In the context of a high basic reproduction number, which statement about induced herd immunity is correct?

• A. It must be low to allow transmission
• B. It can only be achieved through natural infection
• C. It is irrelevant to R0
• D. It must be high to effectively reduce spread

Answer: (D)

Herd immunity threshold is the portion of the population that must be immune to stop sustained transmission. It depends on the basic reproduction number: HIT = 1 − 1/R0. When R0 is large, this threshold is also large, meaning you need a big share of people immune to cut transmission.

With a high R0, the effective reproduction number is Re = R0 × (1 − p), where p is the fraction immune. To push Re below 1 and stop an outbreak, p must exceed 1 − 1/R0. For example, if R0 is 3, about 66–67% need to be immune; if R0 is 5, about 80% would need to be immune. That’s why induced herd immunity must be high to effectively reduce spread.

Immunity can be induced by vaccination or prior infection, but regardless of the route, achieving a high level of immunity is necessary when the pathogen’s R0 is large. The other statements don’t fit because herd immunity is directly related to R0, it can be achieved without only natural infection, and a low level of immunity would not sufficiently reduce transmission.