Varicella outbreak trends in school settings during the voluntary single-dose vaccine era from 2006 to 2017 in Shanghai, China

Abstract

Objective: To investigate varicella outbreak trends among schoolchildren during the voluntary single-dose varicella vaccine (VarV) era in Shanghai, China. Methods: Trends in school varicella outbreaks from 2006 to 2017 were assessed using joinpoint regression models. The impacts of changes in single-dose VarV coverage among schoolchildren and implementation of post-exposure prophylaxis (PEP) strategies on outbreak trends were further analyzed. Results: In total, 265 varicella outbreaks involving 3263 cases were reported in Shanghai from 2006 to 2017. The number of outbreaks showed an increasing trend from 2006 to 2017 (t = 2.62, p = 0.026), especially in kindergartens. The proportion of breakthrough varicella cases among all outbreak-related cases showed an increasing trend from 30.4% in 2008 to 85.7% in 2017 (t = 7.45, p < 0.001). Single-dose VarV coverage among schoolchildren was 88.1%, and showed a significant increase from the 1996 to the 2008 birth cohorts, followed by a non-significant decline from the 2008 to the 2014 birth cohorts. During school outbreaks in which PEP campaigns were conducted, the varicella attack rate was significantly lower than those in outbreaks without PEP campaigns (1.2% vs. 1.4%; Chi-square = 23.35, p < 0.001). Conclusions: Even with high coverage, single-dose VarV is insufficient to prevent school outbreaks. The administration of VarV as PEP is an appropriate intervention for varicella outbreak control prior to implementing a two-dose VarV schedule. Keywords: Varicella, Outbreak, Varicella vaccine, Post-exposure prophylaxis, Breakthrough varicella