Herd immunity refers to which concept?

• A. You must vaccinate everyone
• B. Herd immunity refers to immunity only after recovery
• C. Herd immunity refers to majority of population immune such that transmission unlikely; not everyone needs vaccination
• D. Immunity is lifelong for all

Answer: (C)

Herd immunity occurs when a sufficient proportion of the population is immune to a contagious disease, either through vaccination or prior infection, so the effective number of new infections falls below what’s needed to sustain transmission. The exact fraction needed depends on how contagious the disease is—the more contagious, the higher the immune fraction required. Once this threshold is passed, transmission becomes unlikely even for some people who are not immune, so you don’t have to vaccinate everyone to protect the community.

This matches the idea that a majority being immune can interrupt spread, without requiring 100% vaccination. The other statements aren’t correct because vaccination isn’t limited to those who have recovered, immunity isn’t always lifelong for everyone, and there isn’t a necessity to vaccinate every single person for population protection.