Complex sentence: Besides individual protection from getting ill, some vaccines policies aim also to provide herd immunity which is based on the idea that the pathogen will have trouble spreading when a significant part of the population has immunity against it.
Split into simpler sentences: Besides individual protection from getting ill, some vaccination policies also aim to provide the community as a whole with herd immunity. Herd immunity refers to the idea that the pathogen will have trouble spreading when a significant part of the population has immunity against it.