OBJECTIVETo evaluate the breakthrough varicella infection rate and varicella vaccine effectiveness (VE) among children who received 1-dose varicella vaccine.

METHODSA total of 57 180 subjects for the consecutive 4-year birth cohorts were selected from the local children born between 2007 and 2010 in Yinzhou District, Ninghai County and Yuyao City. And they were followed up for varicella from 2008 to 2013. The recipients of the vaccinations were identified through Ningbo Immunization Information System and data on breakthrough infections among the recipients were collected by using China Information System for Disease Control and Prevention. The breakthrough varicella rate and the VE were calculated and the trends of them were described from 2008 to 2013 among 4-year birth cohorts. The cumulative incidence of varicella was compared between vaccinated and unvaccinated children among the consecutive 4-year birth cohorts.

RESULTSThe rate of varicella vaccine coverage, vaccine cumulative incidence among the cohorts was 96.74% (55 317/57 180) and 0.56% (321/57 180). The breakthrough varicella infection for 4-year birth cohorts was 0.44% (244/55 317), and for each birth cohort was 0.95% (142/14 928), 0.44% (61/13 855), 0.22% (29/13 433) and 0.09% (12/13 101), respectively. It was on the rise from 2008 to 2013 and the 2007 birth cohort of it increased fastest from 0.04% (6/14 928) in 2007 to 0.32% (48/14 834) in 2013. The vaccine cumulative incidence of these who vaccinated 1-dose varicella (the breakthrough varicella infection) was lower than these who were unvaccinated (the incidence: 6.25% (37/592), 3.52% (15/426), 3.69% (17/461) and 2.08% (8/384)) by each birth cohort (χ²= 130.27, P < 0.001 for 2007 birth cohort; χ²= 74.11, P < 0.001 for 2008 birth cohort; χ²= 162.80, P < 0.001 for 2009 birth cohort; χ²= 100.01, P < 0.001 for 2010 birth cohort). The vaccine effectiveness for 4-year birth cohorts was 89.33% (95% CI: 86.7%-92.1%) and for each birth corhort was 84.78% (95% CI: 77.94%-89.50%), 86.82% (95% CI: 77.82%-92.95%), 93.99% (95% CI: 89.27%-96.81%) and 95.60% (95% CI: 89.18%-98.21%), respectively. The effectiveness of each birth cohort declinedgradually from 2008 to 2013 and the 2009 birth cohort of it decreased fastest from 98.86% in 2010 to 66.83% in 2013.

CONCLUSIONSThe 1-dose varicella vaccine effectiveness was good, but breakthrough varicella infection rate was on the rise with time and the VE declined gradually from 2008 to 2013.