Safety Evaluation of Large-Scale Administration of a Novel Human Diploid (SV-1) Cell Line-Derived Varicella Attenuated Live Vaccine in Children 7–12 Years Old

<b>Objectives</b>: Varicella is a highly contagious viral disease affecting children. The SV-1 cell line-based varicella attenuated live vaccine (SV-1VarV) is the first vaccine produced using the human diploid SV-1 cell substrate. This study evaluated the real-world safety of SV-1VarV among school-aged children in Jiangsu Province, China. <b>Methods</b>: A retrospective descriptive study was conducted using data from the Jiangsu Provincial Immunization Program Information System and the Chinese National Adverse Event Following Immunization Information System (CNAEFIS). Children aged 7-12 years who received SV-1VarV between July 2024 and March 2025 were included. The incidence, clinical characteristics, and demographic patterns of Adverse Events Following Immunization (AEFI) were analyzed. Reporting rates were calculated per 100,000 doses. Statistical analyses included chi-square tests, Cochran-Armitage trend tests, and Poisson regression analyses (α = 0.05). <b>Results</b>: A total of 366 AEFI cases were reported following 1,096,117 administered doses (33.4/100,000 doses), of which 364 were adverse reactions (33.2/100,000). General reactions accounted for 97.8% (mainly fever and local reactions), and abnormal reactions accounted for 2.2% (0.73/100,000). No serious adverse events or vaccine quality-related events occurred. Adverse reaction reporting rates declined with increasing age (<i>p</i> < 0.001) and were higher in males than females (36.7 vs. 29.2/100,000; <i>p</i> = 0.001). Poisson regression indicated that older age was independently associated with a lower risk of adverse reaction reporting, whereas sex and dose number were not significantly associated. <b>Conclusions</b>: SV-1VarV demonstrated a favorable safety profile during large-scale use in children aged 7-12 years. Most reactions were mild, self-limiting, and consistent with expected post-vaccination responses. These findings provide robust real-world evidence supporting the continued and expanded use of SV-1VarV in school-aged children to optimize varicella immunization strategies.