1. Introduction Vaccines have saved hundreds of millions of lives and helped eradicate several diseases, especially since national programs for immunization were established and coordinated in the 1960s [1]. The current paradigm for vaccine efficacy is the induction of antigen-specific memory in adaptive immune cells, enabling long-lived protection. In addition, nonspecific effects of vaccination, also referred to as heterologous or pleiotropic effects, have opened new fields of research [2,3].
