Resurgence and clinical evolution of influenza A in Chinese children: shifting epidemiology and serological dynamics across pre-, intra-, and post-pandemic eras (2019–2023)

OBJECTIVE: This study aimed to analyze the epidemiological characteristics and hematologic and inflammatory markers of influenza A in children aged 0–18 years before and after the COVID-19 pandemic to guide future prevention and control strategies. METHODS: We conducted a single-center retrospective analysis of 238,494 children tested for influenza A at Shanghai Children’s Medical Center (2019.1-2023.12). Positivity rates and serological parameters were evaluated across age groups seasons, and years. RESULTS: During the pandemic period, influenza A positivity remained consistently low without seasonal peaks. Conversely, post-pandemic positivity (33.3% in 2023) markedly exceeded pre-pandemic levels (13.7% in 2019), exhibiting winter-spring seasonality with bimodal peaks in February-March and December. School-aged children (≥ 6 years) indicated the highest post-pandemic positivity rate (38.9%). Hospitalization rates among influenza A-positive children inversely correlated with age: 16.7% in neonates (≤ 28 days) vs. 1.4% in infants (29d-3y) vs. 0.4% in preschoolers (3-6y) vs. 0.33% in older children (≥ 6y). Children < 6 years predominated in severe diagnoses and comorbid conditions. CONCLUSIONS: This study delineates the evolving epidemiology of influenza A from 2019 to 2023. Post-pandemic resurgence suggested heightened positivity rates and prolonged seasonal activity exceeding pre-pandemic patterns. Young children and those with comorbidities exhibited greater disease severity requiring hospitalization. Enhanced pediatric influenza A prevention is imperative. CLINICAL TRIAL NUMBER: Not applicable. This study is a retrospective observational study and does not meet the definition of a clinical trial requiring registration.