Adenovirus Types 3 and 7 Drive Winter Pneumonia Surge in Hospitalized Chinese Children

Human adenovirus has long been recognized as one of the most important viral causes of acute respiratory infections in children, yet its shifting genotype landscape at the regional level has remained poorly charted in many parts of China. A new three-year surveillance study from Tianjin, published in BMC Infectious Diseases, now provides one of the most detailed pictures to date of how this virus circulates among hospitalized children, which genetic types dominate, and how those types map onto different clinical outcomes. Drawing on nearly twenty thousand respiratory specimens collected between September 2022 and August 2025, the research team led by Yulian Fang and colleagues at Tianjin Children’s Hospital traced a dynamic epidemic in which one viral species displaced another as the dominant cause of pediatric respiratory disease.

The scale of the analysis is one of its distinguishing strengths. The investigators tested 19,523 respiratory specimens from children admitted to the hospital with acute respiratory infections, screening each sample for human adenovirus using targeted next-generation sequencing, a technique that allows simultaneous detection and initial genetic typing of the virus. Overall, 1,704 samples tested positive, corresponding to a detection rate of 8.7 percent. That figure underscores how substantial a burden adenovirus places on pediatric inpatient services in the region, particularly given that the study period spanned three full respiratory seasons following the disruption of the COVID-19 pandemic years.

Demographic patterns in the data were striking and statistically robust. Detection rates were significantly higher in boys than in girls, a difference the authors report with a P value of 0.002, and age proved to be an even stronger determinant of infection risk. Children between three and six years old showed the highest detection rate of any age group, at 11.9 percent, with significant heterogeneity across age strata overall. This preschool peak is consistent with the epidemiology of adenovirus in many settings, reflecting the combination of close contact in kindergartens and playgrounds, immature immune memory, and the high viral shedding that young children can sustain during respiratory illness.

Beyond simply measuring how often the virus appeared, the team went on to determine which genotypes were responsible. From the positive samples, 1,447 could be successfully genotyped, and the analysis revealed 14 distinct genotypes belonging to five of the seven human adenovirus species, designated A, B, C, E, and F. Two types towered over the rest: HAdV-3 accounted for 51.5 percent of genotyped infections and HAdV-7 for 18.4 percent, followed by HAdV-1 at 12.2 percent, HAdV-2 at 6.6 percent, HAdV-21 at 6.3 percent, HAdV-5 at 2.4 percent, and HAdV-4 at 1.3 percent, with remaining genotypes making up the final 1.3 percent. To confirm the sequencing-based typing, the researchers randomly selected positive samples for conventional PCR amplification of the hexon gene, the principal surface protein-coding region used in adenovirus classification, and performed phylogenetic analysis against reference sequences from GenBank.

Perhaps the most consequential finding is that genotype composition changed systematically with the age of the host. Preschool and school-aged children were infected mainly with HAdV-3 and HAdV-7, both members of species B, whereas infants and toddlers were predominantly infected with HAdV-3 and HAdV-1, the latter belonging to species C. The association between age group and genotype was highly significant, with a P value below 0.001. This age-stratified structure has practical implications: it suggests that the viruses circulating in daycare and school environments differ from those afflicting children in their first years of life, and that any vaccination or prevention strategy may need to account for these distinct transmission niches.

Seasonality added another layer of structure to the epidemiology. Adenovirus was detected year-round, but the detection rate varied markedly by season, peaking in winter at 13.4 percent of tested specimens. The seasonal pattern intersected with genotype in an unexpected way. Species B viruses, particularly HAdV-3 and HAdV-7, dominated the winter months, while species C viruses, especially HAdV-1, were more commonly identified in summer. This genotype-season coupling, significant at P below 0.001, implies that winter respiratory disease admissions driven by adenovirus are not simply more frequent but also caused by a different set of viral types than summer cases, a nuance that could inform the timing of clinical preparedness and laboratory typing efforts.

Temporal analysis across the three study years revealed an inter-annual shift in the viral population that may prove to be the study’s most cited observation. Before August 2023, species C adenoviruses were the predominant circulating group; after that point, species B took over as the dominant species. Such species-level turnover in a community setting is rarely documented with this level of resolution, and it raises questions about what ecological forces, whether waning population immunity, changes in contact patterns, or intrinsic viral fitness differences, drove the replacement. The authors’ data cannot answer that question definitively, but the documented shift provides a benchmark against which future surveillance in Tianjin and comparable regions can be measured.

The clinical correlations are equally important for pediatric practice. The main diagnoses among adenovirus-positive children included pneumonia, bronchitis, tonsillopharyngitis, and upper respiratory tract infections. Genotype-phenotype association analysis showed that HAdV-3 and HAdV-7 were the predominant types in pneumonia cases, whereas HAdV-3 and HAdV-1 were more commonly associated with tonsillopharyngitis and upper respiratory tract infections. Most critically, children infected with HAdV-7 were significantly more likely to develop severe pneumonia, an association with a P value below 0.001. This finding aligns with the established reputation of HAdV-7 as the most virulent of the common respiratory adenovirus types and highlights why molecular typing of adenovirus in hospitalized children is not merely an academic exercise but can carry direct prognostic value at the bedside.

For the study’s authors, the combined epidemiological, seasonal, and clinical picture supports a case for region-specific surveillance and targeted interventions. Because HAdV-3, HAdV-7, and HAdV-1 together accounted for the overwhelming majority of typed infections, they represent the logical priority targets for any multivalent vaccine formulated for this population. No licensed adenovirus vaccine is currently available for routine pediatric use in most countries, and existing military vaccines cover only types 4 and 7. The genotype distribution documented in Tianjin, with its clear dominance of type 3 and its age-specific and seasonal structure, offers exactly the kind of empirical evidence that vaccine developers and public health planners need when weighing which valences to include and how to prioritize rollout.

The study also demonstrates the value of combining modern sequencing platforms with classical molecular methods. Targeted next-generation sequencing enabled high-throughput screening and typing across nearly twenty thousand specimens, while hexon gene PCR and phylogenetic analysis provided confirmatory genotyping anchored to international reference databases. The retrospective design, approved by the Ethics Committee of Tianjin Children’s Hospital with a waiver of written informed consent because de-identified residual specimens were used, allowed the team to assemble a dataset of unusual breadth without additional burden on patients. As respiratory virus surveillance becomes increasingly genomic in the post-pandemic era, this work illustrates how sustained, genotype-resolved monitoring of a single pathogen in a single pediatric population can reveal patterns, from species turnover to genotype-specific severity, that would be invisible to conventional diagnostic testing alone, and it establishes a detailed baseline for tracking how adenovirus evolution continues to shape childhood pneumonia in northern China.

Subject of Research: Molecular epidemiology and clinical characteristics of human adenovirus in hospitalized children with respiratory infections in Tianjin, China

Article Title: A comprehensive analysis of molecular epidemiology and clinical characteristics of human adenovirus among hospitalized children with pneumonia in Tianjin, China, 2022–2025

Article References: Fang, Y., Dong, L., Hou, M., Lei, M., Guan, X., & Cai, C. (2026). A comprehensive analysis of molecular epidemiology and clinical characteristics of human adenovirus among hospitalized children with pneumonia in Tianjin, China, 2022–2025. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14452-7

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14452-7

Keywords: human adenovirus, pediatric pneumonia, acute respiratory infection, HAdV-3, HAdV-7, genotype surveillance, next-generation sequencing, hexon gene, seasonality, vaccine development, Tianjin, BMC Infectious Diseases