Influenza epidemiology among hospitalized children in Stockholm, Sweden 1998-2014

Background: Influenza remains a common reason for the hospitalization of children. There is a need for long term studies that are also population based. We describe the epidemiology of severe influenza in a defined population 1998-2014. Method: Retrospective study of annually collected data of virologically confirmed influenza in hospitalized children 0-17 years living in the catchment area (230,000 children). We gathered information about comorbidity and complications from case records, and compared Influenza A, B and A(H1N1)pdm09 with respect to these factors. Results: A total of 922 children with influenza were hospitalized. The mean rate remained unchanged at 22.5-24.2 per 100,000 children per year. There were two major outbreaks: influenza A(H3N2) in 2003-2004 and the A(H1N1) pandemic in 2009-2010. The proportion of children with influenza B increased from 8% during the first half of the study period to 28% during the second half. The highest admission rate was found in children <3 months of age, 169 per 100,000. Children with influenza B were older than those with influenza A. Comorbidity was found in 34%, complications in 41%, and 11% needed intensive care management. The mortality rate was 0.17/100,000 children. Conclusion: Influenza remains an important reason for the hospitalization of children, especially during the first years of life. The increasing proportion of influenza B may have to be considered when recommending influenza vaccines. (C) 2016 Elsevier Ltd. All rights reserved.