Herd immunity is a theory stating that once a certain proportion of a population are immune to a disease, it is no longer possible for transmission to occur.
This theory is employed in vaccination campaigns. It is not possible to vaccinate every individual – some may have medical complications, others may have objections for ‘personal reasons’. However, if a large enough proportion of a population is vaccinated (between 80 and 95%), transmission between hosts becomes challenging.
Herd immunity is employed with measles vaccination, preventing spread between hosts even where some are not vaccinated.