BHS Epidemiology 5 — Questions and Answers
Question 1: The secondary attack rate (SAR) in a household outbreak measures:
- Total cases divided by total population
- New cases among susceptible contacts exposed to a primary case (Correct answer)
- Cases among those with prior immunity
- Deaths per infected household member
Correct answer: New cases among susceptible contacts exposed to a primary case
SAR is the proportion of susceptible household contacts who develop disease after exposure to a primary case, reflecting household transmissibility.
Question 2: When does the positive predictive value (PPV) of a test increase, assuming fixed sensitivity and specificity?
- When disease prevalence decreases
- When specificity decreases
- When disease prevalence increases (Correct answer)
- When sensitivity decreases
Correct answer: When disease prevalence increases
PPV rises with higher disease prevalence because a greater proportion of positive tests will be true positives in a more affected population.
Question 3: An investigator is studying whether air pollution exposure differs between asthma cases and healthy controls. The primary concern with retrospective exposure assessment is:
- Loss to follow-up
- Differential recall of exposure (Correct answer)
- Ecological fallacy
- Length-biased sampling
Correct answer: Differential recall of exposure
Retrospective assessment risks differential recall because cases may remember or report exposures differently than controls.
Question 4: The population attributable risk percent (PAR%) quantifies:
- The excess risk in exposed individuals due to the exposure
- The proportion of disease in the total population attributable to the exposure (Correct answer)
- The absolute risk difference between exposed and unexposed groups
- The relative reduction in risk after removing exposure
Correct answer: The proportion of disease in the total population attributable to the exposure
PAR% estimates what fraction of the total disease burden could theoretically be prevented if the exposure were eliminated from the population.
Question 5: A randomized trial is described as double-blind. This means:
- Neither the investigators nor the statisticians know the treatment assignment
- Neither the participants nor the outcome assessors know the treatment assignment (Correct answer)
- Neither the sponsor nor the IRB knows the allocation
- Both treatment groups receive identical interventions
Correct answer: Neither the participants nor the outcome assessors know the treatment assignment
Double-blinding means participants cannot change behavior based on treatment knowledge and assessors cannot introduce measurement bias.
Question 6: Which of the following transmission routes is exemplified by Legionella pneumophila outbreaks linked to cooling towers?
- Fecal-oral route
- Direct contact
- Airborne via aerosolized droplet nuclei (Correct answer)
- Vector-borne
Correct answer: Airborne via aerosolized droplet nuclei
Legionella is transmitted through inhalation of aerosolized water droplets from contaminated environmental sources such as cooling towers.
Question 7: In an outbreak of foodborne illness, the food-specific attack rates are used primarily to:
- Calculate the basic reproduction number
- Identify the most likely vehicle of transmission (Correct answer)
- Determine the incubation period distribution
- Estimate herd immunity levels
Correct answer: Identify the most likely vehicle of transmission
Comparing attack rates among those who ate versus did not eat each food item identifies which food is the likely vehicle.
BHS Epidemiology 5
The secondary attack rate (SAR) in a household outbreak measures: