The overall attack rate asks how many learners in the entire school became ill during the outbreak period, regardless of whether they ate the implicated meal. The numerator includes every case that occurred within the defined time frame:
66 learners who ate the meal and became ill, plus
9 learners who did not eat the meal but still developed the same illness. That gives
75 total cases. The denominator is the entire population at risk at the start of the outbreak, which is all
600 learners in the school.
Attack rate = (total new cases ÷ total population at risk) × 100 = (75 ÷ 600) × 100 = 12.5%. The answer is
12.5%.
In an outbreak investigation, the
attack rate is a basic measure of how quickly and widely a disease spreads in a defined group over a specific period. It is expressed as a percentage and is calculated by dividing the number of new cases by the number of people who were at risk at the beginning of the outbreak. The word “attack” reflects the acute, time-limited nature of the event, which fits the school feeding scenario where symptoms appeared within
8 hours after the meal.
The calculation requires careful attention to what belongs in the numerator and denominator.
Key point! The overall attack rate uses the whole school population as the denominator, not only the learners who ate the meal. Some examinees mistakenly choose
27.5% because they compute
66 ÷ 240. That figure is the
attack rate among exposed learners only, also called the food-specific attack rate. It is useful for comparing how much more likely the exposed group was to become ill compared with the unexposed group, but it does not answer the question about the entire school.
A useful way to see the difference is to place the two rates side by side.
| Rate | Numerator | Denominator | Calculation | Result |
|---|
| Overall attack rate (entire school) | All cases: 66 + 9 = 75 | All learners: 600 | 75 ÷ 600 × 100 | 12.5% |
| Attack rate among eaters only | Cases among eaters: 66 | Learners who ate: 240 | 66 ÷ 240 × 100 | 27.5% |
| Attack rate among non-eaters | Cases among non-eaters: 9 | Learners who did not eat: 360 | 9 ÷ 360 × 100 | 2.5% |
The contrast between the exposed and unexposed attack rates is central to outbreak investigation. In this scenario, the attack rate among learners who ate the meal (
27.5%) is much higher than the attack rate among those who did not eat (
2.5%).
A large difference between these two rates suggests that the meal was a likely vehicle of infection, but the overall attack rate for the school must still include every case and every learner. The
9 cases among non-eaters are important because they may represent secondary transmission, recall errors about who actually ate, or another source of exposure. They cannot be ignored when calculating the overall rate.
The
88.0% option is not a meaningful attack rate in this context. It might arise from mistakenly using
66 ÷ 75, which would describe the proportion of all cases that occurred among eaters, not an attack rate. That is a different epidemiological measure and does not reflect the risk of illness in the school population.
Watch out! The wording “counting every learner who had the illness during this period as a case” is the key instruction. It tells you to include both exposed and unexposed cases in the numerator. If the question had asked for the attack rate among those who ate the meal, the denominator would be
240 and the answer would be
27.5%. But because it asks for the overall attack rate among all learners, the denominator is
600 and the numerator is
75.
In field investigations of foodborne outbreaks in schools, such as those described in the provided abstracts, investigators typically define a case, identify all persons who meet the case definition, and then calculate attack rates for both exposed and unexposed groups. The overall attack rate gives a sense of the outbreak’s magnitude in the entire population, while the food-specific attack rates help identify the likely source. Both are needed, but they answer different questions.
The overall attack rate is a population-level measure, while the exposed and unexposed attack rates are subgroup measures used to compare risk.