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A total of 465 healthy infants and adolescents ages 12 months to 17 years without a known history of varicella or recent exposure to varicella-zoster virus VZV were immunized with live attenuated Oka/Merck varicella vaccine from November, 1984, through April, 1989. The vaccine administered was from 1 of 7 production lots containing from 950 to 3265 plaque-forming units and was well-tolerated with few side effects. The seroconversion rate for seronegative subjects was 94.6% (403 of 426). This varied by lot from 85% (950 plaque-forming units) to 100% (3010 and 3265 plaque-forming units). Breakthrough disease after exposure to varicella in seroconverters during 5 to 10 years of follow-up was 18.6% (75 of 403). The breakthrough disease was characterized by a maculopapular rash with a median of 35 lesions, most of which were macules. Breakthrough disease lasted a median of 5 days and the median temperature was 99°F; 65.3% (49 of 75) of subjects were afebrile and 2.7% (2 of 75) of subjects had temperatures of > 102.9°F. Varicella vaccine provides excellent (94.6%) seroconversion, and most children who developed breakthrough disease (18.6%) experienced a modified, milder form of illness than has been observed with natural varicella in unvaccinated subjects.