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Difficulty: MediumMeasures of Disease Frequency

An infection control team is investigating an outbreak of *Listeria monocytogenes* at an assisted-living facility. On the day of a suspected food exposure event, the facility had exactly 300300 residents, all of whom consumed the implicated meal. Over the subsequent four weeks, 4545 residents developed culture-confirmed listeriosis. Among the residents who contracted the infection, 99 died directly from complications of the disease. During this same four-week period, 66 other residents died from conditions entirely unrelated to the outbreak. Based on these data, what is the case fatality rate of listeriosis in this specific outbreak?

  1. A
    3%3\%
  2. B
    5%5\%
  3. C
    15%15\%
  4. 20%20\%Answer
  5. E
    60%60\%

Answer

20%20\%
The case fatality rate represents the lethality of a condition and is defined as the percentage of individuals diagnosed with a specific disease who die from that disease. In this scenario, 4545 residents contracted listeriosis and 99 died from it. Therefore, the case fatality rate is 945\frac{9}{45}, which equals 0.200.20 or 20%20\%.

Step-by-Step Solution

1
Identify the requested metric
The question asks for the case fatality rate (CFR) of listeriosis.
CFR measures the lethality of a disease among those who are infected.
2
Recall the formula for case fatality rate
CFR = (Number of deaths from the specific disease) / (Total number of incident cases of the disease).
The denominator must strictly be the individuals who contracted the disease, not the general population.
3
Extract the relevant variables from the clinical vignette
Total cases of listeriosis = 45. Deaths specifically attributed to listeriosis = 9.
These are the precise values required for the CFR formula. The unrelated deaths (6) and total population (300) are distractors.
4
Calculate the final percentage
9 / 45 = 0.20, which equals 20%.
Applying the extracted data to the statistical formula yields the correct CFR.

Key Concept

The case fatality rate (CFR) is a measure of disease severity, calculated by dividing the number of deaths caused by a specific disease by the total number of individuals diagnosed with that disease during the same time period.
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Topics

Measures of Disease Frequency

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