An index case of measles occurs in a household containing 4 other previously unvaccinated susceptible children and 3 immune adolescents who had past infection. Over the next two weeks, 2 of the susceptible children develop measles. The secondary attack rate is closest to:
- A 29%
- B 43%
- C 50% ✓
- D 67%
Correct answer: C. 50%
Explanation
Secondary attack rate equals the number of secondary cases among contacts divided by the number of susceptible contacts, multiplied by 100. Only the 4 susceptible unvaccinated children are at risk; the 3 immune adolescents are excluded from the denominator. Two secondary cases out of four susceptibles gives 50%. Using all seven contacts yields about 29%, the classic error of failing to restrict the denominator to susceptibles.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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