Efficacy of high-titer live attenuated varicella vaccine in healthy young children.

The efficacy of a high-titer, reformulated varicella vaccine was studied in 513 10- to 30-month-old children. Vaccinees were randomly allocated to 5 groups to receive one of two lots of an original high-titer vaccine, one of two lots of a partially heat-inactivated vaccine, or placebo. Both vaccines were well tolerated. Seroconversion was detected in 100% and 99% of children immunized with the high- and low-titer vaccines, respectively. Sixty-five cases of serologically confirmed varicella-like disease were discovered during follow-up (mean, 29.3 months): 5 in the high-titer vaccine group, 19 in the low-titer vaccine group, and 41 in the placebo group (P < or = .005 for each difference). Thus, the protective efficacy of live attenuated varicella vaccine is dependent on vaccine titer. High-titer varicella vaccine induces excellent protection in healthy young children.