The term herd immunity was first used in 1923, by researchers investigating bacterial infections in mice. The concept itself had been recognized much earlier, though its implications were not fully appreciated until widespread vaccination revealed that, for example, immunizing less than 90 percent of a population against diphtheria could reduce the incidence of disease by 99.99 percent. “That indirect protection occurs is obvious, both in logic and in observation,” the epidemiologist Paul Fine notes in his review of the literature on herd immunity, “Herd Immunity: History, Theory, Practice.”
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