What is the herd immunity threshold and how is it influenced by R0?

• A. Herd immunity threshold is fixed at a single percentage regardless of R0.
• B. The threshold is the proportion immune; higher R0 raises the needed threshold.
• C. Herd immunity occurs automatically without immunity in the population.
• D. Threshold is determined solely by vaccination coverage, not pathogen transmissibility.

Answer: (B)

The herd immunity threshold is the portion of the population that must be immune to stop sustained transmission. It depends on how contagious the pathogen is, captured by R0. When enough people are immune so that the effective reproduction number Re falls below 1, the outbreak can’t keep spreading. In the simplest case, the threshold is p_c = 1 - 1/R0. This means higher R0 requires a larger fraction immune. For example, if R0 is 2, about 50% need immunity; if R0 is 3, about 66.7%; if R0 is 5, about 80%. The idea isn’t a fixed percentage independent of R0; it rises with a more transmissible pathogen. Herd immunity isn’t automatic and comes from immunity in the population, not from nothing. Real-world factors like vaccine effectiveness and uneven mixing can shift the actual coverage needed, but the core relationship remains: higher R0 means a higher herd immunity threshold.