Most americans born before 1970 have a permanent scar on their shoulders from the smallpox vaccine. Why don't younger americans have one?

I believe the answer is: Smallpox has been eradicated by a worldwide vaccination program

Vaccines develop an immunity toward the virus. In order to survive, virus need to obtain a host so they could replicate. Due to the vaccine, the viruses cannot find these hosts. Because of this, smallpox virus started to gradually decreased and eventually eradicated.