A measure of how well an intervention, that is a treatment or vaccine, works under ideal and controlled conditions in clinical trial research.

Efficacy, a term in epidemiology, is an important part of clinical trials. If a vaccine or treatment is shown to be safe and efficacious it will then be rolled out to the general population to test its effectiveness.

Clinical researchers, for example, may report that a vaccine is showing an efficacy rate of 90% among 100 participants in a trial. This means that 90 of the participants who were vaccinated did not get the disease the vaccine is designed to prevent, compared to the placebo group who would not have received a vaccine. This shows that the vaccine works well.

Several COVID-19 vaccine candidates in Phase lll clinical trials have reported efficacy rates of between 70% to 90%, which is beyond early expectations for a COVID-19 vaccine. The developers of at least two candidates have applied for emergency use authorisation for their vaccines.

 TIP: Don’t confuse efficacy with effectiveness. While they both measure the performance of a vaccine or treatment, they are measured in different settings.  



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EpidemiologyClinical TrialsVaccinesCompassionate useCOVID-19