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

## TL;DR

The study found that the clinical course of influenza in hospitalized children changed after the COVID-19 pandemic, with more complications appearing post-pandemic.

## Contribution

The study is the first to show a post-pandemic increase in influenza complications in children, potentially linked to immunity debt and delayed healthcare access.

## Key findings

- Influenza-related complications were more frequent after the pandemic compared to the pre-pandemic period.
- Myositis occurred most frequently during the pandemic, coinciding with a higher proportion of influenza B infections.
- No significant differences were observed in inflammatory markers like CRP, PCT, or neutropenia rates between groups.

## Abstract

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.