Situation: Within 8 hours after a school feeding activity, many learners of a public elementary school with 600 learners developed vomiting and diarrhea. Of the 600 learners, 240 ate the meal served, which consisted of an egg sandwich and a fruit juice. The school nurse works with the municipal health office to investigate. The team interviews all 240 learners who ate the meal: Egg sandwich — ate: 57 ill of 150; did not eat: 9 ill of 90 Fruit juice — drank: 56 ill of 200; did not drink: 10 ill of 40 Which conclusion do the data BEST support?

1 The fruit juice, since it was taken by more of the learners who ate
2 Both foods equally, since each has an attack rate above 25% among takers
3 The egg sandwich, as eaters had a far higher attack rate than noneaters ✓ 정답
4 Neither food, since each has ill learners among those who did not take it

해설

The likely vehicle has a high attack rate among those who took it, a low attack rate among those who did not, and accounts for most cases. For the egg sandwich, the attack rate was 38.0% among eaters (57 ÷ 150) versus 10.0% among noneaters (9 ÷ 90), a relative risk of 3.8. For the fruit juice, it was 28.0% (56 ÷ 200) versus 25.0% (10 ÷ 40), a relative risk of about 1.1, so the juice shows no real difference.

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심화 해설

Comparing attack rates to find the vehicle
In a foodborne outbreak, the likely vehicle is the food with a high attack rate among those who ate it, a low attack rate among those who did not, and a large share of all cases. The food-specific attack rate is the number ill divided by the number exposed. For the egg sandwich: eaters 57 ÷ 150 = 38.0%; noneaters 9 ÷ 90 = 10.0%. For the fruit juice: drinkers 56 ÷ 200 = 28.0%; nondrinkers 10 ÷ 40 = 25.0%. The egg sandwich shows a large difference between eaters and noneaters, while the juice shows almost none.

Using the relative risk
The relative risk (attack rate ratio) divides the attack rate among the exposed by the attack rate among the unexposed. For the egg sandwich it is 38.0 ÷ 10.0 = 3.8, meaning eaters were nearly four times as likely to become ill. For the juice it is 28.0 ÷ 25.0 ≈ 1.1, essentially no difference. The egg sandwich also accounts for most of the cases (57 of 66 ill learners). The onset within 8 hours with vomiting and diarrhea fits a toxin-mediated food poisoning, and eggs are a well-known vehicle when food is prepared or stored improperly.

Food	Attack rate, took it	Attack rate, did not	Relative risk
Egg sandwich	38.0% (57/150)	10.0% (9/90)	3.8
Fruit juice	28.0% (56/200)	25.0% (10/40)	about 1.1

Why the other conclusions are wrong
Choosing the juice because more learners drank it confuses popularity with risk; popular items are taken by most people, ill or not. Blaming both foods because each has an attack rate above 25% looks only at the exposed group and ignores the comparison with nontakers. Ruling out both foods because some ill learners did not take them expects a perfect pattern that real outbreaks rarely show; a few ill nontakers are explained by other exposures, cross-contamination, or misremembered intake. Watch out! The key comparison is always the difference between takers and nontakers of the same food.

Exam takeaway and next steps
The vehicle is the food with the greatest difference in attack rates between those who ate it and those who did not. The team should collect any leftover sandwiches and stool samples for laboratory testing, inspect how the eggs were prepared and stored, interview food handlers, and report the outbreak through the surveillance system. Health teaching on safe food handling for school feeding staff helps prevent recurrence.

임상 시나리오

Food-Specific Attack RatesIdentifying the outbreak vehicle

Attack rate = ill ÷ exposed. Egg sandwich: 38.0% eaters vs 10.0% noneaters, relative risk 3.8.

Fruit juice: 28.0% drinkers vs 25.0% nondrinkers, relative risk about 1.1: no real difference.

The egg sandwich is the likely vehicle. Collect samples, inspect food preparation, and report through surveillance.

Caution

Do not choose a food because many people took it; compare attack rates between takers and nontakers.

핵심 개념

Attack rate — The proportion of exposed people who become ill during an outbreak.
Food-specific attack rate — The attack rate among people who ate, or did not eat, a particular food.
Relative risk — The attack rate in the exposed divided by the attack rate in the unexposed.
Vehicle — The contaminated food, water, or object that carries the agent to people in an outbreak.

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