Background: The COVID-19 pandemic substantially altered the epidemiology of respiratory infections. Its impact on the clinical course of influenza in hospitalised children remains insufficiently characterised. Objectives: We aimed to compare the clinical course, complications, and selected laboratory parameters of influenza in children before, during, and after the COVID-19 pandemic. Methods: This single-centre retrospective study included 553 children hospitalised with laboratory-confirmed influenza between September 2017 and August 2025. Patients were divided into three groups: pre-pandemic, pandemic, and post-pandemic. Clinical complications and inflammatory markers (CRP, PCT, neutrophil counts) were analysed. Results: Influenza-related complications occurred in 59.5% of patients and were significantly more frequent after the pandemic compared to the pre-pandemic period (64.3% vs. 52.9%, p = 0.02). Pneumonia was the most common complication across all groups, but its incidence was lowest during the pandemic. Myositis occurred most frequently during the pandemic and appears to coincide with a higher proportion of influenza B infections. No significant differences were observed in CRP, PCT concentrations, or neutropenia rates between groups. Conclusions: The COVID-19 pandemic influenced the clinical presentation of influenza in children, with a post-pandemic increase in complications. These findings may reflect delayed access to healthcare and the phenomenon of immunity debt, highlighting the need for continued surveillance and preventive strategies.

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.