herd immunity threshold

Herd immunity threshold is the percentage of a population that needs to be immune to a contagious disease—either through vaccination or previous infection—to prevent its spread. When enough people are immune, the disease has fewer opportunities to pass from person to person, protecting those who are not immune. This threshold varies depending on how easily the disease spreads; for highly contagious illnesses, a higher percentage of immunity is needed. Achieving this level of immunity helps control or eliminate outbreaks, safeguarding the entire community, especially those who cannot be vaccinated.