Herd immunity: history, theory, practice.

Herd immunity has to do with the protection of populations from infection which is brought about by the presence of immune individuals. The concept has a special aura, in its implication of an extension of the protection imparted by an immunization program beyond vaccinated to unvaccinated individuals and in its apparent provision of a means to eliminate totally some infectious diseases. It is a recurrent theme in the medical literature and has been discussed frequently during the past decade. This new popularity comes as a consequence of several recent major achievements of vaccination programs, i.e.: the historic success of the global smallpox eradication program; dramatic increases in vaccination coverage stimulated by national programs and by the Expanded Programme on Immunization; the commitment of several countries to eradicate measles; and international dedication to eliminate neonatal tetanus and to eradicate poliomyelitis from the world by the year 2000.'