What is herd immunity?

Herd immunity is a concept that refers to the protection of a population from infectious diseases when a significant proportion of individuals within that population are immunized or have acquired immunity to the disease, either through vaccination or previous infection. This collective immunity decreases the overall amount of the pathogen that can spread among individuals, thereby providing indirect protection to those who are not immune, such as individuals who cannot be vaccinated for medical reasons.

When a large percentage of the population is immune, the chances of an outbreak occurring are reduced, as there are fewer susceptible hosts for the pathogen to infect. This is particularly important for protecting vulnerable populations like infants, elderly individuals, or those with compromised immune systems.

The other choices describe various aspects of immunity or vaccination but do not accurately capture the essence of herd immunity. For instance, the idea of vaccination of all individuals does not account for those unable to receive vaccines due to medical reasons. Similarly, immunity solely from natural infections does not provide the broad protection that vaccination can offer across a population. Lastly, the notion that disease transmission occurs only among vaccinated individuals misrepresents how herd immunity functions, as the goal is to reduce transmission overall in the community, not to isolate it to one group.