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 serological profiles of influenza A in children aged 0–18 years before and after the COVID-19 pandemic to inform prevention and control strategies. Methods We conducted a 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 (33.33% in 2023) significantly exceeded pre-pandemic levels (13.66% in 2019), exhibiting winter-spring seasonality with bimodal peaks in February-March and December. School-aged children (≥ 6 years) demonstrated the highest post-pandemic positivity rate (38.86%). Hospitalization rates among influenza A-positive children inversely correlated with age: 16.67% in neonates (≤ 28 days) vs. 1.42% in infants (29d-3y) vs. 0.36% 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 demonstrated 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.