Vaccination reduces the spread of disease in a community by

• A. Reducing the number of susceptible individuals and breaking chains of transmission, leading to herd immunity.
• B. Only protecting those who receive the vaccine.
• C. Increasing pathogen virulence to prompt quicker immunity.
• D. Eliminating all pathogens from the population.

Answer: (A)

Vaccination reduces the spread of disease by lowering the number of susceptible individuals and breaking transmission chains, which creates herd immunity. When people gain immunity through vaccination, they cannot be infected and cannot pass the pathogen on, so transmission is disrupted. As more people become immune, the chance that an infectious person meets a susceptible person drops, eventually preventing chains of transmission from continuing. This not only protects those who are vaccinated but also helps protect those who cannot be vaccinated or have weaker immune responses—this collective protection is herd immunity. The idea can be seen in how the average number of people an infected person would infect (the effective reproduction number) drops below 1, making sustained spread unlikely.

Other choices don’t fit because vaccines don’t only protect those who receive them; they reduce spread through community protection. Increasing pathogen virulence is not how vaccines work. And eliminating all pathogens from the population is not a realistic outcome of vaccination.