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)

Health officials worry that herd immunity will be threatened as more parents choose not to vaccinate. 卫生官员担心，随着越来越多的父母选择不接种疫苗，群体免疫力将受到威胁。

With herd immunity, there aren't enough susceptibles in the population to sustain an epidemic. 有了群体免疫，人口中就没有足够的易感者来维持流行病。