When I was a child, nearly everybody became ill with chickenpox. Like nearly all kids, when I became ill with it, I stayed home from school about a week and fully recovered.
All that changed in 1995, when the FDA licensed and approved the live attenuated chickenpox (varicella) vaccine in persons aged >12 months. After the vaccine began to be used by most children, the incidence of chickenpox rapidly declined. However, due to continual outbreaks of chickenpox, a second dose of the chickenpox vaccine was added to the childhood immunization schedule in 2006.
Is the chickenpox vaccine effective at significantly lowering the incidence of chickenpox? Yes. Due to the vaccine, there is a significantly lowered incidence of chickenpox.