IS481-based pertussis positivity transition from preschool to school-aged children and the ptxP3 predominance in high-load subset: a digital platform surveillance study in China.

China experienced a massive pertussis resurgence in 2024. We aimed to characterize the epidemiological trends, clinical symptoms, and molecular evolution of Bordetella pertussis using a large-scale, home-based surveillance platform in four Chinese megacities. From May 2024 to August 2025, symptomatic individuals in Beijing, Shanghai, Guangzhou, and Shenzhen were screened via a digital healthcare platform. Bordetella pertussis was defined using RT-PCR targeting IS481. Genomic analysis, including MLVA typing and virulence/resistance gene sequencing (ptxP, ptxA, ptxC, prn, fim3, fim2, tcfA, and 23S rRNA), was performed on a subset of high-load samples. A total of 3,356 (4.9%) participants were positive by IS481 screening. Significant spatiotemporal heterogeneity in positivity was observed peaking in mid-2024, followed by a transition in 2025. Shanghai (7.8%) exhibited a markedly higher positivity rate than Beijing (2.1%) and Guangzhou & Shenzhen (2.6%). Notably, 52.4% (1759/3356) of cases occurred in school-aged children (6-17 years), marking a significant shift in susceptibility from preschool to school-aged children. Clinically, pertussis presented special profile characterized by a high prevalence of cough (85.0%-90.2%) and a low incidence of fever (6.6%-12.8%). Genomic surveillance (n = 63) confirmed the absolute dominance of the erythromycin sensitive-ptxP3 lineage (95.2%) within the high-bacterial-load subset. The IS481-based B. pertussis test among symptomatic digital-platform users reveals a critical shift of pertussis towards school-aged children and the predominance of ptxP3 in the high-load subset. These findings highlight the clinical necessity of immunization strategies targeting evolving susceptible groups and emphasize the need for continued surveillance of circulating B. pertussis variants.