Attack rate
- Measure
The attack rate is the share of exposed people who fall ill in an outbreak; comparing rates by food points to the suspect.
What it is
The attack rate is the cumulative incidence (the risk) over a short, defined period, typically an outbreak. It is computed for the whole exposed population and for subgroups, for example by food eaten: the food with the highest attack rate among those who ate it and the lowest among those who did not is the likely vehicle. The secondary attack rate measures spread among the contacts of the first cases.
Formula and reading
- Attack rate = cases during the outbreak / people exposed at the start × 100.
- Secondary attack rate = cases among contacts / (total contacts − primary cases) × 100.
- RR of a food = attack rate in those who ate it / attack rate in those who did not; an RR near 1 rules that food out.
Worked example
- Data (made up): a wedding has 150 guests and 55 fall ill with vomiting and diarrhoea within 24 hours.
- Overall attack rate: 55 / 150 = 0.367, that is, ≈ 37 %.
- Chicken salad: 80 ate it and 48 fell ill (60 %); 70 did not and 7 fell ill (10 %).
- RR for the salad: 60 % / 10 % = 6.0.
- Rice: 100 ate it and 38 fell ill (38 %); 50 did not and 17 fell ill (34 %); RR = 0.38 / 0.34 ≈ 1.1.
- The salad explains 48 of the 55 cases (87 %) and has the highest RR: it is the likely vehicle.
- Secondary attack rate: 70 other people live in the homes of 20 cases; 14 fall ill the next week: 14 / 70 = 20 %.
- Reading: the salad multiplied the risk by 6, and 1 in 5 household members was infected later at home.
Food-specific attack rates in the example (made-up figures)
| Food | Ate: ill / total | Did not eat: ill / total | RR |
|---|---|---|---|
| Chicken salad | 48 / 80 (60 %) | 7 / 70 (10 %) | 6.0 |
| Rice | 38 / 100 (38 %) | 17 / 50 (34 %) | 1.1 |
Markers
- AR
- Secondary AR
- RR
- Cumulative incidence
How to read it
- In a foodborne outbreak, a food history from everyone who attended (ill and well) is what makes these rates possible.
- A high household secondary attack rate points to stronger hand hygiene, isolation and contact tracing.
Common pitfalls
- Computing the rate among the ill only: the well people who ate and did not eat are also needed.
- Including primary cases in the denominator of the secondary attack rate: it underestimates it.
- Choosing the food with the most cases without looking at who did not eat it: a food almost everyone ate will have many cases even if innocent.
Test yourself
Think of the answer, then tap the question to see it.
Why is the attack rate really a proportion?
Because it divides cases by the people at risk at the start, with no person-time: it is a cumulative incidence (a risk).
What denominator does the secondary attack rate use?
The susceptible contacts of the primary cases, not counting the primary cases themselves.
A food was eaten by 95 % of guests and has RR = 1.0. Is it the likely vehicle?
No: although it has many cases, those who ate it and those who did not fell ill equally; it does not explain the outbreak.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Principles of Epidemiology | Lesson 3 - Section 2 (Centers for Disease Control and Prevention, 2012); Principles of Epidemiology: Lesson 6, Section 2 (Centers for Disease Control and Prevention, 2012).
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Representative model, for educational purposes only.