Two doses of the varicella vaccine provide better protection than a single dose, according to a study funded in part by the National Center for Research Resources and the National Institute of Allergy and Infectious Diseases.
After the Centers for Disease Control and Prevention (CDC) began recommending in 1995 that children receive a single dose of the varicella vaccine, there has been a 90% decrease in the incidence of varicella and corresponding declines in morbidity and hospitalization. But outbreaks continued in schools and day care centers despite high vaccination rates. To combat such occurrences, the CDC recommended adding a second dose to the vaccine schedule in 2006, even though there was limited evidence on whether this would be more protective.
Kuehn BM. Two Doses of Varicella Vaccine. JAMA. 2011;305(8):770. doi:10.1001/jama.2011.183