TABLE 1.

Characteristics of the Children Selected Into Study Cohort, HMOs A and B, 1995–1999

CharacteristicHMO A (N = 80 584)HMO B (N = 8181)
Continuous variables
 Range of age of cohort12 mo–6 y12 mo–5 y
 Mean and range of follow-up time21.3 mo, 1 d–4.6 y14.7 mo, 1 d–3.9 y
 Total follow-up time142 673.85 person-y10 026.3 person-y
 Mean and range of age at varicella vaccination16.8 mo, 12–71 mo15.7 mo, 12–58 mo
Categorical variablesN (%)N (%)
 Females39 154 (48.6)4068 (49.7)
 Number of children with asthma15 208 (18.9)1062 (13.0)
 Number of breakthrough cases268 (0.3)97 (1.2)
 Inhaled steroid prescription 3 mo before varicella vaccination758 (0.9)0
 Breakthrough in 3 mo after inhaled steroid prescription*1843 (2.3)13 (0.2)
 Oral steroid prescription 3 mo before varicella vaccination1827 (2.3)157 (1.9)
 Breakthrough in 3 mo after oral steroid prescription*1770 (2.2)144 (1.8)
 Varicella vaccination before 15 mo of age35 826 (44.5)4789 (58.5)
 Received MMR80 038 (99.3)8139 (99.5)
 Varicella vaccination within 28 d after MMR546 (0.7)42 (0.5)
  • * For noncases, the exposure is calculated relative to the time equivalent to the occurrence of breakthrough in the matched case.