Epidemiology · Reference
What is an attack rate?
The attack rate is the proportion of an at-risk population that develops a disease during a defined period, typically an outbreak. It is a form of cumulative incidence applied to a specific exposed group, widely used to compare risk between those exposed and not exposed to a suspected source.
What the attack rate measures
The attack rate expresses how much of an at-risk group falls ill during a specified period. It is computed as the number of new cases divided by the number of people at risk, usually over the limited time frame of an outbreak, and is often expressed as a percentage. Because the numerator is new cases and the denominator is the population at risk at the start, the attack rate is a form of cumulative incidence (incidence proportion). The word “rate” is conventional but technically a misnomer, since no person-time is involved; it is really a proportion confined to a defined population and period.
Use in outbreak investigation
Attack rates are a workhorse of outbreak investigation, particularly for point-source events such as foodborne outbreaks. Investigators calculate the attack rate among those exposed to a suspected source and among those not exposed, then compare them. A higher attack rate among the exposed points toward that exposure as a possible source, and the comparison can be summarised with measures such as relative risk. By tabulating attack rates across many possible exposures — for example each food item at an event — investigators can identify the exposure most strongly associated with illness.
Related forms
A secondary attack rate measures spread beyond the initial cases: the proportion of susceptible contacts of primary cases who go on to develop disease within the incubation period. It is used to gauge transmissibility, for example among household members.
The food-specific attack rate is the attack rate among those who consumed a particular food. Comparing food-specific attack rates between consumers and non-consumers of each item is the classic analytic step in investigating a foodborne outbreak, helping to single out the implicated vehicle.
Interpreting attack rates
Like any measure, an attack rate is only as clear as its definitions: the case definition, the population counted as “at risk” and the time period must all be stated for it to be interpretable. It describes a particular group during a particular event and does not, on its own, establish causation — a high attack rate among the exposed is a clue, weighed alongside biological plausibility and other evidence. This page defines the measure and its role in outbreak methodology in general terms and does not offer clinical or personal-health advice.
Key facts
At a glance
- Definition: At-risk people who develop disease in a defined period
- Formula: New cases ÷ population at risk (× 100)
- Type: A cumulative incidence (proportion), not a true rate
- Used in: Outbreak investigation (esp. foodborne)
- Variants: Secondary attack rate, food-specific attack rate
Common questions
FAQ
How is the attack rate calculated?+
The attack rate is the number of new cases divided by the number of people at risk over a defined period, usually expressed as a percentage. Because it counts new cases among a population at risk over a limited interval, it is a form of cumulative incidence rather than a true rate.
Why is the attack rate used in outbreak investigations?+
Investigators compare the attack rate among people exposed to a suspected source with the rate among those not exposed. A higher attack rate in the exposed group points toward that exposure as a possible cause, and tabulating rates across many exposures helps identify the most likely source.
What is a secondary attack rate?+
The secondary attack rate is the proportion of susceptible contacts of primary cases who develop the disease within its incubation period. It is used to measure how readily a disease spreads from person to person, for example within households, rather than from a common source.
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