Herd Immunity

**Semantic type:** Organ or Tissue Function

**Definition:** Immunity (via exposure to a pathogen or via vaccination) for a majority of the individuals in a community that serves to protect a naïve segment of that same population who cannot receive vaccination (contraindicated) and is therefore especially vulnerable to disease. The effect is described by the epidemiological principle that if immunity (via successful vaccination) was delivered at random and if members of a population mixed at random, such that on average each individual contacted R0 individuals in a manner sufficient to transmit the infection, then incidence of the infection would decline if the proportion immune exceeded (R0 - 1)/R0, or 1 -1/R0, where R0 is the basic reproductive number.