Varicella outbreak among primary school students--Beijing, China, 2004.

INTRODUCTION

On June 9, 2004, a varicella outbreak was reported in a Beijing primary school affecting approximately 80% of children in one preschool classroom. An outbreak investigation was initiated to identify factors contributing to the high rate of transmission and to assess the effectiveness of control measures.

METHODS

A varicella case was defined as onset of a generalized, vesicular pruritic rash lasting >4 days in a student at the school during January 1-June 26, 2004. Parents of all students in the four lowest grades (K-2) were questioned concerning varicella illness before January 1, 2004. Exposure and vaccination histories of 111 ill students and 120 control students with no history of varicella were compared.

RESULTS

During January 1-June 26, 2004, of 1,407 students, 138 (9.8%) had varicella; 488 (35%) K-2 students had no history of varicella before the outbreak. In five classrooms in which attack rates (ARs) were high (>40%), a primary-case student had remained in school 2 days while ill with a rash. The secondary attack rate (SAR) in these classrooms was 21%, compared with 1.7% in classrooms in which the first ill student was sent home immediately (risk ratio [RR] = 10; 95% confidence interval [CI] = 3.7-29.0). A total of 111 (70%) ill students rode the school bus daily, compared with 120 (33%) control students (odds ratio [OR] =4.9; CI = 2.7-9.0). A total of 73 (33%) ill students had a history of varicella vaccination before January 1, 2004, compared with 32 (69%) control-students (OR = 0.22; CI = 0.08-0.59).

CONCLUSION

Students who were not excluded from school on the first day of rash were key contributors to the spread of varicella in their classrooms. High susceptibility to varicella at school entry indicates that vaccination of susceptible students might be the only effective measure to control this recurrent problem.