A live, attenuated varicella vaccine has been available in the United States for almost 10 years. Current guidelines recommend that a single dose be given to healthy children between 12 months and 12 years of age. The vaccine has been working well with minimal adverse events, but a two-dose regimen has been proposed as a more efficacious plan.

Kuter and associates performed a multicenter trial funded by the manufacturer of the vaccine, Merck, in which children were randomly assigned to receive one or two doses of varicella vaccine three months apart. Children were followed annually for history of varicella exposure, and blood tests were performed for varicella antibody.

More than 2,000 children were randomized to receive one or two doses of varicella vaccine. During the 10-year follow-up period, 60 children in the single-dose group and 17 in the two-dose group developed confirmed varicella more than 42 days after vaccination. The majority of the children who developed varicella after vaccination had mild disease and no infection-related complications. Calculation of vaccine efficacy resulted in rates of approximately 94.4 percent for one dose and 98.3 percent for two doses, a statistically significant difference. Both regimens prevented severe cases of varicella at 100 percent efficacy. The persistence of antibody over time was high in both groups.

The authors conclude that although a single dose of varicella vaccine significantly reduces varicella disease, two doses significantly increase the vaccine efficacy. This finding correlates with that of another study, which demonstrated significant boosting of the varicella immune response with a second injection given four to six years after the first vaccination. Further cost-effectiveness analyses are necessary to determine if a two-dose regimen should be widely recommended.