A state department of public health investigates an outbreak of a novel respiratory virus in a long-term care facility. On November 1, the facility houses 120 residents. Medical records indicate that 20 residents had previously contracted and recovered from the virus during a smaller wave in September, conferring confirmed immunity.
On November 5, an outbreak begins when a resident develops symptoms. Over the next 14 days, a total of 15 new cases of the virus are diagnosed among the residents. During this period, 3 of the newly infected residents die from respiratory failure.
Which of the following best represents the attack rate for the November outbreak among the residents?
- A
- B
- C
- Answer
- E
Answer
The attack rate is .
The correct answer accurately calculates the attack rate by identifying 15 new cases and dividing them by the 100 susceptible residents. The 20 residents with prior immunity are correctly subtracted from the total population of 120 to determine the true population at risk.
Step-by-Step Solution
Key Concept
The attack rate is a specific type of cumulative incidence used during outbreaks, calculated by dividing the number of new cases by the total population at risk (excluding immune or non-susceptible individuals) at the beginning of the outbreak period.