Fewer people are dying

Prior to 1995, on average, 145 Americans under 50 died every year from chickenpox complications.

In 1995, the chickenpox vaccine was introduced, and now that number has dropped from 145 to 66.

The vaccine probably isn't economically worth it. That is, for the cost, you could probably save a lot more lives doing something else, like overhauling dangerous intersections.