Herd immunity

The intention of vaccination is to prevent vaccine-preventable disease, but there are still some immunocompromised individuals in this world. The vaccinated community helps to protect those who are not vaccinated, a concept known as “herd immunity” or “community immunity.” When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease because there is little opportunity for an outbreak. The principle of herd immunity applies to control of a variety of contagious diseases, including influenza, measles, mumps, rotavirus, and pneumococcal disease. Simply put, when a person is vaccinated, they prevent disease from being spread to others in the community, including:

1)Babies too young to receive vaccines
2) Unvaccinated children and adults
3) Pregnant women
4) The elderly
5) Individuals with weakened immune systems, such as those with asthma, chronic illness, or undergoing treatment for cancer
6) Individuals who are allergic to vaccine components

When less than 90% of children are immunized in a particular community, these pockets of low vaccination create an environment where infectious diseases can take hold and spread. Herd immunity can be only acheive when it exceeds the herd immunity threshold, which means a higher percentage of people in the certain commuity must be vaccinated. 

References 

1) http://www.immunizeforgood.com

2) https://www.vaccines.gov/basics/protection/index.html