 Healthy BCG naïve adults in the UK have been selected for this study because of their low baseline level of anti-mycobacterial immunity and therefore reduced ability to suppress BCG growth. Whilst the target population for the heterologous effects of BCG vaccination is infants, the blood volume required in order to optimise the GIA would not be possible to collect from infants. Therefore by undertaking this work in healthy BCG naïve UK adults we can obtain the blood volumes required for this exploratory work in a population of individuals with a similar background mycobacterial exposure to infants in TB high burden, low income countries. 