Herd immunity

Herd immunity (also called herd effect, community immunity, population immunity, or social immunity) is a form of protection from infectious disease. This happens when a large percentage of a population has become immune to an infection, whether through previous infections or vaccination.
When many individuals have immunity, they do not spread the disease any more. This either stops or slows the spread of disease.
While not every single individual may be immune, the group as a whole has protection. This is because there are fewer high-risk people overall. The infection rates drop, and the disease peters out.
Herd immunity protects at-risk populations. These include babies and those whose immune systems are weak and can’t get resistance on their own.