herd immunity

Syllable breakdown

Dictionary meaning

the indirect protection from an infectious disease that occurs when a sufficiently large proportion of a population has become immune, either through prior infection or vaccination, so that the probability of transmission from an infectious to a susceptible individual falls below the threshold needed to sustain an epidemic; the herd immunity threshold varies by pathogen and depends on the basic reproduction number of the disease; for measles, which has an extremely high transmission rate, more than 95 percent of the population must be immune to prevent outbreaks, while diseases with lower transmissibility require lower thresholds; herd immunity through vaccination is the basis of successful elimination and eradication campaigns against diseases including smallpox and polio