Attack Rate Calculator
Attack rates, secondary attack rate and exposure comparison for outbreaks.
5.0 (1 rating)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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Understand the method, inputs and limits behind your result.
- Formula
- Attack rate = cases / people at risk × selected multiplier. Compare exposed and unexposed groups with their separate denominators.
- Variables
- Calculate: Overall attack rate, Exposed vs unexposed (outbreak food or source), Secondary attack rate
- People who became ill
- People at risk
- Exposed: ill
- Exposed: total
- Unexposed: ill
- Unexposed: total
- Secondary cases
- Susceptible contacts. Contacts minus primary cases
- Confidence level: 90%, 95%, 99%
- Units
- Use the units shown beside each input. Counts and ratios are dimensionless unless labeled otherwise.
- Example
- 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.
- Assumptions
- Confidence intervals use exact (Clopper–Pearson or Poisson) methods. Rates are only comparable between populations with similar age structures unless you standardize them.
- Source
- ToolzHive calculation guide and calculator implementation. External reference verification is pending.
- Last reviewed
- Review date not yet recorded.
Methodology & References
The method depends on the selected model and input options. Review the assumptions and references above before interpreting the result.
- Choose overall attack rate, exposed versus unexposed, or secondary attack rate.
- Enter the ill and total counts for each group.
- Read the attack rates, their ratio and confidence intervals.
Interpretation and limitations
- 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.
References
External reference verification is pending. The formula and examples describe the current ToolzHive implementation.
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Clear instructions
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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.
Practical guides
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How to Use Attack Rate Calculator
Calculate attack rates, food- or source-specific attack rates with the attack rate ratio, and secondary attack rates during an outbreak investigation.
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