Herd immunity in a closed population reduces outbreak risk primarily by?

• A. Reducing transmission when enough are immune
• B. Lowering contact rates among nonimmune individuals
• C. Eliminating vaccination needs
• D. Increasing virulence of pathogen

Answer: (A)

Herd immunity reduces outbreak risk by breaking transmission chains through a sufficient level of immunity in the population. When enough people are immune, an infected person is unlikely to contact a susceptible person, so the effective reproduction number falls below 1 and the outbreak cannot sustain itself. In a closed population, there are no new susceptibles entering, so once transmission is interrupted, spread dies out. This is the mechanism behind herd immunity: the immune fraction lowers the overall chance that transmission occurs, not by eliminating vaccination needs, changing behavior of nonimmune individuals, or increasing the pathogen’s virulence. For example, if R0 is 3, about two-thirds need immunity to reach the herd immunity threshold, making large outbreaks unlikely.