Attack Rate Formula Calculator
Helps determine the risk of contracting a disease during an outbreak.
Refer to the text below the tool for more information about AR and secondary AR and their usage.
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).
Attack Rate (%) = No. of new cases of disease during time interval / Population at risk at start of time interval x 100
Secondary Attack Rate (%) = No. of new cases among contacts / Total number of contacts at risk x 100
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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).
Reference
Centers for Disease Control and Prevention: Principles of Epidemiology in Public Health Practice, Third Edition – An Introduction to Applied Epidemiology and Biostatistics
Specialty: Epidemiology
Article By: Denise Nedea
Published On: October 26, 2020 · 12:00 AM
Last Checked: October 26, 2020
Next Review: October 26, 2025