we can keep improving this resource, which you can also quickly access from the homepage of the Parents PACK website.

Want to let others know about the dictionary? This downloadable flyer can be used for easy posting or sharing.

A to F

AAFP

This acronym stands for American Academy of Family Physicians. This is a professional organization of family doctors. The group focuses on primary care, preventive medicine, and improving health systems to better the lives patients, families and communities.

AAP

This acronym stands for American Academy of Pediatrics. This is a professional organization of pediatricians that advocate for the health and well-being of infants, children, adolescents, and young adults. The organization also develops clinical guidelines, policy statements, and educational resources for pediatric care.

ACIP

This acronym stands for the Advisory Committee on Immunization Practices. This is a group of individuals charged with reviewing the data about vaccines and providing guidance in the form of recommendations to the Centers for Disease Control and Prevention (CDC).

ACOG

This acronym stands for American College of Obstetricians and Gynecologists. This is a professional organization of physicians who specialize in obstetrics and gynecology. The organization provides guidelines on women’s reproductive health, pregnancy care and gynecologic practices.

ACP

This acronym stands for American College of Physicians. This is a professional organization of internal medicine physicians (internists). They provide education, participate in advocacy, and develop evidence-based clinical guidelines.

Active immunity

Immunity that develops after receiving a vaccine or getting infected with a virus or bacteria in the community. This type of immunity includes immunologic memory, so a person is often protected for years and sometimes for life.

Adaptive immune response

Part of the immune response that develops when responding to a particular (or specific) pathogen.

Adjuvant

This is a chemical added to a vaccine to improve the immune response. Including an adjuvant often allows for fewer or lower doses of a vaccine.

Adverse event

A negative health effect that occurs after a medical procedure, treatment or other intervention (e.g., vaccine). While it is tempting to assign causality when two things happen close in time, the time proximity does not prove anything about the causal relationship between the events.

Allergy

An immune system response to a substance that is normally harmless, such as food, pollen, or medication, including in rare instances vaccine ingredients. Reactions can cause symptoms, like itching, rash, swelling, or sneezing.

Anaphylaxis

A rare, but serious, allergic reaction that occurs quickly after exposure to an allergen (within minutes). The reaction can cause difficulty breathing, swelling of the throat and a drop in blood pressure. It is considered a medical emergency.

Vaccines can be a rare cause of these reactions if someone has an allergy to an ingredient in the vaccine. For this reason, people are recommended to stay at the location where the vaccine was given for 15-30 minutes, so that if such a reaction occurs, trained medical personnel are available to help.

Antibodies

Proteins made by cells of the immune system called B cells. These proteins help identify foreign invaders and stop them from making us sick. Antibodies can circulate in the blood, but they are also found in other areas of the body. There are five types: IgG, IgM, IgA, IgE and IgD. Each has unique roles.

Antigens

Parts of viruses and bacteria, as well as other agents, that cause the generation of an immune response. In vaccines, these are the parts that cause an immune response to generate immunologic protection.

Arthritis

Inflammation of the joints. Symptoms typically include pain and discomfort in the joints that are swollen.

Association

When talking about scientific data, an association means that two things are related, but importantly, this relationship does not tell us anything about whether one causes the other. For example, going to the movies and eating popcorn are associated, but going to the movies does not cause one to eat popcorn, nor does eating popcorn cause one to go to the movies.

Attenuated

Weakened; related to vaccines, this word describes a kind of vaccine in which the antigen is live, or able to replicate in a person, but it has been changed in a way such that it causes an immune response to be generated without causing severe illness.

Autoimmunity

A situation in which a person’s immune system sees a part of one’s self as “foreign” and mounts an immune response against it.

Booster dose

An additional dose of a vaccine that is given to strengthen the immune response and maintain protection against a potential pathogen. This dose “reminds” the immune system that it has seen this antigen in the past.

Catch-up immunization

A vaccination schedule designed for someone who has missed one or more recommended doses of vaccine.

Causation

When talking about scientific data, causation means that two things are related because one causes the other. For example, buying a movie ticket and going to the movies are causally related because you must buy a ticket to get into the movie theater.

CDC

This acronym stands for the Centers for Disease Control and Prevention. This federal agency is part of HHS. It was formed in 1946 in Atlanta, GA to stop the spread of malaria. Over time, the agency grew, and their work expanded to include conducting disease surveillance, coordinating outbreak responses, providing public health education, and conducting research. They also develop health guidelines and support state and local health departments.

CIDRAP

This acronym stands for the Center for Infectious Disease Research and Policy. This research organization is based at the University of Minnesota and provides independent, evidence-based information on infectious disease outbreaks, public health preparedness, and emerging health threats. In 2025, they started the Vaccine Integrity Project to ensure that vaccine recommendations remain science based.

