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.