You can protect people who were never vaccinated at all

When enough of a population is immune to a disease, its chains of transmission break down even for people who aren't immune, because an infected person is unlikely to meet anyone left to infect. The threshold depends on how contagious the disease is — measles, among the most contagious diseases known, needs roughly 95% coverage. It's why vaccination protects infants and others who can't be vaccinated themselves.

— P. Fine, K. Eames & D. L. Heymann, Herd Immunity: A Rough Guide — Clinical Infectious Diseases, 2011