Outbreak of varicella in highly vaccinated kindergartens.

[Objective] To investigate an outbreak of varicella in highly vaccinated kindergartens in Beijing. [Methods] Epidemiological analysis was made on the data from outbreak of varicella among 100% vaccinated kindergartens in Chaoyang District in 2007. [Results] ① 9 clinical cases were reported. 3 of these were moderate,the others were mild,and distributed in 2 classes on 2 floors,and significant differences were observed between the attack rates in the 2 classes. ② Varicella vaccination coverage rate of the exposure population was 100%. The median age at vaccination of vaccinated students was 15 months,and the median time since vaccination was 49 months. The vaccination interval time of 9 cases was among 3 to 4 years. The probability that varicella happening of people with vaccination interval ≥4 years was 2 times as those vaccinated ﹤4 years. ③ We detected antibody of varicella and found there were 4 cases with positive IgM antibody,and 46 cases with positive IgG antibody. Mean concentration of IgG antibody of cases was(2 000.00 ± 509.09) mIU /ml,and mean concentration of IgM antibody was(13.28 ± 24.31) U /ml. [Conclusion] ① Although breakthrough disease is mostly mild,varicella in vaccinated persons is contagious. ② 100% varicella vaccination coverage is not sufficient to prevent the outbreak. ③ In response to increasing reports of varicella outbreaks among highly vaccinated populations,we propose that,besides vaccination,isolation and exclusion are effective in preventing and controlling the epidemic situation of varicella,too. ④ There are two problems which we should keep on investigating on. The first one is how long the immunoprotection time of homemade live attenuated vaccine is. The second one is if we should launch two doses of preventive inoculation of varicella vaccine.