Vaccine Effectiveness

The degree to which a vaccine reduces disease risk under real-world conditions in a general population, unlike efficacy measured under trial conditions.

Source: Institute of Medicine. Initial National Priorities for Comparative Effectiveness Research. National Academies Press; 2009. doi:10.17226/12648.

Vaccine effectiveness measures how well a vaccine reduces the risk of disease in everyday use, among the general population under ordinary conditions. It captures the vaccine's real-world performance, which can fall short of the efficacy measured in a trial, because in practice storage may be imperfect, people may miss doses, and those vaccinated differ from the selected trial participants. This makes it the truer guide to what a vaccine actually achieves once rolled out. Real-world protection from a vaccine is what it captures. The Institute of Medicine describes such measures.

Source: Institute of Medicine

Vaccine effectiveness differs from vaccine efficacy in the conditions of measurement: effectiveness measures the reduction in disease risk under real-world conditions in a general population, while efficacy measures it under the controlled conditions of a clinical trial. So effectiveness and efficacy differ in real-world versus trial conditions, which is why the two can differ, since real-world use involves varied populations and conditions unlike the controlled setting of a trial, and effectiveness reflects how the vaccine performs in actual practice, whereas efficacy reflects its performance under ideal trial conditions, making the distinction important for understanding a vaccine's real-world impact.

Source: Institute of Medicine 2009

Vaccine effectiveness is measured in real-world conditions because how well a vaccine works in actual use, among a general population under everyday conditions, can differ from its performance in a controlled trial, so real-world measurement shows its practical impact. So vaccine effectiveness is measured in real-world conditions to capture actual performance, which is why it complements efficacy, since the varied populations and conditions of real use may affect how well the vaccine works, and measuring effectiveness in practice shows the vaccine's real-world impact, providing information beyond the efficacy measured under the ideal conditions of a trial.

Source: Institute of Medicine 2009

Vaccine effectiveness might differ from efficacy because real-world conditions differ from trial conditions in factors such as the population vaccinated, how the vaccine is stored and administered, and other variables, which can affect how well the vaccine works in practice. So effectiveness may differ from efficacy owing to real-world factors, which is why the two are distinguished, since the controlled conditions of a trial may not match everyday use, and factors such as a more varied population or differences in delivery can make real-world effectiveness differ from the efficacy measured in a trial, making effectiveness important for understanding how the vaccine actually performs.

Source: Institute of Medicine 2009

Vaccine effectiveness is important because it shows how well a vaccine actually reduces disease in real-world use, which is what matters for its impact on the population, complementing the efficacy measured in trials. So vaccine effectiveness matters for real-world impact, which is why it is measured, since the practical benefit of a vaccine depends on how it performs in actual use, and knowing its effectiveness in the general population under real conditions informs the assessment of its value and its contribution to controlling disease, providing information about real-world performance beyond what trial efficacy alone can show.

Source: Institute of Medicine 2009