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Abstract: Using data from the Immunization Information System (IIS) and the Infectious Disease Reporting System (IDRS), we have described the epidemiology and analyzed the risk factors for breakthrough varicella in the Fengtai district of Beijing, China. From 2008 to 2011 the breakthrough varicella infection rate (BVR) was 0.65%, 0.85%, 1.08% and 1.56% respectively, thus demonstrating a distinct increasing tendency (P <0.001). We also compared the incidence of varicella-related illnesses between the breakthrough varicella cases and unvaccinated cases, and found that the severity of illness in the breakthrough cases was milder than that of unvaccinated cases. Moderate-to-severe cases (P =0.008, OR=0.676, 95% CI=0.505–0.904) were differently distributed in the breakthrough varicella cases and unvaccinated cases. In this study, we demonstrated that floating status (P =0.031, OR=1.96, 95% CI=1.06–3.62), contact history (P <0.001, OR=2.67, 95% CI=1.59–4.69), time since vaccination (P =0.006, OR=1.59, 95% CI=1.14–2.21), age at vaccination (P =0.010, OR=0.59, 95% CI=0.39–0.88) and combined vaccination (P =0.026, OR=3.99, 95% CI=1.18–13.54) were associated with the occurrence of breakthrough varicella, and among these, floating status is a novel risk factor. This study should provide useful information for the evaluation of the performance of varicella vaccination and prevention in PR China. [Copyright &y& Elsevier]