A second dose of chicken pox vaccine should be given to all children at ages 4 to 6 years old. This is the recommendation of the Advisory committee on Immunization Practices of the Centers for Disease Control and Prevention.
The second dose of the vaccine can be given earlier than 4 years of age provided at least 3 months have passed between the two doses.
The first dose in recommended between 12 to 15 months old.
Pediatric News, August 2006, Vol. 40 No 8, front page