Based on available clinical data, the underlying medical conditions were classified into nine groups: no underlying disorders, respiratory disorders, immunological disorders (including haematological malignancies), neurological disorders, intellectual disability (including Down syndrome), cardiovascular disorders, kidney and/ or liver pathology, other non-specified malignancies and metabolic disorders. Distinction was made between patients with single and multiple underlying disorders. Descriptive statistics were calculated for all available clinical and epidemiological characteristics. No statistical significant differences between the two seasons were found for these variables.