# The Impact of the COVID-19 Pandemic on the Epidemiology of Influenza in Hospitalised Children in the Years 2017–2025

## TL;DR

The study shows how the COVID-19 pandemic changed how often and when children were hospitalized with influenza in Poland from 2017 to 2025.

## Contribution

The study reveals how the pandemic altered influenza seasonality, virus types, and hospitalization trends in children.

## Key findings

- Hospitalizations were lowest during the pandemic (2021/22) and highest in the post-pandemic period (2024/25).
- Influenza A was most common but less so during the pandemic compared to before and after.
- The pandemic shifted influenza seasonality to a bimodal pattern, unlike the usual January–March peak.

## Abstract

Background: The COVID-19 pandemic significantly altered the circulation of respiratory viruses, including influenza. This study aimed to compare the epidemiology and clinical characteristics of paediatric influenza before, during, and after the pandemic. Methods: We retrospectively analysed 553 children aged 0–18 years hospitalised with laboratory-confirmed influenza at a paediatric infectious disease centre in Bydgoszcz, Poland, between September 2017 and August 2025. Patients were stratified into pre-pandemic (A), pandemic (B), and post-pandemic (C) periods. Epidemiological indicators, influenza type, age, sex, and hospital stay duration were assessed using χ2 and non-parametric tests. Results: Hospitalisations varied across seasons, lowest in 2021/22 (n = 18) and highest in 2024/25 (n = 175). Seasonal peaks occurred January–March in groups A and C, whereas group B showed a bimodal pattern in December and March–April. Influenza type A predominated in all periods, though less during the pandemic (56.7% vs. 89.2% pre-pandemic and 73.2% post-pandemic). Median hospital stay decreased from 5 days pre-pandemic to 4 days during and after the pandemic. None of the hospitalised children were vaccinated. Conclusions: The COVID-19 pandemic influenced influenza seasonality, virus type distribution, and hospitalisation patterns in children. Observed shifts highlight the importance of ongoing surveillance and targeted vaccination strategies to mitigate influenza burden in the post-pandemic period.