Assessment of a school exclusion policy during a chickenpox outbreak.

Two Ohio schools experienced an outbreak of over 200 cases of chickenpox during the period from October 5 to December 21, 1988, despite adherence to the 1986 American Academy of Pediatrics' recommendation that children be excluded from school for 1 week or until all lesions have crusted. In grades kindergarten through four, the attack rate for susceptibles was 51% (167/329). With the use of person-time analysis, classmates of a child with chickenpox in grades kindergarten through four were 3.6 times more likely to become a case 12-17 days (the range of one incubation period) after the last day the child with subsequent chickenpox was in class than at any other time during the 2.5-month study period (95% confidence interval (Cl) 2.4-5.4). This was even more pronounced during the first half of the outbreak (relative risk (RR), 10.8; 95% Cl 4.4-26.5). Cases were not more likely to aggregate 12-17 days after a child returned to school after having chickenpox (RR, 0.9; 95% Cl 0.5-1.5). No cases occurred in classmates 12-17 days after the 15 children absent less than 5 days returned to class. Because substantial chickenpox transmission may occur before rash onset, exclusion practices may have a limited effect on outbreak control and increase the indirect costs associated with chickenpox.