Vaccine efficacy refers to the measure of how well a vaccine prevents disease in a population under ideal and controlled conditions, such as in clinical trials. It is typically assessed by comparing the incidence of a specific disease in vaccinated individuals versus unvaccinated individuals. Factors such as the type of vaccine, dosage, timing of administration, and individual factors like age, health status, and immune response can all impact vaccine efficacy. Understanding vaccine efficacy is crucial for determining the overall effectiveness of vaccines in preventing the spread of infectious diseases and protecting public health.