Sporadic and Severe Lower Respiratory Illness-related Human Adenovirus Type 21 Infection in Southern China

Abstract Background: Human adenovirus type 21 (HAdV-21) is an important member of HAdV species B, but our understanding of this type is limited. Methods: We screened HAdV and 17 other common respiratory pathogens for 1,704 pediatric patients (≤14 years old) hospitalized with acute respiratory illness in Guangzhou, China in 2019. HAdV-21 infections were further confirmed by molecular typing from HAdV-positive patients, and their clinical manifestations, genomes, infectivity and pathogenicity in vitro were analyzed.Results: 151 of 1,704 cases (8.9%) were positive for HAdV, making it the third most frequently detected pathogen. Two sporadic HAdV-21 infections were identified in June and September. Both HAdV-21-positive patients presented with severe lower respiratory illness and had similar initial symptoms at onset of illness. The genome structure of HAdV-21 was found to be similar to that of other members of HAdV species B. The phylogenetic analysis showed that it was closely related to HAdV-B21 strain BB/201903 (MN686206) isolated in Bengbu, China in 2019, suggesting the possibility of the same source, and attention need to be paid to its prevention and control. In vitro, the infectivity and pathogenicity of HAdV-21 were lower than the main epidemic types 7 and 3. Plaques formed by HAdV-21, -7, and -3 were significantly different in shape and size (p < 0.05), with plaques formed by HAdV-21 being the smallest and with poorly defined edges. There was no significant difference between the plaques of the HAdV-21 isolates (p > 0.05). Conclusions: This study provides an important reference for the in-depth understanding of the epidemiology and pathogenicity of HAdV-21, and suggests the necessity of HAdV-21 research, prevention and control.