A family of 10 members contains one index case of measles. Of the remaining 9 members, 3 develop measles within one incubation period of the index case. The secondary attack rate of measles in this household is:
- A 30%
- B 33% ✓
- C 40%
- D 43%
Correct answer: B. 33%
Explanation
Secondary attack rate equals the number of new cases among contacts divided by the number of susceptible contacts exposed to the primary case, multiplied by 100. The index case must be removed from both numerator and denominator, leaving 3 secondary cases out of 9 exposed susceptible contacts: 3 divided by 9 times 100 gives 33%. Option A comes from wrongly using 10 as the denominator, and option D arises from keeping the index case in the numerator as well.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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