Cirrhosis

Scarring of the liver, which causes abnormal liver function.

Clinical trials

These are studies in people that test whether a vaccine or other product is safe and works as expected. The studies can vary in size based on how far along in development the product is, and they can vary on their main focus, such as safety or whether the product works, depending on the stage of development during which they are completed.

Find out more:

CMS

This acronym stands for Centers for Medicare & Medicaid Services. This federal agency administers U.S. healthcare programs and services, including Medicare, Medicaid, and the Children's Health Insurance Program (CHIP). It is part of HHS.

Community immunity

As more people in a community become immune to an infection, the less chance it has to spread. This protects those in the community who are not able to be protected. This concept is also called herd immunity.

Congenital rubella syndrome (CRS)

An outcome of infection with rubella during pregnancy. Babies who survive often suffer permanent damage, including deafness, damage to the eyes, mental delays, diabetes, autism, heart murmur or other heart damage, damage to the liver or spleen, or lack of head growth.

Contraindication

A reason that a person is advised not to get a vaccine or medical treatment. Some examples related to vaccines include a medical condition, a severe allergy to a vaccine ingredient, or a severe reaction to a previous dose of the same vaccine.

Correlation

Describes a situation in which two things are observed together (e.g., getting a vaccine and experiencing some medical event). Importantly, this joint occurrence does not tell us anything about the relationship between the two events. Different types of relationships that could exist include one event causing the other or both events being caused by something else.

Dehydration

Excess water loss.

Empyema

Accumulation of pus in the lungs or between the lung and chest wall. Symptoms can include chest pain (which worsens when breathing in deeply), dry cough, excessive sweating (night sweats), fever and chills, tiredness, shortness of breath, and weight loss.

Encephalitis

Inflammation of the brain, often caused by infection.

Encephalopathy

Damage or malfunctioning of the brain due to infections, toxins, trauma or disease. People with this condition can experience confusion, memory loss or personality changes.

Epidemic

Widespread disease in a specific geographic area.

Epiglottitis

Swelling of the epiglottis, the flap of skin that covers the windpipe during swallowing. Symptoms can include trouble breathing, suffocation and sometimes, death. Before there was a vaccine, Haemophilus influenzae type b (Hib) was the most common cause.

EUA

This abbreviation stands for emergency use authorization. This designation allows for temporary use of a new or repurposed medical product during a public health emergency. By allowing the product to be used before it has completed the typical approval process, the vaccine or medication can be made available more quickly. After the emergency has passed, the manufacturer must apply for a regular license, called a biologics license application or BLA, in order to continue distributing the product.

Evidence to Recommendations (EtR) framework

This is a specific process used by the ACIP to consider the scientific data when making vaccine recommendations. The areas considered include the importance of the public health problem (e.g., number of people affected and effects of a disease), the potential benefits and harms of the intervention (e.g., giving a vaccine), the values of the people who will be affected by the policy (e.g., what the public thinks about preventing the disease and accepting the vaccine), acceptance of the intervention (e.g., do the people who will give the vaccines feel it’s important and what are the ethics of giving the vaccine), resource use (e.g., the monetary costs to save a life or prevent hospitalization), equity (e.g., the impact of the recommendation on different populations), and feasibility (e.g., considerations for implementing the recommendation).

FDA

This acronym stands for the Food and Drug Administration. This federal agency regulates food safety, pharmaceuticals, vaccines, medical devices, and cosmetics. The FDA was started in 1906 to ensure that products are safe, effective, and properly labeled. This agency is part of HHS.

G to L

GAVI

This acronym stands for the Global Alliance for Vaccines and Immunization. This organization is a public–private partnership that works to increase access to vaccines in low- and middle-income countries. Their efforts include funding immunization programs, strengthening health systems, and improving global vaccine equity.

GRADE Analysis

This is a specific way to evaluate the quality of scientific studies. The letters stand for Grading of Recommendations Assessment, Development and Evaluation. The process includes defining a strong research question (see “PICO question” definition) and conducting a review of the scientific literature (see “Systematic literature review” definition) to find and evaluate all the relevant scientific papers that have been published about the topic. This process enables scientists to not only identify the body of literature but also look at each study to compare the different findings and consider why there may be differences among them by considering the strength of the research design and the risk for bias in the results.

Guillain-Barré syndrome (GBS)

Guillain-Barré syndrome (GBS) – A condition in which the immune system attacks the nervous system. It often causes muscle weakness that can last for months or years. This condition can occur after an infection or more rarely, after receipt of some vaccines.

