Objective  To evaluate the effectiveness of emergency vaccination of varicella vaccine by comparing the attack rate and secondary attack rate of vericella in non-vaccination and vaccination groups.

  Methods  We collected the data on varicella incidence and emergency vaccination of varicella vaccine among kindergarten children, primary school pupils, and high school and vocational school students in Shanghai city between June 2013 and December 2015. We compared the secondary attack rate of varicella in non-vaccination and vaccination groups and analyzed the difference in the secondary attack rate among the reported varicella epidemics with different varicella incidence.

  Results  During the period, totally 1 684 varicella epidemics were reported, involving 1 591 569 exposed people and 1 563 540 people receiving emergency vaccination exposed to 1 619 epidemics. The secondary attack rate and the incidence rate by the end of the epidemic of the emergency vaccination groups was significantly lower than those of the non-vaccination groups. The incidence rate at the time of conduction of emergency vaccination was positively correlated with the secondary attack rate and the overall incidence rate by the end of the epidemic among the emergency vaccination groups (P = 0.002 and P < 0.01). The secondary attack rate and the overall incidence rate by the end of the epidemic among the groups receiving emergency vaccination after the identification of the first initial varicella case were both significantly lower than those among the groups receiving emergency vaccination after the identification of the first two initial varicella cases (χ² = 150.79, P < 0.01; χ² = 661.67, P < 0.01); while, the secondary attack rate and the overall incidence rate by the end of the epidemic among the groups receiving emergency vaccination after the identification of the first two or more initial varicella cases were both significantly lower than those among the groups not receiving emergency vaccination (χ² = 4.15, P = 0.04; χ² = 29.43, P < 0.01).

  Conclusion  Emergency vaccination of varicella vaccine as early as possible is highly effective in decreasing secondary attack rate and the overall incidence of a varicella epidemic; although the effectiveness is the best for the emergency vaccination conducted immediately after the first initial varicella case report, it is still effective for the emergency vaccination conducted after the report of the first two or more initial varicella cases.