Influenza A/H3N2 and Its Co-infection with Other Respiratory Pathogens: Higher Pneumonia Rates and Prolonged Hospital Stays in Pediatric Patients

Background: In pediatric populations, influenza viruses such as influenza B, influenza A/H1N1, and influenza A/H3N2 present unique clinical challenges due to their distinct characteristics. Co-infections with other respiratory pathogens can lead to more severe disease progression in children, although the specific impacts of these co-infections are not yet fully understood. Objectives: To explore the differences in clinical characteristics among children with single infections of influenza B, influenza A/H1N1, and influenza A/H3N2, and to assess the impact of co-infections with other respiratory pathogens on the severity of disease in children. Methods: This retrospective study evaluated the severity of pediatric influenza hospitalizations during 2022 - 2023 by correlating virus types/subtypes with clinical outcomes, including pneumonia incidence, mechanical ventilation requirements, PICU admissions, and duration of hospital stay. Results: The study included 1,380 pediatric patients with influenza: 343 with influenza A/H1N1, 678 with A/H3N2, and 359 with influenza B. In children aged six and older, influenza A/H3N2 infection resulted in higher pneumonia rates and longer hospital stays compared to influenza A/H1N1 and influenza B (P < 0.05). Laboratory result differences were also observed between single infections of influenza A and B in this age group. The co-infection rate for influenza A/H1N1 was 3.5%, significantly lower than that for H3N2 (11.9%) and influenza B (10.6%) (P < 0.05). Clinical differences were noted between single and co-infections of influenza A/H3N2 and B, with co-infections showing higher pneumonia rates and longer hospital stays compared to single infections (P < 0.05). Conclusions: In children aged six and older, influenza A/H3N2 is associated with higher rates of pneumonia and longer hospital stays. Co-infections involving Influenza B or A/H3N2 with additional respiratory pathogens further increase the risk of pneumonia and extend the duration of hospitalization.