When a person gets vaccinated against a disease, their risk of infection is also reduced – so they are far less likely to spread the disease to others. As more people in a community get vaccinated, fewer people remain vulnerable, and there is less possibility for passing the germ on from person to person. Lowering the possibility for a germ to circulate in the community protects those who cannot be vaccinated due to other serious health conditions. This is called “herd immunity.”
“Herd immunity” exists when a high percentage of the population is vaccinated, making it difficult for infectious diseases to spread, because there are not many people who can be infected. But herd immunity only works if most people are vaccinated.