0 a situation in which a disease cannot spread through a group of people because enough of them are immune (= unable to get a disease), usually because of having been vaccinated (= given an injection):
One reason why immunization works is the herd immunity it gives.
We vaccinate the masses, causing herd immunity, to protect the relatively few who would otherwise become ill and suffer devastating consequences.
Herd immunity occurs when enough people in a population are immunized against a disease so that it also protects the unvaccinated.
