The live attenuated Bacillus Calmette-Guerin (BCG) vaccine protects against extrapulmonary infection with Mycobacterium tuberculosis and against leprosy [1, 2]. It has been shown that vaccination with BCG also leads to non-specific protective effects: early administration of BCG vaccination could lead to reduced child mortality, mainly as a result of reduced neonatal sepsis, respiratory infection, and fever. [3-5] Furthermore, BCG has also been used in patients with bladder cancer, to induce an improved reaction of the immune system, which prevents tumor progression and dead. [6] Recently, it was shown that BCG vaccination can induce epigenetic changes in human monocytes, which result in higher pro-inflammatory cytokines response upon restimulation with non-related pathogens in vitro. [7] The observed effects are proposed to be due to modulation of the human innate immune system, in a process called 'trained immunity'. Upon stimulation with a pathogen, the innate immune system becomes primed and is able to react faster and more efficient to a secondary (and non-related) stimulus. [7, 8]