BCG vaccine

The BCG vaccine teaches the immune system to recognise TB by introducing a weakened cousin of the bacterium that causes disease in cattle. Like a dress rehearsal, it lets the body practise its defences against a harmless stand-in, so it can respond faster if it later meets the real organism.

Its clearest benefit is in babies and young children, where it substantially reduces the risk of the most dangerous forms of childhood TB — TB meningitis and miliary (bloodstream-spread) disease. For this reason it is given routinely soon after birth in many countries where TB is common, usually as a single injection into the skin of the upper arm.

Its protection has real limits. BCG is much less reliable at preventing ordinary lung TB in adults, and its effect wanes over time, so it does not stop TB transmission in the community. It can also cause a positive tuberculin skin test, complicating later screening, which is one reason blood-based tests are often preferred in vaccinated people.