Herd immunity: recent uses in vaccine assessment.

Human communities defend themselves against specific infectious agents in a way that extends beyond the simple sum of the immune status of its individuals. By analogy with individual immunity to specific agents, the community level of immunity may vary from complete susceptibility to full protection. Herd immunity has been used to name this community property, which is the result of evolution through natural selection, leading to relationships between two species, typical of prey-predator systems. Varying uses of the term herd immunity led to the use of other expressions, such as herd protection, herd effect and community immunity. Knowledge derived from observational studies and models on herd immunity has supported decisions on the choice of vaccines and vaccination strategies for the benefit of populations. This knowledge is most likely to be extended in the future, with far-reaching effects.