This Attack Rate Formula Calculator is the ideal app for Epidemiologists because it can help determine the risk of contracting a disease during an outbreak.

Attack Rate in Epidemiology

Attack rate is a term used in epidemiology, usually in the setting of outbreaks, to define the risk of contracting a disease during a specified period (i.e. outbreak duration).

The overall attack rate refers to the total number of new cases divided by the total population:

Attack Rate (%) = No. of new cases of disease during time interval / Population at risk at start of time interval x 100

The secondary attack rate can help document the difference between community transmission and disease transmission in a closed population, for example a household.

Secondary Attack Rate (%) = No. of new cases among contacts / Total number of contacts at risk x 100

AR is similar to cumulative incidence but is used in the context of an epidemic, to create hypothetical predictions about an outbreak or to report on food poisoning incidents rather than in studies or for reporting of chronic diseases.

The “at risk population” is defined as those individuals who are susceptible to develop the disease carried by the attacking pathogen (who have no immunity to it) and so are at a higher risk of becoming ill than others.

Specific attack rates may be defined in relation to age, gender, residence, occupation or other factors. Two attack rates may be compared to each other as a relative risk (division) or as a risk difference (subtraction).