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Efficacy is the ability to produce a desired amount of a desired effect. In a medical context it indicates that the therapeutic effect of a given intervention (e.g. intake of a medicine, an operation, or a public health measure) is acceptable. 'Acceptable' in that context refers to a consensus that it is at least as good as other available interventions to which it will have ideally been compared to in a clinical trial. For example, an efficacious vaccine has the ability to prevent or cure a specific illness in an acceptable proportion of exposed individuals.
The concept of 'self-efficacy' is an important one in the self-management of chronic diseases because doctors and patients often do not follow best practice in using a treatment. For instance, a patient using oral contraceptive pills to prevent pregnancy may sometimes forget to take a pill at the prescribed time; thus, while the perfect-use failure rate for this form of conception in the first year of use is just 0.3%, the typical-use failure rate is 8%.