What does the term "herd immunity" refer to?

The term "herd immunity" refers to the protection of a population from disease spread when a sufficient proportion of individuals are immune. This immunity can come from vaccination or previous infections. When a large portion of the population is immune, the spread of infectious diseases is reduced, ultimately protecting those who may be vulnerable and cannot be vaccinated, such as infants, elderly individuals, or those with certain health conditions.

The concept relies on the idea that diseases cannot spread easily within a community if enough people are immune, thereby creating an indirect protection effect for those who are not immune. This threshold varies depending on the disease and its contagiousness; for some, a higher percentage of immune individuals is necessary to establish herd immunity effectively.

In contrast, the other options focus on aspects such as vaccination trends, individual beliefs about vaccination, or immunity from previous infections, but do not capture the essential principle of community-level protection that herd immunity embodies.