BCG or Bacillus Calmette Guerin is a vaccine used for prevention of Tuberculosis(TB) in humans since 1921.Approximately 100 million children were administered (vaccinated) with BCG in almost 170 countries in the world.The BCG vaccine is prepared from a strain of attenuated,live tuberculosis bacilli' Mycobacterium Bovis. That has lost it's virulence in humans by being specially cultered in an artificial medium for years.The bacilli have retained enough strong antigenecity to become an effective Vaccine for the prevention of tuberculosis in human beings.

BCG Vaccine is one of the most widely used vaccines in the world. Its efficacy against TB meningitis in the pediatric age-group is established; but its efficacy against pulmonary TB is appears to be variable. Many countries use BCG for prevention of TB. But a few countries like U.S, they do not use BCG for routine vaccination. This is probably because they believe in detecting the active disease by a reliable Mantoux test, instead of practicing the routine vaccination against a relatively rare condition i.e, the incidence of TB is very low.

Efficacy:-The most controversial aspect of BCG is its variable efficacy when used in various clinical trials. This appears to depend on the geography. The efficacy of 14% to 84% upto 5 years after immunization. However a clinical trials conducted in South India showed no protective effect.

But BCG vaccine seems to be very effective in preventing miliary TB & TB meningitis. Hence it is still extensively used in countries where efficacy against pulmonacy TB is negligible.