In immunology, herd immunity refers to a situation in which a high percentage of a population is immune to a disease, essentially stopping the disease in its tracks because it cannot find new hosts. You may also hear this concept referred to as “community immunity.” The threshold for herd immunity varies, depending on the disease, with more virulent agents requiring vaccination of a higher percentage of the population to create the desired immunity. In addition to being used in disease prevention, community immunity is also utilized to fight ongoing outbreaks.
Most vaccination policies are focused on creating community immunity. Many countries require vaccinations in childhood, for example, protecting children from common diseases and ensuring that when these diseases enter the population, they cannot prey on children or adults, who have been previously vaccinated or exposed to the disease. The creation of herd immunity is especially important in crowded environments which facilitate the spread of disease, like schools.
Immunologists try to prevent the outbreak of diseases by creating widespread immunity, but they are not always successful. Sometimes a disease mutates or is entirely new, or a batch of vaccinations is faulty, or a large percentage of the population fails to get vaccinated, creating a situation in which an outbreak can occur, because much of the population is vulnerable. In the event of an outbreak of a major disease, agencies like the World Health Organization (WHO) can dispatch teams within days to figure out the cause of the outbreak and develop a vaccine, in the hopes of creating herd immunity to halt the outbreak in its tracks.
For some diseases, immunity thresholds are as low as 50%, especially when combined with good hygiene. In other instances, up to 90% of the population may need to be vaccinated to create the desired protection. It is also extremely important to receive regular boosters, as some vaccines lose their efficacy over time, leaving people vulnerable to an outbreak. Herd immunity led to the eradication of smallpox, and it explains why diseases such as polio and diphtheria are rare in developed nations with established vaccination policies.
The concept of herd immunity is often used to encourage reluctant parents to vaccinate their children. In addition to ensuring that their children are protected from fully preventable diseases, childhood vaccinations also benefit society at large by creating widespread protection from disease. Likewise, adults may be reminded to receive boosters to help protect their communities.