Herd immunity threshold (HIT) for a disease with basic reproduction number R0 = 5 is:
- A 60%
- B 80% ✓
- C 50%
- D 75%
Correct answer: B. 80%
Explanation
Herd immunity threshold = 1 − (1/R0) = 1 − (1/5) = 1 − 0.2 = 0.8 = 80%. When ≥80% of the population is immune (through vaccination or prior infection), the effective reproduction number Reff falls below 1 and the epidemic cannot be sustained. For measles (R0 = 12–18), HIT is 92–95%; for COVID-19 original strain (R0 ≈ 2.5), HIT was approximately 60%.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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