Herd immunity is a concept by which an infectious agent can no longer spread across a population because a sufficiently large percentage of individuals have immunity, either from a history of exposure to the wild-type of the infectious agent, or due to acquired immunity from vaccination with a vaccine designed to protect against the wild-type. From an individual perspective, herd immunity can afford protection to one who is not immune.
This type of immunity exists within populations to varying degrees depending on the number of individuals who are immune to the infectious agent. Other factors that influence the spread of pathogens include attributes of the infectious agent, attributes of the vaccine, the route, mode and rate of transmission of the infectious agent, the lethality of the infectious agent, and the diversity of types of the specific pathogen for which immunity is sought.
Regardless of whether immunity is achieved via a vaccine, or via natural infection, the total percentage of individuals who are immune can have an effect of slowing down the rate of spread of infection via natural transmission. Herd immunity is sought, in part, due to concern over individuals who are immunocompromised because they are more likely to contract a circulating pathogen in a population in which the infectious agent is relatively free to spread. It is generally thought that there is an inverse relationship between the percentage of a population that is immune, and the chance that a person who is not immune will come into contact with an individual who can transmit the virus.
Either through natural immunity or through vaccination, sufficient herd immunity can, in principle, bring the rate of transmission of an infectious agent to zero, with immune individuals collectively acting as a barrier to the spread of the infectious agent, until it “burns out.” Unless there is a natural reservoir, herd immunity can be a factor that can contribute to the eradication of infectious agents.