Herd immunity is related to what measure in a population?

• A. The average age of the population
• B. The mutation rate of the pathogen
• C. The proportion of resistant animals in the population
• D. The geographic distribution of cases

Answer: (C)

Herd immunity depends on the proportion of resistant (immune) individuals in the population. When a sufficient fraction is immune, the pathogen cannot spread efficiently, so transmission dies out or is greatly slowed. This is described by the effective reproduction number, R_eff = R0 × (proportion susceptible); herd immunity is reached when R_eff drops below 1, meaning the immune proportion exceeds 1 − 1/R0. For example, if R0 is 3, about two-thirds of the population would need to be immune to prevent sustained transmission. This measure is about immune proportion, not the average age, mutation rate, or geographic spread, though those factors can influence how easily immunity prevents spread. Vaccination or prior infection both contribute to the resistant fraction and help protect others in the population.