Abstract: Objective To explorer the profile and epidemiological characteristics of hospitalized children with acute
respiratory tract infection (ARI) in Tianjin, china, from 2013 to 2018. Methods Sputum and/or throat swab specimen and
serum sample were obtained from children within 24 hours after admission. Respiratory bacteria were identified by
biochemical reaction and Analytical Profile Index. IgM antibodies against nine common respiratory pathogens including
influenza virus A (IFA), influenza virus B (IFB), parainfluenza virus serotype 1,2,3 (PIVs), respiratory syncytial virus (RSV),
adenovirus (ADV), mycoplasma pneumonia (MP), chlamydia pneumonia(CP), coxiella burnetii (COX) and legionella
pneumophila serogroup 1 (LP-1) were detected by indirect immunofluorescence antibody test (IFAT). Total antibodies
against MP were further semi-quantified by passive particle agglutination test (PPAT), titer ≥ 1∶160 was confirmed as recent infection. Results A total of 7 832 children hospitalized with ARI were included in the study, of whom 5 353 children were completed bacterial identification, 7 449 children were performed with IFAT and PPAT, and 4 970 children with all above etiological tests. Of the 7 832 children enrolled in the study, 3 291 (42.0%) were positive for one or more pathogen. Twenty percent of the 5 353 patients were performed with bacterial identification with at least one bacterium. The top five common
bacteria were streptococcus pneumoniae (5.3%), Klebsiella pneumoniae (4.0%), Escherichia coli (2.7%), staphylococcus aureus (1.9%) and haemophilus influenzae (1.6%) respectively. Among the five tested viruses, IFB (2.8%) and PIVs (0.6%) were the most commonly identified. MP was the most commonly detected pathogen throughout the year, with a mean positive rate of 29.3% (2 241/7 449) and the seasonal peak form September to December. The presences of more than one pathogen were found, atypical pathogenic/bacterial, atypical pathogenic/viral and bacterial/viral combinations were detected in 3.8, 3.0 and 0.6% respectively. Logistic regression analysis showed that co-infection was independently associated with the increased risk of progression to severe pneumonia (OR=1.916；95%CI: 1.415-2.595；P＜0.01). Conclusion This study suggests that MP and co-infection combined pathogen are the most commonly identified pathogen in the hospitalized children with ARI from 2013 to 2018, with a rising tendency in recent years. S. pneumoniae, K. pneumoniae, E. coli, S. aureus and H. influenzae are commonly indentified bacteria in children with severe lower respiratory tract infection. Co-infection is associated with the increased risk for progression and exacerbation of illness.

Key words: acute respiratory tract infection, pneumonia, etiology, children, epidemiology