The 'Basic Case Reproduction Number' (R₀) of measles is approximately 12–18. After achieving 95% vaccination coverage, which threshold phenomenon explains why measles transmission is interrupted?
- A Individual immunity prevents any measles virus replication
- B Measles virus mutates to a less virulent form as population immunity increases
- C Herd immunity threshold is reached when enough immune individuals break transmission chains, protecting unvaccinated individuals ✓
- D The virus reservoir in the environment is eliminated by community-level vaccination
Correct answer: C. Herd immunity threshold is reached when enough immune individuals break transmission chains, protecting unvaccinated individuals
Explanation
Herd immunity (community immunity) occurs when a sufficient proportion of the population is immune, such that transmission chains are broken even before reaching susceptible individuals. The herd immunity threshold (HIT) = 1 − (1/R₀). For measles with R₀ = 15, HIT = 1 − 1/15 = 93.3%. Achieving >95% measles vaccination coverage exceeds this threshold, providing indirect protection to unvaccinated individuals (neonates, immunocompromised persons) who cannot be vaccinated.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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