Question

The number of susceptible persons who develop the disease within the range of the incubation period after contact with a primary case is the:

• A. Secondary attack rate
• B. Case fatality rate
• C. Primary attack rate
• D. Tertiary attack rate

Hint:

New cases among contacts of the index case within one incubation period.

Answer 1

The Correct Option is A

Step 1: The stem describes new cases arising among contacts of an index (primary) case, within one incubation period - this is the classic definition of the Secondary Attack Rate (SAR). Step 2: SAR is calculated as \[SAR = \frac{\text{number of exposed contacts developing disease within the incubation period}}{\text{total number of susceptible contacts exposed}} \times 100.\] Step 3: SAR is a good measure of the infectivity or communicability of an agent and is widely used in household and close-contact studies. Step 4: The distractors are wrong - case fatality rate measures deaths among cases, the primary attack rate refers to the initial cases in the source population, and tertiary attack rate is not a standard epidemiological term. Hence the answer is secondary attack rate.