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Sensitivity — USMLE Step 3 MCQ

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HardPreventive Medicine, Epidemiology, BiostatisticsSensitivityUSMLE Step 3

A vaccine cohort records 12 infections during 2,400 vaccinated person-years and 30 infections during 1,500 unvaccinated person-years. What is the vaccine effectiveness based on incidence-rate ratio?

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Correct answer: A — 75%

The best answer is “75%”. The vaccinated incidence rate is 12/2,400 = 0.005 per person-year and the unvaccinated rate is 30/1,500 = 0.020. The incidence-rate ratio is 0.25, so vaccine effectiveness is (1 − 0.25) × 100 = 75%. Comparing case counts alone would ignore unequal person-time.

Reference: CDC Principles of Epidemiology in Public Health Practice: https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/SS1978.pdf