To increase efficiency in the prevention of varicella, this study conducted in-depth research to unravel the reasons for outbreak of epidemic varicella among people who had received single dosage of varicella vaccine. There were 1377 students out of total 1450, who were inoculated with single dosage of varicella vaccine (94.97%). Statistical analysis of probability of getting varicella during outbreak epidemic varicella showed that 74 people (5.1%) had the disease. Out of the 74, there were 67 people who had been previously inoculated with a single dosage of varicella vaccine, indicating that there was significant correlation between vaccination and probability of getting varicella during outbreak period. This implied that vaccinated individuals still had high risk of getting the disease. The number of skin lesions was significantly different (t=-10.68, p<0.0001) between the different groups. Although single dosage of vaccine could not effectively protect people from varicella, those who had been inoculated with the vaccine lived higher quality lives than those who had never been inoculated. Thus, varicella-infected individuals vaccinated with a single dose of varicella vaccine had milder symptoms, indicating alleviation of the suffering of the patients.