There are two key measures that determine vaccine effectiveness: clinical trial efficacy and real-world effectiveness

  • Trial efficacy is measured in a structured clinical trial. These trials involve groups of diverse people and measure how much a vaccine reduces their risk of getting sick. If a vaccine has high efficacy, that means there were significantly fewer people in the vaccinated group who got sick compared with the unvaccinated group. 
  • Vaccine effectiveness measures successful immune protection under real-world conditions. If a vaccine has high effectiveness, there is clear evidence that the vaccine protects the public against severe infection, symptomatic illness, hospitalization, and death. 

Measuring vaccine effectiveness is a complex process. Oftentimes vaccine effectiveness data are not identical to vaccine efficacy data. This is because vaccine effectiveness takes into account the real world conditions in which vaccines are actually administered. Without context these statistics can oversimplify the number of factors used to determine outcomes. Vaccine effectiveness varies based on location, population, and specific health outcomes. The CDC and other public health researchers routinely monitor and evaluate data for vaccine effectiveness.

Added December 14, 2023