A retrospective epidemiological study was performed by using hospitalization discharge registers from the national information system for hospital data (CMBD, Conjunto Mínimo Básico de Datos), from the Spanish Ministry of Health. All the registers for the Canary Islands, with an avarage population of 2,033,201, reported from January 1st, 2005 through December 31st, 2014 were selected. The national information system for hospital data (Conjunto Mínimo Básico de Datos; CMBD) includes an estimated 98% of discharges in public hospitals, covering 99.5% of the Spanish population. [9,10,11].
The 9th International Classification of Diseases ICD-9-CM (CIE-9-MC)  codes for herpes zoster (053.0, 053.1, 053.10,053.11, 053.12, 053.13, 053.19, 053.2, 053.20, 053.21, 053.22,053.29, 053.7, 053.71, 053.79, 053.8, 053.9) were selected. All herpes zoster related hospitalizations in a) any diagnostic position and b) first diagnostic position were analyzed..
This data base has been extensively used in epidemiological studies evaluating the burden of disease and changes in epidemiology of different diseases like varicella, respiratory syncytial virus or anaphylaxis, among others [13,14,15].
For each case, information on age, sex, outcome (recovery, death…) and length of stay was obtained.
The unit of analyses was the hospital discharge. The annual hospitalization rate (annual number of hospital admissions per 100,000 population), average length of hospital stay (ALOS), mortality rate (annual number of deaths at hospital per 100,000 population) and case-fatality rate (annual number of deaths at hospital/annual number of hospital admissions; %) were calculated. Data from the age- specific annual municipal population registries (corrected by the CMBD coverage) were used as the denominator. It was assumed that the distribution by age of the population covered by this data base was equal to the distribution of the general population of the Canary Islands.
All analyses were stratified by age group: < 50 years, 50–54 years, 55–59 years, 60–64 years, 65–69 years, 70–74 years, 75–79 years, 80–84 years and ≥85 years. We also reported the crude and age-standardized (EU27 + EFTA 2013 standard population) total hospitalization, total mortality, and total case fatality rates. ANOVA or Kruskal Wallis tests were used to evaluate differences in means and distributions. Poisson regression was used to assess differences in the hospitalization rate over the study period.
In all tests the significance level used was p < 0.05. Statistical analyses were performed using the IBM Statistical Package for Social Sciences (IBM SPSS/PASW for windows, version 21.0, Chicago, IL, USA).
The patient information was anonymized and de-identified prior to the analysis. Local ethics committee (Comité de Ética de la Investigación de la Universidad Rey Juan Carlos) ruled that no formal ethics approval was required in this particular case.