Background: Under the pressure of the outbreak of respiratory Human Adenovirus (HAdV) infections in children in Northern Vietnam in the end of 2022, this study was initiated to identify the HAdV subtype(s) responsible for the outbreak in relation to the clinical features of the patients and examine the risk factors of more severe cases. Methods: The study was conducted on pediatric patients tested positive with HAdV using multiplex real-time PCR between October and November 2022. Nasal swab samples were used for sequencing to identify HAdV subtypes and clinical data were collected retrospectively. Results: Among 97 successfully sequenced samples, the predominant subtypes were HAdV-B3 (83%), HAdV-B7 (16%) and HAdV-C2 (1%). Lower respiratory manifestations were found in 25% of patients (5% with severe pneumonia). There was no significant association between HAdV type and clinical features except that those infected with HAdV type 3 exhibited higher WBC and neutrophil % (p<0.001). Co-infection of HAdV with ≥1 other respiratory viruses or bacteria was found in 70.8% of those with lower respiratory illnesses (OR (95%CI); p-value vs. those without =5.21 (1.60, 19.36); 0.0084 after adjusting for age at hospital visit, sex, birth delivery method, day of disease at hospital visit), and in 100% of those with severe pneumonia vs. 33% of those without (p=0.005).   Conclusion: HAdV-B3 and HAdV-B7 were predominant in the outbreak. Co-infection of HAdV together with other respiratory viruses or bacteria was a strong risk factor for lower respiratory tract illnesses and severe pneumonia. The findings advocate the advantages of multi-factor microbial panels for the diagnosis and prognosis of respiratory infections in children.