A live virus attenuated vaccine was developed from the OKA strain (isolated from vesicles in the OKA child, 3 years of age, presenting with a benign varicella). The vaccine was completed after 11 propagation at 34 degrees C on HELF cells. The vaccine is a lyophilized preparation of a live attenuated virus, obtained by propagation of the virus in MRC-5 human diploid cell culture, titering between 1500 and 5000 cfu (colony forming unit per milliliter). This attenuated virus, contrary to the wild virus, is thermo-sensitive (39 degrees C). The rate of seroconversion after administration of one vaccine dose in healthy children, from 12 months to 12 years of age, ranges from 93% to 100% (Table 1). In controlled studies, the efficiency for the prevention of varicella in children reaches at least 85% with a 2-4 years follow-up. The prevention of severe forms is at least 95%.