What is herd immunity?

Herd immunity refers to the situation when a significant portion of a population becomes immune to a particular disease, which reduces its spread and provides indirect protection to those who are not immune. This immunity can stem from previous infections or vaccinations, and the threshold for achieving herd immunity can vary depending on the pathogen. When enough individuals in a community are immune, the overall likelihood of disease transmission decreases, thereby protecting vulnerable members who may not be able to receive vaccines or who may not respond adequately to vaccination.

In the context of the other options, a complete absence of disease in a population does not accurately describe herd immunity, as herd immunity simply reduces disease incidence without eliminating it completely. Additionally, individual immunity from a vaccine is essential for contributing to herd immunity, but it does not encompass the collective aspect that is critical for the definition. Lastly, while government mandates for vaccinations can enhance herd immunity, they are not a defining characteristic of the concept itself. Thus, the essence of herd immunity is captured best by the idea that a large percentage of the population is immune, leading to broader community protection.