Most vaccine-preventable diseases are spread from person to person. If one person in a community gets an infectious disease, he or she can spread it to others who are not immune. But a person who has been vaccinated is less likely to get that disease and spread it to others. This helps protect people who aren’t immunized, including those who can’t be vaccinated and those for whom vaccination was not successful. For very few people, underlying illnesses may affect their immune system response and in turn, they may need additional vaccination or protection. The more people who are vaccinated, the fewer opportunities a disease has to spread.

Source: Center for Disease Control and Prevention

If one or two cases of disease are introduced into a community where many people are not vaccinated, outbreaks will occur, like the outbreak of whooping cough at a Traverse City, Mich. school in 2014.

Depending on the disease and how easily it can spread, vaccination rates may need to be as high as 80 to 95 percent for community immunity to take place.

Learn more about community immunity and how vaccines protect everyone.

CDC: What Would Happen If We Stopped Vaccinations?
The College of Physicians of Philadelphia: History of Vaccines
Children’s Hospital of Philadelphia: Vaccines and the Immune System