An Outbreak of Human Adenovirus Infection Among Children Post COVID-19 Pandemic in Southern China.

Human adenovirus (HAdV) is an important pathogen causing pediatric acute respiratory illness (ARI) before COVID-19 pandemic. Since January 2023, China has reported ARI outbreaks attributed to Mycoplasma pneumoniae and other respiratory viruses. In this study, a total of 276 737 pediatric patients with ARI were enrolled from January 2023 to April 2024. HAdV-positive samples were identified by real-time PCR. The monthly frequency of HAdV infection remained below 0.9% through July 2023, with a gradual rise to 1.03% in November and 2.92% in December. A significant surge was observed in the early months of 2024, with HAdV-positive rate spiking to 10.81% (3025/27992) in January and 10.75% (1393/12954) in February. The adenovirus outbreak peaked during the third to fifth week of 2024, with the highest recorded ratio of 23.86% (131/549) on January 29, 2024. Among the 600 samples that were typed, the predominant subtypes were HAdV-B3 (96%) and HAdV-C1 (2.67%). Phylogenetic analysis of two HAdV-B3 strains revealed close genomic similarity to the strain BJ20170281, which was isolated in Beijing, China, in 2017. HAdV-B3 has triggered an outbreak in Southern China in winter, an atypical epidemic season, following the COVID-19 pandemic. Ongoing surveillance of circulating pathogens including HAdV is necessary to prevent ARI outbreaks.