Molecular Epidemiology, Viral Load and Clinical Severity of Human Adenovirus-Associated Respiratory Infections in Hospitalized Children in Shanghai, 2021-2023.

Human adenoviruses (HAdVs) are important causes of pediatric acute respiratory tract infections (ARTIs), with disease severity potentially varying by type. The impact of COVID-19-related non-pharmaceutical interventions (NPIs) and their relaxation on the relative prevalence of various HAdV types, RNase P-normalized viral load distributions, and associated clinical outcomes remains incompletely defined. We conducted a retrospective cohort study of children hospitalized for ARTIs at Shanghai Children's Hospital from 2021 to 2023, integrating epidemiologic trends, clinical phenotypes, HAdV typing and RNase P-normalized HAdV DNA quantification by droplet digital PCR (ddPCR). Multivariable logistic regression was used to identify independent correlates of severe community-acquired pneumonia (SCAP). Among 19,537 hospitalized children, HAdV was detected in 600 (3.07%), with annual detection rates of 2.85%, 2.36% and 3.44% in 2021, 2022 and 2023, respectively (p = 0.002). A marked type replacement occurred, shifting from species C (predominantly HAdV-108) in 2021-2022 to species B (predominantly HAdV-3) in 2023. Among 507 HAdV-positive cases with complete data, post-pandemic cases in 2023 involved older children and showed higher proportions of species B infection, Mycoplasma pneumoniae (MP) co-infection, and SCAP (14.74% vs. 3.08% in 2021-2022; p < 0.001). In multivariable analyses, MP co-infection (aOR 4.16) and HAdV-B species (aOR 3.34) remained independently associated with SCAP. RNase P-normalized viral loads were highest in acute tonsillitis and comparatively lower in severe pneumonia and in patients with comorbidities or complications (p < 0.05), supporting a limited role of viral burden alone in severe outcomes. In conclusion, hospitalized pediatric ARTIs at Shanghai Children's Hospital during 2021-2023 showed a post-pandemic shift toward species B predominance and more severe phenotypes, closely linked to MP co-infection. Integrated molecular surveillance and routine MP screening may support earlier risk stratification for severe HAdV-associated respiratory disease in children.