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How did herd immunity contribute to the eradication of smallpox?

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Immune stimulation by activated helper T cells — immune system Stimulation of immune response by activated helper T cells. Activated by complex interaction with molecules on the surface of a macrophage or some other antigen-presenting cell, a helper T cell proliferates into two general subtypes, T_H1 and T_H2. These in turn stimulate the complex pathways of the cell-mediated immune response and the humoral immune response, respectively.

Herd immunity played a critical role in the eradication of smallpox by ensuring that a sufficient proportion of the population was protected, leaving the causative virus with too few susceptible hosts to sustain its spread.1

This was achieved through massive global vaccination campaigns coordinated by the World Health Organization. A key strategy in this effort was ring vaccination, where public health officials identified and isolated infected individuals and then vaccinated their close contacts—as well as the contacts of those contacts—to create a protective "ring" of immune individuals around each case.12

Because smallpox had no nonhuman reservoir and infected persons were generally only contagious after symptoms appeared, this strategy effectively interrupted chains of transmission. By successfully reducing the density of susceptible individuals, the global community achieved the threshold of herd immunity necessary to eliminate the disease, leading to its official declaration of eradication in 1980.21