(herd immunity) Immunity (via exposure to a pathogen or via vaccination) for a majority of the individuals in a community that serves to protect a na�ve 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.

(herd immunity) Protection occurring when so many people in a region are immune to an infectious disease that it can't spread to others.

The state of a community in which a significant proportion of its population becomes immune to an infectious disease, and the risk of person-to-person transmission decreases. Individuals who are not immune and susceptible are indirectly protected because ongoing disease spread is very small.

Herd immunity, state in which a large proportion of a population is able to repel an infectious disease, thereby limiting the extent to which the disease can spread from person to person. Herd immunity can be conferred through natural immunity, previous exposure to the disease, or vaccination. An...