Vaccine effectiveness refers to how well a vaccine works in preventing disease in real-world conditions. It is usually measured by comparing the rate of disease in vaccinated individuals to that in unvaccinated individuals. High effectiveness means that the vaccine significantly reduces the risk of getting the disease, while lower effectiveness indicates a smaller impact. Factors influencing vaccine effectiveness include the type of vaccine, the population's health, and the presence of new variants of the virus. For example, the effectiveness of the COVID-19 vaccine may vary based on the specific strain of the virus and the time since vaccination.