A local public health department is investigating a community outbreak of a highly contagi… — USMLE Step 3

A local public health department is investigating a community outbreak of a highly contagious viral respiratory illness originating from a single elementary school. Initial contact tracing identifies $20$ primary cases, all of whom are students at the school. These $20$ children live in $20$ distinct households. Demographic analysis reveals that the total number of individuals residing in these $20$ households is $105$ . Medical record review confirms that among the household members who are not primary cases, $15$ individuals have documented immunity due to recent vaccination and are considered non-susceptible. During the recognized incubation period following exposure to the primary cases, $14$ new cases of the identical respiratory illness develop among the household members.

What is the secondary attack rate (in percentage) of this viral illness among the susceptible household contacts?

Answer: 20 %

Answer

The secondary attack rate measures the frequency of new cases of a disease among the contacts of known primary cases over a specific period. It is a critical metric for assessing the infectivity of a pathogen in closed environments like households. The calculation strictly requires defining the susceptible population exposed. In this scenario, there are

105

total residents, but

20

are the primary cases introducing the pathogen, leaving

85

household contacts. Of these

85

contacts,

15

are immune, leaving exactly

70

susceptible contacts. With

14

new secondary cases developing, the rate is

14

divided by

70

, which equals

0.20

, or

20\%

Step-by-Step Solution

Calculate the total number of household contacts at risk of exposure.

105 total residents - 20 primary cases = 85 household contacts.

Primary cases are the source of the exposure in the household and must be excluded from the denominator of at-risk contacts.

Determine the true number of susceptible household contacts.

85 household contacts - 15 immune individuals = 70 susceptible contacts.

Secondary attack rate measures the spread of disease specifically among those who are capable of contracting it; therefore, immune individuals must be removed from the denominator.

Calculate the secondary attack rate.

(14 secondary cases / 70 susceptible contacts) * 100 = 20%.

The formula for secondary attack rate is (Number of secondary cases / Number of susceptible contacts) * 100.

Key Concept

Secondary attack rate is calculated by dividing the number of new cases developing during the incubation period by the total number of susceptible contacts, excluding primary cases and immune individuals.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic