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.
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.
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.
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.
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.
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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.
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.
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.
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.
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.
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.
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.
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.
Find out more about mRNA vaccines.
Watch a video about how mRNA vaccines work.
Learn more about other applications for mRNA technology.
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.
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.
Preservative
This is an ingredient added to a vaccine to make sure it does not become contaminated.
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.
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.
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.
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
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).
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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).
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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 Nov. 19, 2025