Hemorrhage

Rapid loss of blood.

Hepatitis

Inflammation of the liver.

Herd immunity

Another name for community immunity. See “community immunity" for definition and more information.

HHS

This acronym stands for the U.S. Department of Health and Human Services. This department of the U.S. government oversees matters related to health, including public health policy, programs and research. This agency oversees the CDC, FDA, NIH, and CMS.

Hypotonic-hyporesponsive episode

A condition in which a person becomes listless and lethargic with poor muscle tone. This condition can last for several hours. While this is a rare, but scary, condition, it does not cause permanent effects. A small number of infants can experience this after receipt of the pertussis vaccine.

Immune memory

A type of immunity in which the immune system “remembers” a foreign invader or pathogen that the person has been exposed to in the past. This type of immunity develops after a prior infection or because of vaccination. Immune responses of this type occur more quickly and are stronger than immune responses that occur after a first exposure.

Immune system

The body system that protects us against infections. This system operates throughout our entire body all the time — 24 hours a day, 7 days a week — without us thinking about it.

Immunogenicity

The ability of something, like a vaccine, to cause a person’s immune system to make an immune response.

Immunogens

The parts of a vaccine to which the immune system responds.

Incubation period

The time between when someone is exposed to a pathogen and the appearance of the first symptoms of disease.

Informed consent

The process of providing information about a medical procedure so that the patient, or their legal representative, can decide whether to accept (i.e., consent to) the procedure.

Innate immune response

The first line of defense of our immune system. These responses occur quickly and are not specific for any particular pathogen.

Jaundice

Yellowing of the skin or eyes.

M to S

Manufacturing byproducts

Small amounts of substances from the production process that remain in the final product. In vaccines, cell debris, antibiotics or fragments of DNA can remain, but the quantities are so small that they cannot cause negative health effects, particularly since protein purification processes have improved over time. One example is egg proteins in the influenza vaccine. In the past people with severe egg allergies could not get the influenza vaccine because of residual egg proteins, but now, they can.

Memory cells

Cells of the immune system that are created during an adaptive immune response. A small number of these cells remain after an infection is cleared. They monitor the body for future encounters with the same pathogen.

Meningitis

Inflammation of the lining of the brain and spinal cord (meninges). Symptoms typically include fever, stiff neck, headache, seizures, nausea, vomiting, confusion and sensitivity to light. This condition can result from both viral and bacterial infections. Bacterial meningitis tends to be more severe.

Milligram

A milligram is one-thousandth of a gram. A gram is the weight of one-fifth of a teaspoon of water or about the weight of one raisin.

mRNA

RNA stands for ribonucleic acid. RNA is involved with making proteins, so we have RNA in every cell. Three main types of RNA do this work in cells, including mRNA. The “m” stands for “messenger” because this type of RNA delivers the instructions for how to make the protein to the part of our cell responsible for this work.

In relation to vaccines, mRNA represents a type of vaccine that delivers the instructions for making the protein that our immune system needs to recognize to protect us against a pathogen.

Multisystem inflammatory syndrome (MIS-C, MIS-A)

A condition in which an individual’s immune system attacks parts of their body following infection with the virus that causes COVID-19. The condition can occur in adults (MIS-A) or children (MIS-C). Early versions of SARS-CoV-2, the virus that causes COVID-19, caused this condition in about 6 or 7 people of 1 million infected. That rate has decreased over time. By 2024, about 8 of 100 million infected people experienced this condition.

MIS-A and MIS-C occur about four to six weeks after COVID-19 infection, and it can happen to anyone who got infected even if they did not get very sick. Symptoms can vary, but multiple organs are typically affected. Fever, shortness of breath, and chest pain or heart palpitations may be the first signs of this condition.

Myocarditis

Inflammation of the heart. Symptoms can include pressure or pain in the chest; severe tiredness; shortness of breath; irregular heartbeat; dizziness; achiness or swelling of the legs, ankles or feet.

NIH

This acronym stands for the National Institutes of Health. This is the primary medical research agency of the U.S. It is part of HHS, and historically, it was one of the world’s leading centers for biomedical and public health research, directly employing over 21,000 people in 2024 and distributing almost $37 billion in research grants to researchers across the country.

Observational studies

A type of scientific research in which the scientists monitor a situation without introducing an experimental parameter. For example, if someone is trying to determine whether a vaccine is causing a particular side effect or condition, they may look at the health records of a group of patients during a particular time period and compare the rates of the side effect or condition in those who did or did not receive the vaccine.

Oophoritis

Inflammation of the ovaries.

Orchitis

Swelling of the testicles.

