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

No. of new cases of disease during time interval
Population at risk at start of time interval
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Number of new cases among contacts
Total number of contacts at risk
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1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


 

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

Last Checked: October 26, 2020

Next Review: October 26, 2025