A family has 6 members, one of whom develops measles and serves as the index case. During the subsequent expected incubation period, 2 previously susceptible siblings develop measles. The secondary attack rate of measles in this household is:
- A 33.3%
- B 40% ✓
- C 66.7%
- D 50%
Correct answer: B. 40%
Explanation
Secondary attack rate = number of secondary cases divided by number of susceptible contacts, multiplied by 100. The index case is excluded from the denominator, leaving 5 susceptible household members, of whom 2 developed disease: (2/5) x 100 = 40 percent. Option A incorrectly uses all 6 members as denominator. This measure is used to assess contagiousness and the effectiveness of control measures within closed groups such as households.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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