Vaccine Effectiveness and Clinical Characteristics of Breakthrough Varicella During Outbreaks Among Students in Qingdao, China

OBJECTIVE: In the context of varicella outbreaks occurring in schools, this study focused on the efficacy of the varicella vaccine and the severity of the illness. METHODS: Our study conducted from 2017 to 2023 in 11 schools collected data using questionnaires on student demographics, clinical presentation, varicella history and immunization status completed by parents or teachers. Students with varicella were in the case group, whereas healthy classmates comprised the control. Conditional logistic regression analysis was utilized to determine vaccine effectiveness (VE). RESULTS: Of the 1947 students, 23.5% were unvaccinated, 53.4% had received 1 dose of the vaccine, and 23.1% had received 2 doses. A total of 123 varicella cases were reported, and 71 were breakthrough cases. The median interval since last vaccination was 9 years after 1 dose and 4 years after 2. The adjusted VEs of 1 and 2 doses in the outbreak case-control study were 67% and 74%. The VE difference between 1 and 2 doses was not significant ( P = 0.376). The number of rashes differed by vaccine status ( P < 0.001), and the minority breakthrough cases had >50 rashes. The fever duration was significantly lower in breakthrough cases than in unvaccinated cases ( P < 0.05). For other clinical manifestations, varicella cases who received a single dose versus those who received 2 doses did not exhibit any difference. CONCLUSIONS: Varicella outbreaks were still recorded in Qingdao because of insufficient immunization rates. The vaccination coverage may be expanded to enhance VE.