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

## TL;DR

A 17-year study in Norway found that influenza hospitalization rates and patient age increased during the 2009 H1N1 pandemic and post-COVID-19 period, but disease severity remained unchanged.

## Contribution

This study provides a longitudinal analysis of influenza in children, including pandemic periods, with insights into age shifts and disease severity.

## Key findings

- Hospitalization rates for influenza increased during the 2009 H1N1 pandemic and post-COVID-19 period.
- Post-COVID-19, children hospitalized with influenza had milder disease and fewer severe complications.
- The median age of influenza-positive children doubled during the 2009 pandemic and post-COVID-19 period.

## Abstract

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.

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.

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<.001) compared to 2010–2020. Post-COVID-19, fewer children had lower respiratory tract infections (aOR 0.32, 95% CI 0.20–0.53) and severe complications (aOR 0.24, 95% CI 0.07–0.82).

Compared to other influenza seasons, disease severity during the 2009 H1N1 influenza pandemic was unchanged, but post-COVID-19 children referred to hospital with IFV A/B had milder disease.

The online version contains supplementary material available at 10.1186/s12879-026-12818-5.