susceptible population to a point where infections can't spread rapidly before infected individuals can be identified and quarantined if necessary. That is what "herd immunity" is. It's not everyone immune, it's the greater population being protected because infection rates can't skyrocket. The propagation rate is stonewalled by immune individuals before it can reach a sustained exponential growth rate. That allows outbreaks to be contained to a small, isolated, and easily traceable population. And the fewer people who get vaccinated, the less effective the vaccines will be for protecting everyone smart enough to get one. In a world where geographic boundaries are almost non-existent, everyone should get vaccinated for all communicable diseases with FDA approved vaccines.