Mycobacterium tuberculosis (M.tb) is a pathogen with worldwide preponderance that infects humans and causes the transmissible disease tuberculosis (TB). An estimated one-third of the world's population is latently infected with M.tb, carrying a 10% lifetime risk of developing active life-threatening disease. In 2016, there were 10 million new cases worldwide and 1.7 million people died of TB. Co-infection with human immunodeficiency virus (HIV) greatly increases the risk of TB reactivation and death. Diagnosis is challenging and drug treatment is often harmful, costly and complex. For these reasons, it is essential to develop a more effective vaccine against TB.~An improved understanding of the nature of protective immunity in humans would significantly improve rational vaccine development. Whilst host immunity, particularly systemic adaptive immunity, has been well characterized in murine models, the understanding of the immunological events that occur in humans during acute infection is limited. In particular, the knowledge of human mucosal responses to M.tb. is limited. This is primarily due to the difficulties in studying early disease processes in the lung. Consequently, the majority of human studies have investigated immune responses ex-vivo in peripheral blood or after in-vitro infection of cell lines. A better understanding of the immune components that exist at the respiratory mucosal surfaces in humans could lead to interventions that prevent infection at the point of entry.~TB043 is a clinical challenge trial primarily to evaluate the safety of BCG challenge administered by the aerosol inhaled route in healthy, BCG naive (Group 1-7) as well as historically BCG vaccinated UK adults (Group 8) . The trial will also look to evaluate and compare the amount of BCG recovered from the lungs as various points after challenge, allowing investigation into the immune components at mucosal surfaces.
TB043 is a clinical challenge trial primarily to evaluate the safety of BCG challenge administered by the aerosol inhaled route in healthy, BCG naive (Group 1-7) as well as historically BCG vaccinated UK adults (Group 8) . The trial will also look to evaluate and compare the amount of BCG recovered from the lungs as various points after challenge.