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.
A to F
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).
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.
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.
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.
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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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).
G to L
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.
Immunogenicity
The ability of something, like a vaccine, to cause a person’s immune system to make an immune response.
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
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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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).
Reviewed October 7, 2025