I'm not a medical person but from how it's been described to me and from what I've read, learnt over the past year or so, the vaccine enables the body to fight off any viral infection which in turn prevents the virus from reproducing, in the persons body, to a high enough load where it has the ability to be transferred to, and infect, other people. If most of the population is vaccinated then any virus transfer, if it occurs, is less and weaker so eventually the virus dies out, or only pops up occasionally. That's why most of us are still vaccinated for these virus's in our childhood and why these virus's are no longer prevalent in our communities.
From the web
"How long does protection from hepatitis B vaccine last? Studies indicate that immunologic memory remains intact for at least 30 years among healthy people who initiated hepatitis B vaccination at >6 months of age. The vaccine confers long-term protection against clinical illness and chronic hepatitis B virus infection.
"The chickenpox vaccine is a live vaccine and contains a small amount of weakened chickenpox-causing virus. The vaccine stimulates your immune system to produce antibodies that will help protect against chickenpox"
"Poliovirus vaccine is an active immunizing agent used to prevent poliomyelitis (polio). It works by causing your body to produce its own protection (antibodies) against the virus that causes polio"