Osteomyelitis

Infection of the bones. Symptoms typically include fever; chills; irritability; and swelling, warmth, redness, or pain in the infected area.

Otitis media

An infection of the middle ear. Symptoms in children typically include ear pain (especially when lying down), tugging or pulling at an ear, difficulty sleeping, crying more than usual, irritability, difficulty hearing or responding to sounds, loss of balance, fever, fluid drainage from ear, headache and loss of appetite.

Pancreatitis

Inflammation of the pancreas. Symptoms typically include abdominal pain, pain that radiates to the back and may increase after eating, fever, increased pulse rate, nausea, vomiting and tenderness when touching the abdomen.

Pandemic

A worldwide epidemic.

Passive immunity

Immunity that is “borrowed” from someone else. Examples include maternal antibodies received before birth or in breast milk and immunoglobulin treatments.

Pericarditis

Infection of the lining of the heart (pericardium). Symptoms typically include chest pains, low-grade fever, dry cough, abdominal swelling, weakness and fatigue, and shortness of breath when reclining.

PICO question

This is a kind of research question that is designed to include the affected Population, Intervention, Comparator, and Outcomes. It is used for clinical research. For example, this kind of question is developed for a GRADE analysis when evaluating the literature to make vaccine recommendations.

Placebo

Something given to the control group in a study so that the participants, and often the research team, don’t know who got the experimental treatment. Placebos can be saline or diluent without an active ingredient or even another medication or vaccine. The choice of placebo depends on both biological parameters of what is being studied as well as ethical considerations.

Plasmids

Circular pieces of DNA in cells into which other DNA can be inserted. Inserted DNA can be a gene for a protein of interest, like insulin. Once the plasmid is introduced into a cell, the inserted gene can make the protein. This technique is called recombinant DNA technology and is used to make some vaccines, such as the hepatitis B vaccine, among others.

Platelets

Cells in the blood that enable it to clot.

Pneumonia

Inflammation of the lungs. Symptoms typically include fever, chest pain, chills, cough, shortness of breath, rapid breathing, weakness, increased heart rate, nausea, vomiting and headache.

Postherpetic neuralgia (PHN)

Long-lived, debilitating pain that can occur as a result of shingles infection. Pain can be so severe that everyday activities are disrupted.

Precaution

A reason that a person may not be able to get a vaccine. Some examples include due to a current moderate or severe illness or receiving a medical treatment that could interfere with the response to the vaccine, such as chemotherapy. People should talk to their doctor to decide together whether they can get the vaccine or not.

Preservative

This is an ingredient added to a vaccine to make sure it does not become contaminated.

Primary immune response

The immune response generated the first time a person’s immune system stops an infection by a particular (specific) pathogen.

Prospective study

A study in which the participants are identified at the beginning and monitored for some effect over time.

Randomized controlled trials (RCT)

A study design in which experimental and control groups are included, and participants are assigned to each group randomly. If only the researcher knows who is in each group, it is a single-blinded study. More commonly, neither the researcher nor the participant knows who is in each group; this is called a double-blinded study.

Recommendations

These are guidance that indicate how vaccines should be used, by whom and when. These are developed by the ACIP, and the director of the Centers for Disease Control and Prevention (CDC) determines whether to accept them. Recommendations do not mean a vaccine is required, but they are typically based on considerations of scientific evidence and medical considerations, so these are often more important than requirements when considering how to protect ourselves against vaccine-preventable diseases.

Recurrent respiratory papillomatosis (RRP)

A condition in which tumor-like growths occur in the throat. The growths can cause hoarseness and difficulty breathing, talking and swallowing. RRP can also cause a disease of the lungs that resembles cystic fibrosis. RRP is a rare complication of HPV infection.

Requirements

Also known as mandates, these are laws about vaccines. They are made by state governments. While recommendations are considered when making these laws, they are also the result of political and financial considerations at the state level. While these are often the focus for families (What vaccines do we need to get?), they can be less aligned with best health practices that decrease an individual’s chance of being infected with a vaccine-preventable disease.

Residual manufacturing byproducts

These are ingredients that remain in vaccines in very small quantities after the production process.

Retrospective study

A study in which the activities have already occurred, such as looking at children who were or were not vaccinated in infancy and are now 5 years of age to see whether the vaccinated group had higher rates of a particular problem, such as attention-deficit/hyperactivity disorder (ADHD).

SCDM

This acronym stands for shared clinical decision-making. Historically, this was a special type of vaccine recommendation reserved for vaccines where widespread use may not be warranted, due to things like high costs or a need to vaccinate huge numbers of people to prevent a single case of disease, but which could benefit some people because they are at higher risk for the infection.

