“Herd immunity is the protection of an individual from infection by virtue of the other members of the population being incapable of transmitting the virus to that individual. Herd immunity can be achieved by immunizing a population with a vaccine that interrupts transmission, such as the live, attenuated polio vaccine, but not with a vaccine that does not interrupt transmission, such as the killed polio vaccine.”

Review of Medical Microbiology and Immunology, 13th Edition. by Warren Levinson. Page 259.