The varicella vaccine is a live but weakened form of the chickenpox (varicella-zoster) virus. Most health organizations recommend that children receive 2 doses of varicella vaccination at 12 to 15 months of age and 4 to 6 years old.
When a weak form of the virus is given in the form of a vaccination, the body's immune system reacts by destroying the virus and building up disease-fighting proteins (antibodies) to fight the virus. The antibodies to that virus stay in the body and will keep the person from becoming sick if exposed to the virus again. The person is then considered immune to chickenpox.
WebMD Medical Reference from Healthwise