[Review of the concept of herd immunity, in the context of COVID-19 epidemic and the development of vaccines].

The concept of herd immunity is briefly reviewed, showing that some popularized ideas do not correspond to the original concept. The relationship with the basic and effective reproductive numbers is established. It is pointed out that the threshold for the herd effect does not indicate the number of individuals that will be infected in an epidemic. The relationship with the effective vaccination threshold and its relationship with the effectiveness of the vaccine are established. The reducing effect of the herd immunity threshold produced by the heterogeneity of transmission and mixing in the population and the existence of isolated subpopulations are analyzed, which could be important and could explain the low levels of post-epidemic seroprevalence in some places helping to mitigate new outbreaks.