Herd immunity occurs when a significant portion of a population becomes immune to an infectious disease, either through vaccination or previous infection, reducing the spread of the disease. This concept is crucial for protecting vulnerable individuals who cannot be vaccinated, such as infants, elderly individuals, and those with compromised immune systems. When enough people are immune, the transmission of the pathogen is slowed or stopped, making it harder for the disease to spread within the community. Vaccination plays a critical role in achieving herd immunity, especially in the case of diseases with high transmission rates, such as measles or COVID-19.

The percentage of the population that needs to be vaccinated to achieve herd immunity varies depending on the disease's contagiousness. For example, diseases like measles require a higher vaccination rate to prevent outbreaks. Achieving herd immunity not only protects individuals but also helps to prevent epidemics and maintain public health safety, ensuring the overall well-being of the community.