Attack rate

Last revised by Daniel J Bell on 23 Apr 2020

The attack rate, also known as the incidence proportion, is an epidemiological term for a proportion that shows the number of new cases of a disease that develop over a specified time period from a specified population present at the beginning of an outbreak of disease.

For an infectious disease, the secondary attack rate relates to the cases of the disease in contacts with those infected. The secondary attack rate is related to R0, the basic reproductive number, but is not the same. There are complex functions that can predict an attack rate from the R0 of a disease in some cases, however there is not an entirely predictable mathematical relationship 3. In diseases that are seasonal, e.g. influenza, seasonal attack rates may differ by time of year. Some specific types of food poisonings do seem to follow a seasonal pattern in not only incidence but may also have a variable attack rate 2.

NB: Infants born with a disease would be included in the numerator of the attack rate for it, but not the denominator.

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