What Is the Difference between Vaccine Efficacy and Vaccine Effectiveness?

Dr. Hank Bernstein explains why there may be a difference between clinical trial findings and those found when the general population gets a vaccine.


What Is the Difference between Vaccine Efficacy and Vaccine Effectiveness?

Hank Bernstein, MD: I'm Hank Bernstein and I'm speaking on behalf of the Vaccine Education Center at Children's Hospital of Philadelphia. The question, “What's the difference between vaccine efficacy and vaccine effectiveness?” These are two confusing terms.

Both vaccine efficacy and vaccine effectiveness measure the impact of the vaccine on specific outcomes, such as hospitalizations or death. An important difference is that efficacy is the term we use when we want to know whether the vaccine is doing what it's supposed to do for a particular individual. It measures specific outcomes in ideal situations like a clinical study, where all aspects of the study are well designed and well controlled. For example, if a participant has an underlying medical condition that may interfere with the results of the study, he or she are often excluded. So, efficacy answers the questions, "Does the vaccine work? And does it do what it's supposed to do?”

On the other hand, vaccine effectiveness answers the question, “Does the vaccine work in the real world?” In fact, a vaccine may be efficacious in a study, but it doesn't work as well in the community. Effectiveness measures the rates of specific outcomes between vaccinated and unvaccinated people in the general population.

Related Centers and Programs: Vaccine Education Center

Last Reviewed on Mar 16, 2021