You are correct in that the most common chickenpox vaccine, VARIVAX, contains a weakened form of the chickenpox virus. Typically, if you catch the chickenpox virus in the wild it can lie dormant in the body and cause shingles later in life. However, the weakened form of the virus present in the vaccine is quickly cleared out by the immune system, and will not reach the viral load necessary to lead to shingles down the line. In fact, because 99% of children who receive the chickenpox vaccine will never get chickenpox, and the remaining 1% will only develop mild symptoms, this immunization is an effective preventative treatment for shingles. Hope this helps!