Chickenpox vaccine protects against infection from the varicella zoster virus, which causes chickenpox infection. The virus is easily spread by sneezing and coughing, or by contact with weeping chickenpox blisters. You can even catch the chickenpox virus from touching clothing or other objects that have fluid from the blister on them.
Note: There is another varicella vaccine called Zostavax®. This is to protect against shingles and is not given to children.
The chickenpox vaccine is a live vaccine which is made using chickenpox viruses that have been weakened (or attenuated), before being included in the vaccine. After vaccination, the weakened vaccine viruses replicate (grow) inside you. This means a very small dose of virus is given to stimulate a response by your immune system.
Live attenuated vaccines don't usually cause disease in vaccinated people who have a healthy immune system. When a live attenuated vaccine does cause any illness, it is usually milder than if you had caught the disease. Live attenuated vaccines given by injection are generally effective after one dose.
Chickenpox – Disease and Vaccine, New Zealand
(The Immunisation Advisory Centre, NZ)