CIC Outbreak Investigation 5 — Questions and Answers
Question 1: In an outbreak investigation, the population attributable risk percent (PAR%) is useful because it estimates:
- The probability that a single exposed individual will develop the disease
- The proportion of disease in the total population attributable to the exposure (Correct answer)
- The relative increase in risk among exposed vs. unexposed individuals
- The number of cases prevented by removing the exposure source
Correct answer: The proportion of disease in the total population attributable to the exposure
PAR% estimates what proportion of all disease in the population is attributable to the exposure, which is valuable for prioritizing public health interventions.
Question 2: A unit-level outbreak investigation reveals that three nurses cared for all cases but no other cases occurred among patients of other nurses. This finding most strongly suggests:
- The pathogen is highly virulent and affects all exposed patients equally
- Healthcare worker carriage or technique as a potential transmission source (Correct answer)
- Environmental contamination limited to those nurses' assigned rooms
- A surveillance artifact due to selective testing by those nurses
Correct answer: Healthcare worker carriage or technique as a potential transmission source
When cases cluster around specific healthcare workers, it raises the hypothesis of HCW carriage or technique-related transmission as a contributing factor.
Question 3: Which type of bias occurs when cases are more likely to recall an exposure than controls in a case-control outbreak study?
- Selection bias
- Recall bias (Correct answer)
- Information bias due to interviewer technique
- Confounding bias
Correct answer: Recall bias
Recall bias occurs when cases, motivated by their illness, remember exposures more completely than controls, potentially inflating the apparent association.
Question 4: When a healthcare facility experiences an outbreak of Legionella pneumophila, what is the most critical environmental source to investigate first?
- Patient room air-handling units and HEPA filters
- Potable water system including cooling towers and hot water distribution (Correct answer)
- Soil and construction sites near the facility
- Contaminated respiratory therapy nebulizers in use
Correct answer: Potable water system including cooling towers and hot water distribution
Legionella amplifies in water systems, and the potable water distribution system and cooling towers are the primary environmental sources to sample and remediate.
Question 5: In the investigation of a nosocomial outbreak, which laboratory result would most strongly support clonal transmission from a single source?
- All isolates share the same antibiogram pattern
- All isolates are indistinguishable by whole genome sequencing with fewer than 5 SNP differences (Correct answer)
- All isolates are the same species and Gram-stain morphology
- All isolates produce the same virulence factor on culture media
Correct answer: All isolates are indistinguishable by whole genome sequencing with fewer than 5 SNP differences
Isolates with fewer than 5 SNP differences by WGS are considered highly related and strongly support clonal transmission from a common source.
Question 6: Which of the following best describes the role of the ICP when the local health department assumes jurisdiction over an outbreak investigation?
- The ICP's role ends and they defer entirely to the health department
- The ICP collaborates by providing facility data, implementing control measures, and serving as the internal liaison (Correct answer)
- The ICP independently continues a parallel investigation without sharing data
- The ICP suspends all infection control activities pending health department guidance
Correct answer: The ICP collaborates by providing facility data, implementing control measures, and serving as the internal liaison
The ICP serves as the critical internal liaison, providing case and exposure data, implementing recommended controls, and coordinating between the facility and health department.
Question 7: A hospital reports a cluster of post-operative surgical site infections following cardiac bypass surgery over six weeks. The MOST informative initial analysis would be:
- Calculating the overall SSI rate and comparing to national NHSN benchmarks
- Creating a line list and epidemic curve stratified by surgeon, OR suite, and procedure date (Correct answer)
- Immediately culturing all surgical instruments and OR surfaces
- Reviewing antibiotic prophylaxis compliance rates for all cardiac cases
Correct answer: Creating a line list and epidemic curve stratified by surgeon, OR suite, and procedure date
A line list and epidemic curve stratified by surgeon, OR suite, and date allows the ICP to identify person, place, and time patterns that generate hypotheses about the source.
CIC Outbreak Investigation 5
In an outbreak investigation, the population attributable risk percent (PAR%) is useful because it estimates: