Effectiveness
According to the American Academy of Pediatrics, clinical trials have demonstrated that varicella vaccine is 70% to 90% effective for preventing varicella and more than 95% effective for preventing severe varicella.
Furthermore, follow-up evaluations took place in the United States of children immunized that revealed protection for at least 11 years. Also, studies were conducted in Japan which indicated protection for at least 20 years.
People who do not develop enough protection when they get the vaccine may develop a mild case of the disease when in close contact with a person with chickenpox. In these cases, people show very little sign of illness. This has been the case of children who get the vaccine in their early childhood and later have contact with children with chickenpox. Some of these children may develop a mild chickenpox also known as breakthrough disease.
Read more about this topic: Varicella Vaccine