Attack Rate Calculator
Attack rates, secondary attack rate and exposure comparison for outbreaks.
Enter your values to see the result.
Confidence intervals use exact (Clopper–Pearson or Poisson) methods. Rates are only comparable between populations with similar age structures unless you standardize them.
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- Relative Risk Calculator — Get the full set of risk measures.
- Case Fatality Rate Calculator — Measure severity among cases.
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Clear instructions
Find steps, examples and limitations below.
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How to use the Attack Rate Calculator
Attack rates, secondary attack rate and exposure comparison for outbreaks.
- 1 Choose overall attack rate, exposed versus unexposed, or secondary attack rate.
- 2 Enter the ill and total counts for each group.
- 3 Read the attack rates, their ratio and confidence intervals.
Example and practical tips
38 of 60 people who ate the chicken salad fell ill against 4 of 90 who did not: an attack rate ratio of 14.25.
Frequently asked questions
What is an attack rate?
The proportion of an exposed population who become ill during an outbreak. It is a cumulative incidence over a short period.
What is a secondary attack rate?
The share of susceptible contacts of a case who become ill, which measures person-to-person spread.
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