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Coverage of a public health program is the proportion of persons targeted by a specific program who actually received the intended service or benefit. Coverage is a very common indicator in both emergency and non-emergency situations. Vaccination programmes are very frequently evaluated by measuring coverage.
Note that it is a outcome (or process, depending on your preferred evaluation terminology) indicator. It is not an impact indicator. Nonetheless, many studies have proved the efficacy of currently used vaccines. They have proved that, if an individual person gets the vaccine according to recommendations, that person will have a substantially reduced likelihood of getting the disease or a lower severity or duration of the disease. As a result, if most people in a population are vaccinated against a specific disease (that is, the "outcome" indicator "coverage" is high), that disease should become less common (the level of the "impact" indicator of disease incidence or mortality should decline). This connection between high coverage and decline in disease has been demonstrated for most vaccines in many populations.