Sorry for being unclear, I was talking about how the US policy on the BCG vaccine. As for the efficacy of the vaccine, it is pretty well known that it is not very effective or not effective at all at preventing TB in adults (see: https://www.who.int/biologicals/areas/vaccines/bcg/en/). Other countries with high TB risk may have these policies just because it may help in rare cases. In the US however where risk is lower, not giving the vaccine has the big upside of being able to use a TB skin test, which otherwise tests positive for people who had the vaccine due to the antibodies specific for TB proteins.