Herd immunity and how it is achieved?

• A. It is protection of a population when a sufficient proportion is immune, reducing transmission; achieved through vaccination or natural infection.
• B. It is the protection provided only by vaccination.
• C. It is achieved only after an outbreak has ended.
• D. It is the immunity of individuals against all pathogens.

Answer: (A)

Herd immunity occurs when enough people in a population are immune to a particular infectious agent that the spread of that pathogen is unlikely, which protects those who are not immune. This protective effect arises because the number of susceptible hosts becomes too small to sustain transmission, so outbreaks are less likely or less severe.

Immunity can come from vaccination or from people who have recovered and gained natural immunity. Vaccination is the safer way to build this protection without illness, but natural infection can contribute to the overall level of immunity too. It isn’t confined to the end of an outbreak; it builds up as immunity rises in the community. And it isn’t about being immune to all pathogens—it's about reducing transmission of a specific pathogen at the population level.