The definition adopted here is a "quantitative" one, i.e. it refers to herd immunity as a phenomenon that leads to the reduction of transmission of an infection in a population.

The term is sometimes used in a "qualitative" sense, i.e. it indicates the presence of sufficient immune individuals in a population (above a specified threshold) which leads to the disappearance of the infection.

The former use of the term is broader. The latter reflects the development of the concept of herd immunity in the context of research on disease elimination through vaccination programmes.