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BackgroundNorwegian health authorities do not recommend universal pediatric vaccination against seasonal influenza. We aimed to estimate the incidence of influenza by age and underlying medical conditions in hospitalized Norwegian children aged

ObjectiveTo evaluate risk factors for severe disease in children under 59 months of age hospitalized with respiratory syncytial virus (RSV) infection.Study designWe prospectively enrolled 1,096 cases of laboratory confirmed RSV infection during three consecutive RSV seasons in 2015–2018. Potential risk factors for severe disease were retrieved through patient questionnaires and linkage to national health registries. Need for respiratory support (invasive ventilation, bi-level positive airway pressure, or continuous positive airway pressure), and length of stay exceeding 72 h were used as measures of disease severity. Associations were investigated using multivariable logistic regression analyses. Multiple imputation was used to avoid bias and inference induced by missing data.ResultsRisk factors associated with a need for respiratory support included age younger than 3 months of age [aOR: 6.73 (95% CI 2.71–16.7)], having siblings [aOR: 1.65 (95% CI 1.05–2.59)] and comorbidity [aOR: 2.40 (95% CI 1.35–4.24)]. The length of hospital stay >72 h was significantly associated with being younger than 3 months of age [aOR: 3.52 (95% CI 1.65–7.54)], having siblings [aOR: 1.45 (95% CI 1.01–2.08)], and comorbidity [aOR: 2.18 (95% CI 1.31–3.61)]. Sub-group analysis of children younger than 6 months of age confirmed the association between both young age and having siblings and the need for respiratory support.ConclusionIn a large cohort of children