Epidemiology & Prevention USMLE Step 1 Practice Question
A 45-year-old woman participates in a randomized influenza vaccine trial. She receives the inactivated flu vaccine and remains asymptomatic throughout the season with normal vital signs (BP 120/80, HR 72, RR 16, Temp 98.6°F, SpO2 98%). In the vaccinated group (n=1,000), 6 develop symptomatic infection confirmed by rapid antigen testing. The unvaccinated control group (n=1,000) has 30 symptomatic cases. No adverse vaccine reactions occurred. Which calculation determines this vaccine's efficacy?
Answer choices
- A40%
- B20%
- C60%
- D94%
- E80%Correct answer
- F6%
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