How does herd immunity slow the spread of an infectious disease?

Herd immunity occurs when a large proportion of a population becomes immune to an infectious disease, either through vaccination or previous exposure. This state slows the spread of disease by reducing the number of susceptible individuals, which minimizes the likelihood that an infected person will come into contact with someone who can contract and transmit the pathogen.12

By lowering the density of susceptible hosts, the disease lacks enough people to sustain its transmission, effectively protecting those who are not immune. The threshold required to achieve this varies by disease; highly contagious illnesses like measles require a higher percentage of immune individuals than less contagious ones.2

Mass immunization is a critical tool for reaching these thresholds, as it is often logistically impossible to vaccinate an entire population. When herd immunity is maintained, it can lead to the control or even eradication of diseases, such as the global elimination of smallpox. Conversely, declining vaccination rates can weaken herd immunity, leading to the resurgence of preventable diseases.2