Herd immunity is the resistance to an infectious agent of an entire group or community (and, in particular, protection of susceptible persons) as a result of a substantial proportion of the population being immune to the agent. Herd immunity is based on having a substantial number of immune persons, thereby reducing the likelihood that an infected person will come in contact with a susceptible one among human populations, also called community immunity.1
Vaccines provide active immunity to disease. Vaccines can trick your body into believing it has a disease, so it can fight the disease. Here is how a vaccination works:
The vaccine is administered. It contains antigens to a specific disease.
The immune system identifies the antigens in the vaccine as foreign invaders.
The immune system then develops antibodies to neutralize the antigens.
The immune system stores these antibodies for future use in case the person is ever exposed to the disease.
When a vaccine is given to a significant portion of the population, it protects those who receive the vaccine as well as those who cannot receive the vaccine. This concept is called “herd immunity.” When a high percentage of the population is vaccinated and immune to a disease, they do not get sick — so there is no one to spread the disease to others. This herd immunity protects the unvaccinated population from contagious (spread from person to person) diseases for which there is a vaccine.2
People who depend on herd immunity
Some people in the community rely on herd immunity to protect them. These groups are particularly vulnerable to disease, but often cannot safely receive vaccines:
People without a fully-working immune system, including those without a working spleen
People on chemotherapy treatment whose immune system is weakened
People with HIV
Newborn babies who are too young to be vaccinated
Many of those who are very ill in hospital
For these people, herd immunity is a vital way of protecting them against life-threatening disease.3