• Significance of the study: Human adenovirus (HAdV) is an important pathogen causing acute respiratory infections (ARIs). This study investigated its prevalence in China during the period of 2020–2023 to elucidate the epidemiological trends both during and after the coronavirus disease 2019 (COVID-19) pandemic. The findings are intended to provide a basis for future HAdV surveillance and control efforts. • Scientific question: The COVID-19 pandemic led to notable changes in the epidemic patterns of several respiratory viruses. However, data on the prevalence of HAdV in China are limited during and after COVID-19. • Evidence before this study: HAdV is frequently detected in patients with ARIs or pneumonia. Prior to the COVID-19 pandemic, the overall detection rates of HAdV infection in ARIs and pneumonia cases were 5.62 % and 5.69 %, respectively, in 2019, and outbreaks were frequently reported, drawing great public concern. • New findings: This study revealed that the overall HAdV detection rate during the COVID-19 epidemic was lower than in the pre-epidemic period but rebounded in 2023. The HAdV epidemic in China during the 2020–2023 period was likely to be driven primarily by viral infections in the southern region. Additionally, the seasonal pattern of HAdV appeared to be closely related to the targeted control measures implemented against COVID-19 in China.

What is already known about this topic? Recent years have witnessed increasing reports of acute respiratory outbreaks caused by novel recombinant human adenovirus (HAdV). In November 2023, researchers at the Medical University of Hannover in Germany identified HAdV-B114. What is added by this report? The six strains of HAdV isolated in this outbreak were 100% homologous and also had the highest homology with the GenBank reference OR853835 for HAdV-B114 at 99.9%, 100%, and 99.8% in penton base gene hexon gene, and fiber gene. This is the first documented outbreak in China associated with HAdV-B114. What are the implications for public health practice? Given that genetically recombined HAdVs can pose serious threats to human health and trigger emerging infectious disease outbreaks, continuous monitoring — particularly of novel recombinant types based onpenton base gene, hexon gene, and fiber gene sequences — should be prioritized to enable early detection and effective public health response.

Human adenovirus type 14 variant p1 (HAdV-14p1) has been associated with severe respiratory infections worldwide. However, no cases of HAdV-14p1 infection have been reported in China since 2019. HAdV-14 was unexpectedly identified in 2024 in a pediatric patient hospitalized with bronchopneumonia in Chongqing, China. This study aims to elucidate the genetic characteristics of this strain and determine its phylogenetic relationship with previously circulating domestic and international strains. Whole-genome sequencing was performed on the HAdV-14 strain Chongqing2024-115, followed by comprehensive phylogenetic and genetic variation analyses using all publicly available global HAdV-14 genome sequences from the GenBank database. Genome-based phylogenetic analysis classified global HAdV-14 strains into two well-supported clades: the HAdV-14 prototype clade and the variant HAdV-14p1 clade. The latter was further subdivided into three distinct subclades (I–III). Chongqing2024-115 clustered closely with earlier HAdV-14p1 subclade III and isolates from Beijing (2012) and Gansu (2013), showing high sequence identity (99.7%–99.9%). Compared with the genome of the HAdV-14 prototype strain (de Wit, AY803294), all three HAdV-14p1 subclades shared 99 specific nucleotide variations, including a six-nucleotide deletion in the knob domain of the fiber gene (nt751–756). The identification of subclade-specific variations and mutations unique to strain Chongqing2024-115 indicates ongoing microevolution within the HAdV-14p1 clade. Our results confirm that the contemporary HAdV-14p1 strain shares a common genetic ancestry with earlier Chinese subclade III strains. Considering the association between HAdV-14p1 and severe disease, sustained surveillance is essential to understand better its prevalence, evolution, and impact on public health in China.

Introduction Recent sentinel surveillance has revealed a rising prevalence of human adenovirus type 21 (HAdV-21) among HAdV infections in China. This study aimed to elucidate the molecular features of currently circulating HAdV-21 strains in China. Methods Whole-genome sequencing (WGS) was performed on 23 HAdV-21 strains isolated from acute respiratory infection cases, 56.5% involving lower respiratory tract infections, across 7 Chinese sentinel surveillance provincial-level administrative divisions (PLADs) (2023–2024). These sequences, along with 50 previously reported HAdV-21 genomes from 6 countries (1956–2019), were integrated into a WGS dataset for comprehensive phylogenetic, genetic variation, and recombination analyses. Results WGS categorized the HAdV-21 strains into 3 subtypes: HAdV-21a, HAdV-21b, and historical HAdV-21p (isolated in the 1950s). HAdV-21a (1956–2024, involving 5 of the 6 countries) and HAdV-21b (2005–2024, involving 3 of the 6 countries) exhibited extensive spatiotemporal distributions. Recent Chinese strains (2023–2024) belonged to HAdV-21a and HAdV-21b (HAdV-21a/b), showing extremely high genetic homology with Chinese 2019 strains (genetic distance: 0.00007) and global strains (distance: <0.00040). Phylogenetic analysis confirmed that HAdV-21a/b shared a common ancestor and maintained a highly conserved genome despite decades of circulation. Sequence variation analysis identified shared and subtype-specific mutations in these two subtypes. Recombination pattern analysis further revealed that HAdV-21a/b acquired an HAdV-3-derived fragment in the E4 region (breakpoint: nt32,843). Conclusions Recombinant HAdV-21a/b subtypes have co-circulated in China in recent years with remarkable genetic conservation. Enhanced surveillance is essential to quantify associated disease burden and guide targeted prevention and control strategies.