Risk factors for severe lower respiratory tract infection among children under 5 years of age hospitalized with respiratory syncytial virus.

In 2023, a resurgence of acute lower respiratory tract infections (ALRTI) linked to respiratory syncytial virus (RSV) was noted following the COVID-19 pandemic. Patients' outcomes, disease severity, and demographics all changed. This study aims to identify risk factors for severe disease and the impact of other viral co-infection on severity, which has not yet been extensively studied in China. This observational cohort study comprised cases of RSV-associated ALRTI among children younger than 5 years who were admitted to the hospital from January to December 2023. Demographic characteristics, co-infection status, and laboratory parameters were compared between severe and mild groups. Logistic regression analysis was used to identify risk factors for severe RSV-ALRTI and viral co-infection, with the corresponding 95% CI and P value. Receiver Operating Characteristic (ROC) curve was drawn to analyze the efficacy of specific risk factors. A total of 1036 cases with a median age of 16 (6, 38) months; 662 boys (63.9%) were admitted with RSV-associated ALRTI; 764 (73.7%) were younger than 3 years. The cohort suggests out-of-season epidemic of RSV. Severe disease was documented for 405 cases (39.1%), with a younger median age (10 (4, 32) and a higher proportion with a history of prematurity and low-body weight (P < 0.01). The most common co-infections were bacterial co-infection (18.9%) and co-infection with other respiratory viruses (15.5%). Age combined with neutrophil proportion, concentration of lactic dehydrogenase (LDH) and Na⁺ had predictive value for severe RSV-ALRTI (P < 0.001, AUC: 0.723, 95% CI: 0.691-0.754). Bacterial co-infection, particularly with Streptococcus pneumoniae, was identified as an independent risk factor for severe RSV-ALRTI (OR = 1.587, 95% CI: 1.054-2.389). There was no significant correlation between co-infection with other viruses and severe RSV-ALRTI. Patients with viral co-infection displayed stronger humoral immune activation (higher IgG, IgM, IgA, and C3 levels) and elevated serum IgM was an independent risk factor (OR = 2.641, 95%CI: 1.868-3.734), demonstrating predictive value for identifying co-infection subtypes. In this cohort study of younger than 5 years old children without serious underlying diseases hospitalized with RSV in 2023, severe RSV disease was more likely among infants. Prematurity, low body weight, younger age and bacterial co-infection were the main risk factors. Age combined with neutrophil proportion, concentration of LDH and Na⁺ had predictive value for severe RSV-ALRTI. Increased serum IgM had certain predictive value for other viral co-infection.