As more people become vaccinated against COVID-19, the term “herd immunity” has emerged as a popular buzzword. The earliest use of the term can be traced back to a study examining bacterial transmission between a “herd” of mice in 1923. It was discovered that herd immunity occurs when a significant portion of a population becomes immune to an infectious disease. The principle of herd immunity has been critical to eradicating certain diseases. For example, when the World Health Organization launched its campaign to eradicate smallpox in the 1960s, widespread vaccination was key to halting the spread of the disease.
By preventing the spread of disease within communities and protecting those who cannot be vaccinated, herd immunity disrupts chains of infection. Below we review what herd immunity is, how it is developed, and why it's important.
What is herd immunity?
In short, herd immunity or “community immunity” refers to a population’s level of immunity against an infectious disease. When a high portion of a population gains immunity to an infectious disease, like measles, the risk of transmission from person to person drastically declines. Whether it’s active or passive immunity, the presence of antibodies helps the body fight off infection. The key to community immunity is the limited likelihood of maintaining transmission because there are too few susceptible hosts— those who have been vaccinated or have already had the infection cannot contract and spread the virus. As a result of this ripple effect, unimmunized individuals are indirectly protected.
How is herd immunity achieved?
Herd immunity can be reached through two ways: natural infection and recovery or mass vaccination. Although community immunity can occur when enough people have developed protective antibodies after recovering from a disease, immunization creates immunity without causing illness or resulting complications.
It is also important to note that achieving herd immunity not only relies on vaccines or infection, the percentage of a population that has developed immunity also plays a significant role. Herd immunity threshold varies by disease and how contagious disease is. In some instances, herd immunity can be induced with as little as 40% of the population immunized. Conversely, the herd immunity threshold for a highly transmissible disease like measles is an estimated 95%.
Why is the ripple effect of herd immunity important?
Herd immunity is crucial because it uniquely offers widespread protection, including the most vulnerable members of our communities. For certain people, this includes infants, pregnant women, and other individuals who have medical conditions that prevents them from vaccination, herd immunity reduces risk of infection from infectious disease. Building community immunity also helps prevent outbreaks of vaccine preventable diseases.
While staying up to date with routine immunization can directly benefit individuals, herd immunity creates a ripple effect that positively impacts the most vulnerable members of society.