Herd immunity refers to what concept?

• A. Group resistance to invasion and spread of an infectious agent based on the immunity of a high proportion within the group
• B. Immunity of a single individual
• C. Immunity conferred only by vaccines
• D. Herd immunity is unrelated to proportion immune

Answer: (A)

Herd immunity describes a situation where a large portion of a population is immune to a contagious disease, which protects the entire group by reducing transmission. When many people are immune, the pathogen has fewer opportunities to spread, so even those who aren’t immune gain indirect protection. The important part is the high proportion of immune individuals; once that threshold is reached, transmission chains are broken more readily. The exact threshold depends on how easily the disease spreads and how effective the immunity is, so highly contagious diseases require a larger immune proportion to achieve protection. Immunity can come from vaccination or prior infection, and it’s the collective level of immunity that matters, not just the immunity of an individual.