incidence rate

Chapter: 9b
A quotient (rate), with the number of new cases of a specified disease diagnosed or reported during a defined period of time as the numerator, and the number of persons in a stated population in which the cases occurred as the denominator. This is usually expressed as cases per 1,000 or 100,000 per annum. This rate may be expressed as age- or sex-specific or as specific for any other population characteristic or subdivison (see Morbidity rate and Prevalence rate).

Attack rate, or case rate, is an incidence rate often used for particular groups, observed for limited periods and under special circumstances, as in an epidemic, usually expressed as percent (cases per 100). The secondary attack rate in communicable disease practice expresses the number of cases among familial or institutional contacts occurring within the accepted incubation period following exposure to a primary case, in relation to the total of determinable. Infection rate expresses the incidence of all infections, manifest and inapparent.

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