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.