Secondary immune response

The immune response generated during any subsequent exposure to a particular pathogen after the first time (See “primary immune response”). These responses are faster and more targeted than primary immune responses. Secondary immune responses are also referred to as memory immune responses.

Sepsis

Bloodstream infection. Symptoms typically include sudden fever, drop in blood pressure, shock, organ failure and a rash that looks like bleeding under the skin.

Seroconversion

This relates to the presence of antibodies measured in the blood. It can be used to determine if someone generated an immune response after getting a vaccine. If the measurement of these antibodies goes from negative to positive, the person would be described as having “seroconverted.”

Stabilizer

This is an ingredient added to a vaccine to ensure that it maintains its integrity during storage and transport.

Subacute sclerosing panencephalitis (SSPE)

A rare, progressive neurological disorder that can occur in people who had measles. This condition often occurs years after the original infection and is fatal.

Systematic literature review

This type of research procedure is used to evaluate a body of evidence to understand what most of the findings indicate, why the outliers might be different, how well the studies were designed, and whether the results might have biases. By completing this process, investigators can have an informed understanding of the quality of evidence on which they are relying to make recommendations or policies.

T to Z

Thrombocytopenia

Low platelet count caused by a number of different health problems. Sometimes occurs following measles infection or vaccination, but the condition is temporary.

Thrombosis with thrombocytopenia syndrome (TTS)

A rare, but potentially life-threatening reaction identified in recipients of COVID-19 adenovirus-based vaccines (J&J/Janssen and AstraZeneca). These COVID-19 vaccines are no longer available in the U.S.

When this condition occurred, it resulted in blood clots in association with low platelet counts. The condition typically occurred between one and three weeks after receipt of the vaccine and caused symptoms, such as severe headache, shortness of breath, severe abdominal pain, unexplained leg pain, chest pain, easy bruising, or small red spots on the skin.

Tolerance

The immune system's ability to distinguish between “self” and “non-self” proteins so that it does not create immune responses to “self” proteins.

Toxin

A harmful protein.

Toxoid

An inactivated toxin.

Vaccine effectiveness

This term describes how many people will be protected by a vaccine when it is used “in the real world,” meaning when it is used by the public. This is distinct from vaccine efficacy (see “vaccine efficacy” definition).

Find out more:

Vaccine efficacy

This term describes how many people were protected by a vaccine when it was tested in a clinical trial. Because clinical trials are well-controlled scientific studies, these measurements are often higher than vaccine effectiveness (see “vaccine effectiveness” definition).

Find out more:

Vaccine Integrity Project

The Vaccine Integrity Project is a non-governmental initiative led by the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota. It was launched in 2025 in response to concerns about threats to vaccine policy, public trust, and the systems that support immunization in the U.S.

VAERS

This acronym stands for the Vaccine Adverse Event Reporting System. This is part of the vaccine safety surveillance system in the U.S. This program collects reports of negative effects that people experience after vaccination that might have been caused by vaccination. Anyone can submit a report, and they do not need evidence that the vaccine was the cause. Public health investigators review each submission and if a concerning trend emerges, they turn to other systems to determine if the vaccine is causing the condition. Importantly, this is a warning system, so it cannot tell us whether a vaccine might be causing a negative effect.

VICP

This acronym stands for the Vaccine Injury Compensation Program. This program was established in 1986, and its purpose is to pay people who were injured by vaccines. The benefits of this program for individuals are that they have a lower requirement of proof. However, if they don’t agree with the outcomes, they can still take the case to regular court.

VIS

This acronym stands for Vaccine Information Statements, which are informational sheets about vaccines that are supposed to be given to people before they are vaccinated. The sheets describe the risks and benefits of the vaccine so that people can make informed decisions. The sheets also include information about what to do if a serious side effect occurs and how to find out about the VICP program or file a claim.

VRBPAC

This acronym stands for the Vaccines and Related Biological Products Advisory Committee. This is a group of individuals charged with reviewing the data about vaccine clinical trials and providing guidance to the Food and Drug Administration (FDA).

VSD

This acronym stands for Vaccine Safety Datalink. This is part of the vaccine safety surveillance system in the U.S. The program includes health systems throughout the country. If a vaccine safety concern arises, studies can quickly be conducted by evaluating health records to determine whether the condition of concern is greater in those who got the vaccine compared with those who didn’t.

WHO

This acronym stands for World Health Organization. This agency is part of the United Nations, and it’s responsible for international public health. They work to coordinate global health responses, set international health standards, conduct disease surveillance and support countries as they develop or improve their health systems.

Reviewed March 18, 2026