The chickenpox vaccination protects against the varicella zoster virus which causes chickenpox. Chickenpox is most commonly transmitted through direct contact from an infected person, and generally only starts showing signs between one to three weeks after infection. Chickenpox is highly contagious and is most common in children below the age of ten, in whom it usually causes mild symptoms. Adults, smokers, pregnant women and the immune suppressed can have much more severe reactions to chickenpox.
The vaccination has proven to be highly effective in children at over 95% protection while adult vaccination has achieved a 75% protection rate. Even if you contract chickenpox after the vaccine is administered it can severely reduce the symptoms.