Hospitalized children with influenza virus: a 17 year-long observational study including the 2009 H1N1 influenza pandemic and COVID-19 pandemic

BACKGROUND: Influenza viruses (IFV) have major impacts on children's health. We studied IFV A/B epidemiology and disease severity during a 17 year-long period including the 2009 H1N1 influenza pandemic and post-COVID-19 in 2021-2023. METHODS: Nasopharyngeal samples from children referred with fever or respiratory symptoms to a Norwegian hospital from 2006 to 2023 were tested with PCR for IFV A/B and 17 other pathogens. We compared IFV hospitalization rates and disease severity before and during the 2009 influenza pandemic, and before and after the COVID-19 pandemic, respectively, using age-adjusted logistic regression analyses. RESULTS: From 2006 to 2023, 502 children with IFV A/B were included (IFV A 74%, IFV B 26%). Two-hundred-fifty-one (50%) were hospitalized > 24 h. During the 2009 H1N1 influenza pandemic rates of hospitalization > 24 h in children ≤ 5 and > 5 years of age increased from 44 to 72, and 4 to 20 per 100,000, respectively, compared to 2006-2009. The median age of IFV positive children doubled from 23 to 49 months (p=.006), but no other clinical changes occurred. Post-COVID-19 (2021-2023), the hospitalization rates only increased in children > 5 years from 12 to 25 per 100,000, and the median age almost doubled from 33 to 62 months (p