This vaccine is a live but weakened form of the chickenpox (varicella) virus. Most health experts recommend that all children 12 months of age and older who have not had chickenpox get one dose at 12 to 15 months and a second dose at 4 to 6 years old.

Adolescents and adults who are not already immune to the chickenpox virus need two doses at least 4 weeks apart.

When a weak form of the virus is injected, the body's immune system reacts. It destroys the virus and makes disease-fighting proteins (antibodies) to fight the virus. The antibodies to that virus stay in the body and will keep the person from getting sick or getting severe symptoms if exposed to the virus again. The person is then considered immune to chickenpox.