Attack Rate

The attack rate is the proportion of people who become ill with a disease. These rates are used in the investigation of an acute outbreak of disease to determine what exposures contributed to the illness. It is calculated as the number of people who became ill divided by the number of people at risk for the illness:

A case definition must first be developed. Case definitions may be based on a constellation of clinical signs (e.g., fever with vomiting and/or diarrhea) or on serology (e.g., the presence of antibodies to the etiologic agent). Those people who meet the case definition are identified; the number of people who meet the case definition is the numerator of the attack rate. The denominator of the attack rate is the number of people at risk of becoming ill. As the attack rate is used in the investigation of an outbreak, those at risk are the people who had the opportunity to be exposed such as those who attended the same event as those who became ill.

The time over which cases are collected is defined by the specifics of the outbreak: those who develop the symptoms over a set period after an event. The period over which new cases occur can be a clue to the identification of the causative agent.

Those who did or did not get sick are interviewed to determine their exposures, and attack rates for those who were or were not exposed are calculated. It may be impossible to determine the causative exposure from exposure-specific attack rates alone, as many of the subjects will have exposure to multiple potential agents. A relative attack rate must be calculated for each exposure: