Respiratory pathogen epidemiology in children under 5 years hospitalized with acute respiratory infections in Qingdao, China: a retrospective, single-center, repeated cross-sectional surveillance study.

BACKGROUND: During the coronavirus disease 2019 (COVID-19) pandemic, non-pharmaceutical interventions (NPIs) reduced the transmission of severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) while simultaneously affecting the prevalence of respiratory pathogens to varying degrees. Currently, epidemiological characteristics vary across different regions and among different pathogens, and there is a lack of continuous surveillance data following the outbreak. This study aims to analyze the epidemiological characteristics of six respiratory pathogens in hospitalized children under the age of 5 years with acute respiratory infections (ARIs) in the Qingdao region following the COVID-19 pandemic, thereby providing a reference for developing prevention and treatment strategies for respiratory infections.

METHODS: Children aged <5 years who were hospitalized with ARIs between May 1, 2023, and June 30, 2025, were enrolled in this retrospective, single-center, repeated cross-sectional surveillance study. Nucleic acid testing was performed to detect influenza A virus (FluA), influenza B virus (FluB), respiratory syncytial virus (RSV), adenovirus (ADV), human rhinovirus (HRV), and Mycoplasma pneumoniae (MP). Detection results were analyzed by age group. Between-group comparisons were conducted using the Chi-squared test with Bonferroni correction.

RESULTS: A total of 16,613 cases were included, with an overall pathogen detection rate of 50.62% (8,409/16,613). The three most frequently detected pathogens were RSV (19.74%), MP (16.81%), and HRV (10.97%). RSV demonstrated the highest detection rate in children aged 0-<1 year (25.95%), accounting for 56.37% (1,849/3,280) of all RSV-positive cases. The detection rate of MP increased progressively with age, reaching 35.34% among children aged 4-<5 years. Detection rates for the remaining pathogens were lowest in the 0-<1 year age group. Months with the highest monthly RSV polymerase chain reaction (PCR) positivity were May 2023 (66.46%), January 2024 (43.48%), and April 2025 (42.78%). Months with the highest monthly MP PCR positivity were October 2023 (29.28%) and September 2024 (54.90%), with persistently elevated monthly detection proportions recorded from August 2023 to December 2024. Months with maximal monthly positivity for FluA, RSV, and MP showed visual separation on temporal plots; HRV and MP exhibited concurrent high monthly positivity during October-November 2023.

CONCLUSIONS: RSV and MP were the most frequently detected pathogens among the six tested targets in hospitalized children aged <5 years with ARIs in Qingdao, China. Infants aged <1 year exhibited the highest RSV PCR positivity rate, while MP PCR positivity rose progressively with age. Months with maximal RSV and MP detection proportions showed visually separated temporal maxima. FluA and FluB maintained low epidemic intensity throughout the study period, while HRV circulated continuously year-round.