Plain-language definitions of key terms used in vaccine science, clinical trials, regulation, and public health.
A substance added to vaccines to enhance the immune system's response, making the vaccine more effective. Common examples include aluminum salts.
Any health problem that occurs after vaccination, regardless of whether it was caused by the vaccine. Serious adverse events are rare.
A protein produced by the immune system to identify and neutralize pathogens like viruses and bacteria. Vaccines stimulate antibody production without causing disease.
The component of a vaccine that triggers the immune system to create protective antibodies. It may be a weakened or inactivated pathogen, or a piece of it.
An additional dose of a vaccine given to "boost" immunity that may have waned over time. Common examples include Tdap boosters and annual flu shots.
Research studies conducted in phases (I-IV) to test vaccines for safety and effectiveness in humans before approval.
A condition that makes a particular vaccine unsafe for a specific person. Examples include severe allergies to vaccine ingredients.
How well a vaccine works under ideal, controlled conditions in clinical trials. Effectiveness refers to how well it works in real-world settings.
Protection achieved when a sufficient percentage of a population is immune, making disease spread unlikely. This protects those who cannot be vaccinated.
The body's protection against a specific disease. Can be acquired through infection recovery or vaccination.
Messenger RNA — a type of genetic material that provides instructions for cells to make proteins. COVID-19 mRNA vaccines teach cells to make the spike protein.
Stages of vaccine testing: Phase I (safety), Phase II (dose/response), Phase III (efficacy), Phase IV (post-licensure monitoring).
Vaccine Adverse Event Reporting System — a national safety monitoring system where anyone can report health events after vaccination. Does not prove causation.
A vaccine that uses a harmless virus to deliver genetic instructions for making a pathogen protein. Examples: Johnson & Johnson COVID-19 vaccine.
Vaccine Injury Compensation Program — a federal "no-fault" program that provides compensation for people injured by certain vaccines.
The gradual decrease in protective antibodies over time, which is why some vaccines require boosters.