Children hospitalized with Varicella in Denmark: Sensitivity of the National Patient Register

BACKGROUND: Varicella, common in childhood and most often self-limiting, may cause complications including bacterial superinfection, pneumonia and encephalitis. Universal childhood varicella vaccination has been introduced in several countries, but is controversial in Europe. In Denmark, varicella is not part of the national immunization program and there is no national surveillance of varicella. The primary aim of the study was to describe the epidemiology and clinical characteristics of children hospitalized with varicella in Denmark. The secondary aim was to validate the sensitivity and completeness of the Danish National Patient Register.

METHODS: Active surveillance of children hospitalized with varicella was carried out at four pediatric departments. In the Danish National Patient Register we identified all children discharged with an ICD10 code of varicella from the four departments. We used a capture-recapture analysis to estimate the "true" number of hospitalized children with varicella.

RESULTS: By active surveillance, we identified 86 children eligible for clinical description. In 87 % of cases, the children were 0-4 years of age. Complications were identified in 69 % of patients, including one child with post-varicella cerebral angiopathy. In NPR, we identified 125 children with a discharge diagnosis of varicella. By capture-recapture analysis, the sensitivity of the National Patient register was estimated to be 74 %.

CONCLUSIONS: Varicella can cause serious complications including cerebral angiopathy in children in Denmark. The national patient register will be a useful tool for estimating hospitalization incidence, but will underestimate the true number of hospitalizations.