A vaccine with 70% efficacy is deployed in a population where baseline attack rate (without vaccine) is 20%. The attack rate in vaccinated individuals will be approximately:
- A 14%
- B 50%
- C 6% ✓
- D 10%
Correct answer: C. 6%
Explanation
Vaccine efficacy (VE) = (Attack Rate Unvaccinated − Attack Rate Vaccinated) / Attack Rate Unvaccinated. Rearranging: Attack Rate Vaccinated = Attack Rate Unvaccinated × (1 − VE) = 20% × (1 − 0.70) = 20% × 0.30 = 6%. Vaccine efficacy of 70% means the vaccine reduces the risk of infection by 70% relative to unvaccinated individuals.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.