The safety provided by a vaccine is unfortunately not binary. It triggers a response in most people, not all. Vaccines aim primarily to reduce the number of deaths, and then the number who become seriously ill. I don't know of any vaccine that can prevent replication of a virus in the body of a person who picked up an infection, and can thus transmit to other people
We simply don't have the fabled magic weapon that the press sometimes like to portray - we have only something that is pretty good, not 100% perfection. There will inevitably be a need to add other layers of defence to protect a population as a whole