What is herd immunity, and what vaccination coverage threshold is typically needed for measles to prevent sustained transmission?

• A. Herd immunity means everyone must be vaccinated; threshold 60%
• B. Herd immunity means only unvaccinated are protected; threshold 70%
• C. Herd immunity occurs when a sufficient proportion is immune, reducing transmission; measles threshold 90-95%
• D. Herd immunity requires 100% vaccination; threshold 100%

Answer: (C)

Herd immunity happens when enough people in a population are immune—through vaccination or prior infection—so disease transmission is unlikely to sustain itself. This protects those who aren’t immune because the chains of transmission are broken.

For measles, the needed level of immune individuals is very high, about 90-95%. Measles is extremely contagious (high R0), so only when most people are immune can outbreaks be prevented and transmission halted. Vaccination coverage around this range reduces the chances of sustained spread and protects the whole community, including those who can’t be vaccinated or don’t have full protection.

The other statements misstate the idea or propose unrealistically low (or impossible) thresholds; herd immunity isn’t that everyone must be vaccinated, nor that only the unvaccinated are protected, and it doesn’t require 100% vaccination.