CLINICS 2011;66(7):1177-1182 two-tailed p,0.05 was considered to be statistically sig- nificant. All of the analyses were conducted using the Statistical Package for the Social Sciences (SPSS) for Windows, Version 13.0 (SPSS, Inc., Chicago, Illinois). RESULTS The socio-demographic data between the study and comparison cohorts are shown in Table 1. The female to male ratio was 9:1, and the mean age (¡SD) was 34.8¡14.3 years for the study cohort and 34.8¡15.0 years for the control cohort (p = 0.678). Approximately two-thirds of the patients were aged 18 to 44 years. The distributions of the insurance levels, urbanization levels, and geographic regions of residence were significantly different between these two cohorts. The co-morbid medical conditions and prescription medications of the two cohorts are shown in Table 2. The study cohort exhibited greater proportions of patients with lymphoma, leukemia, liver cancer, and renal disease. In addition, greater proportions of patients were found to receive medication for systemic corticosteroids, azathiopr- ine, cyclophosphamide, methotrexate, cyclosporin, and hydroxychloroquine compared to the control cohort. The study cohort had a significantly greater CCI; however, this cohort had a lower proportion of patients with diabetes mellitus. Among the SLE patients, age was found to be negatively correlated with average daily corticosteroid dose (Pearson correlation coefficient -0.118, p,0.001), and age was found to be positively correlated with CCI (Pearson correlation coefficient 0.106, p,0.001) (data not shown). Thus, the Table 1 - Demographic data of patients with systemic lupus erythematosus (SLE) and of the control cohort. SLE patients (n = 10337) Non-SLE Patients (n = 62022) Variable Gender Male Female Age groups, years ,18 18-24 25-34 35-44 45-54 55-64 .64 Urbanization level 1 2 3 4 5 Geographic region Northern Central Southern Eastern Insurance level (NT$) 0-6360 6361-15500 15501-20300 .20300 1052 (10.2) 9285 (89.8) 955 (9.3) 1844 (17.8) 2724 (26.4) 2342 (22.7) 1438 (13.9) 650 (6.3) 384 (3.7) 3824 (37.0) 3444 (33.3) 1497 (14.5) 921 (8.9) 651 (6.3) 4943 (47.8) 2745 (26.6) 2524 (24.4) 125 (1.2) 3047 (29.5) 2319 (22.4) 2583 (25.0) 2388 (23.1) 6312 (10.2) 55710 (89.8) 5730 (9.3) 11064 (17.8) 16344 (26.4) 14052 (22.7) 8628 (13.9) 3900 (6.3) 2304 (3.7) 25220 (40.7) 20612 (33.2) 7929 (12.8) 4911 (7.9) 3350 (5.4) 33484 (54.0) 13889 (22.4) 13783 (22.2) 866 (1.4) 15307 (24.2) 15782 (25.4) 15437 (24.9) 15766 (25.4) p-value 1.000 1.000 ,0.001 ,0.001 0.867 ,0.001 0.001 ,0.001 ,0.001 ,0.001 ,0.001 ,0.001 0.130 ,0.001 ,0.001 ,0.001 0.830 ,0.001 1179 Risk of herpes zoster in SLE Chen HH et al. younger groups of SLE patients were administered larger average daily doses of corticosteroids (Table 3). The incidence rates with a 95% CI for HZ were 37.7 (35.5- 40.0) cases per 1000 patient-years in the SLE patients and 5.1 (4.8-5.4) cases per 1000 patient-years in the non-SLE controls (Table 4). In the controls, the age-specific incidence rates were higher in the older groups, irrespective of gender. In contrast, among the SLE patients, the incidence rates peaked in the youngest group (,18 years), regardless of gender. Patients with SLE had a significantly greater risk of HZ than the patients in the control cohort (RR 7.37, 95% CI 6.75- 8.04, p,0.001) (Table 5). After adjusting for the possible confounding factors that are shown in Tables 1 and 2, the SLE patients had a significantly greater risk of HZ than the control cohort (RR 2.45, 95% CI 1.77-3.40, p,0.001). After stratifying by age, the adjusted RRs for HZ in the SLE patients compared with those of the non-SLE controls reached statistical significance in most of the age groups and peaked in those aged 18 to 24 years. Stratified by gender, the male SLE patients exhibited larger RRs for HZ than the female SLE patients (RR 7.51, 95% CI 2.89-19.52 vs. RR 2.10, 95% CI 1.48-2.99). A log-rank test revealed that the SLE patients had a significantly lower risk of three-year HZ-free survival rates than the non-SLE controls (p,0.001). The duration between the index date and the date of HZ onset for the SLE patients ranged from one day to 1094 days, and the median and mean (¡ SD) duration were 436 days and 476¡331 days, respectively. The survival curves for the two cohorts, which were determined using the Kaplan-Meier survival analysis, are illustrated in Figure 1. DISCUSSION The present study has determined the incidence occur- rences for HZ and the risk ratios for HZ among patients with SLE compared with those without SLE using nation- wide population-based data. This study is the largest and first nationwide population-based cohort study regarding HZ infections in SLE patients. The incidence rate for HZ among the non-SLE patients (5.1 cases per 1000 patient- years) is consistent with those among the normal population that were reported in previous studies (1.2 to 4.9 cases per 1000 person-years).6-7 However, the incidence rate for HZ among the SLE patients in the present study (37.7 cases per 1000 patient-years) was larger than those among the SLE patients in previous cohort studies (16-22 cases per 1000 patient-years).2-3,8 Using an administrative database instead of reviewing medical records prevents underestimation. The adjusted RR for HZ in the SLE cohort was 2.45, which is in agreement with a previous study in Taiwan that reported that patients with HZ had a rate ratio of 2.12 for co-morbid SLE compared with that of controls.7 However, this result was further adjusted for the presence of prescription medication and the CCI, which can minimize the potential confounding effects of concomitant medications and other co-morbidities. Previous studies have indicated that the incidence of HZ infection increases with age in the general population.7,13 The risk of HZ is greatest in subjects who are over 80 years of age and lowest in those who are younger than 20 years of age.13 In addition, the annual risk of HZ infection increases significantly in the geriatric population, with approximately two-thirds of individuals who are over 60 years developing Values are the number of patients (percentage), unless otherwise indicated.