Vaccine efficacy refers to the ability of a vaccine to prevent disease in controlled clinical trials. It is usually expressed as a percentage, indicating how much the vaccine reduces the risk of getting the disease compared to those who are unvaccinated. For example, if a vaccine has an efficacy of 90%, it means that vaccinated individuals are 90% less likely to contract the disease than those who did not receive the vaccine. Efficacy is determined by comparing the number of cases of the disease in the vaccinated group versus the unvaccinated group during the trial. Factors such as the population studied, the timing of the vaccine, and the specific strain of the virus, like SARS-CoV-2, can influence efficacy results. Understanding vaccine efficacy helps public health officials make informed decisions about vaccination programs and disease prevention strategies.