 Since 1927, it has been observed that BCG-vaccinated neonates have lower all-cause mortality rates. This heterologous or non-specific effect within the first 6-12 months of life has been demonstrated in randomised and observational studies in low income countries with high childhood mortality rates. The most consistent effect is reduced neonatal mortality due to fewer cases of neonatal sepsis, respiratory infection and fever. The main limitation of these studies is the risk of confounding inherent in their cross sectional and observational designs. It is essential that we determine the cogency of this effect, as potential BCG replacement vaccines must be non-inferior to BCG in this regard. 