Explain herd immunity and factors that influence it.

• A. People with immunity never spread disease; factors: seasonality.
• B. Herd immunity is only about animal populations; factors: farm density.
• C. Proportion immune reduces transmission; influenced by vaccine effectiveness, coverage, population mixing.
• D. It is achieved only with immunity in 100% of the population; factors: age.

Answer: (C)

Herd immunity is reached when enough people in a population are immune that the disease can no longer spread effectively, causing transmission to fall below the level needed to sustain outbreaks. The main idea is that the proportion of immune individuals reduces the chance that an infectious person will transmit to a susceptible one, lowering the effective reproduction number. The specific portion required depends on how contagious the disease is; more contagious diseases need a larger share immune. The statement captures the essential factors that shape this protection: how well the vaccine works (vaccine effectiveness), how many people are immune (coverage), and how people mix with each other (population mixing). If vaccine effectiveness isn’t perfect or immunity isn’t evenly distributed among groups, you can still achieve herd protection as long as the effective immune fraction is high enough to bring transmission under control.