Vaccine Efficacy

The degree to which a vaccine reduces disease risk among vaccinated individuals under the controlled conditions of a clinical trial.

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

Vaccine efficacy measures how well a vaccine reduces disease risk among vaccinated people under the controlled conditions of a clinical trial. In that setting, doses are given correctly, participants are selected and monitored, and storage is assured, so efficacy shows what the vaccine can achieve when everything is done ideally. It is the counterpart to effectiveness, which measures performance in messy real-world use, and it usually gives the higher figure of the two. A vaccine's protection under ideal trial conditions is what it captures. The Institute of Medicine describes such measures.

Source: Institute of Medicine

Vaccine efficacy is measured in a clinical trial by comparing the disease risk among vaccinated participants with that among unvaccinated or control participants under the trial's controlled conditions, showing the reduction in risk the vaccine provides. So vaccine efficacy is measured by comparison under trial conditions, which is why it reflects the controlled setting, since a trial compares vaccinated and control groups to determine the reduction in disease risk, and measuring efficacy this way, under controlled conditions, provides evidence of the vaccine's protective effect in the ideal setting of the trial, forming part of the evidence for the vaccine's approval and use.

Source: Institute of Medicine 2009

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

Source: Institute of Medicine 2009

Vaccine efficacy is measured in trials because the controlled conditions of a trial allow the vaccine's protective effect to be assessed rigorously by comparing vaccinated and control groups, providing clear evidence of how well it reduces disease risk. So vaccine efficacy is measured in trials for rigorous evidence, which is why controlled conditions are used, since a trial's design allows the reduction in disease risk attributable to the vaccine to be determined reliably, and measuring efficacy under these conditions provides the evidence of the vaccine's protective effect needed for its approval, before its real-world effectiveness is assessed in practice.

Source: Institute of Medicine 2009

Vaccine efficacy is important because it provides rigorous evidence, from controlled trials, of how well a vaccine reduces disease risk, forming part of the basis for its approval and expectations of its performance. So vaccine efficacy matters as trial evidence of protection, which is why it is measured, since establishing the vaccine's protective effect under controlled conditions is needed to support its approval and use, and knowing the efficacy provides a rigorous measure of the vaccine's ability to reduce disease, which, together with real-world effectiveness, informs the understanding of how well the vaccine works and its value.

Source: Institute of Medicine 2009