Epidemiological characteristics of breakthrough varicella infection during varicella outbreaks in Shanghai, 2008-2014

The outbreaks of varicella occurring in kindergartens and schools are increasingly notified in Shanghai despite the implementation of one-dose varicella vaccination. We analyzed surveillance data on the notified outbreaks of varicella in Minhang District of Shanghai during 2008-2014. A total of 13 511 varicella cases and 154 outbreaks involving 1558 (11.5%) cases were reported. Annual attack rates of outbreak-associated varicella in outbreak classes were 5.5%-12%. The mean age of the outbreak-associated cases was 8.6 +/- 3.1 years. Among 1558 outbreak cases, 660 (42.4%) received one-dose varicella vaccine previously. The proportions of breakthrough varicella infection during outbreaks ranged from 21.5% in 2008 to 86.1% in 2014. Annual breakthrough infection rates in outbreak classes ranged from 5.4% to 7.4%. Breakthrough cases as index cases results in 9.7% of outbreaks, and the average duration of outbreaks was significantly longer in vaccinated cases as index cases than in unvaccinated cases as index cases (11.3 +/- 5.8 days vs. 8.6 +/- 6.1 days, P < 0.05). The mean time of breakthrough infection since vaccination was 6.2 +/- 2.3 years (range 0.6-13.4 years). One-dose varicella vaccination cannot prevent the varicella outbreaks in kindergartens and schools. A second dose of varicella vaccine should be recommended for children.