MVA85A is a promising new vaccine designed to prevent tuberculosis (TB) by dramatically boosting pre-existing responses to BCG, the only licensed vaccine against the disease at present. In BCG vaccinated individuals it induces a strong immune response. However little is known about the evolution of that response or of the kinetics of T-cells (the immune cells that respond to the vaccine) immediately following vaccination. Crucially, the outcome of this process may determine long term protection from disease.