Two doses of the Oka/Merck live attenuated varicella vaccine were tested for efficacy in a randomized, double-blind, placebo-controlled trial in healthy children aged 1–14 years; out of 956 enrolled children, 914 were seronegative and 468 received the vaccine, 446 placebo[2]. The vaccine caused few clinical reactions and was well tolerated; no clinical evidence of vaccine virus transmission from vaccinated children to siblings was found[2]. About eight weeks after vaccination, 94% of initially seronegative vaccinated children had detectable varicella antibodies[2]. During the nine-month follow-up period, there were 39 clinically diagnosed cases of varicella in the participants, of which 38 were laboratory-confirmed; all these cases occurred in the placebo group[2]. No vaccinated child developed varicella during the study period, indicating 100% vaccine efficacy against the disease in that population (P < 10^(-9))[2]. Thus, the main findings are high seroconversion (94%) and complete prevention of clinical varicella during 9 months of follow-up in this study[2].