Weekly dynamics, subtype replacement and regional heterogeneity of influenza activity in China, 2024–2026: A national sentinel surveillance analysis

Objective National influenza surveillance in China is reported weekly, but recent post-pandemic patterns integrating regional timing, subtype/lineage replacement and outbreak activity have not been quantitatively characterized in a continuous national series. We aimed to reconstruct and quantify weekly influenza dynamics in China from 2024/W01 to 2026/W21. Methods We performed a retrospective secondary time-series analysis of 125 publicly available, aggregate weekly reports from the Chinese Center for Disease Control and Prevention and Chinese National Influenza Center. Values were reconciled against official reports when working datasets differed or quality-control checks identified inconsistencies. Positivity was recalculated from positive and tested specimens. Regional differences were evaluated across paired weeks using heteroskedasticity- and autocorrelation-consistent Newey-West standard errors and exploratory lead-lag relationships were assessed using cross-correlation of first-differenced weekly series. Results Nationally, 1,796,636 influenza-like illness specimens were tested and 253,219 were influenza-positive (weighted positivity, 14.1%); national positivity peaked in 2025/W49 at 49.8%. Overall weighted positivity was 14.6% in Southern China and 13.3% in Northern China. Across 125 paired weeks, Southern China had a mean positivity 4.41 percentage points higher than Northern China (95% CI, 2.23–6.59; P < 0.001) and a mean ILI percentage 0.77 points higher (95% CI, 0.48–1.05; P < 0.001). First-difference positivity changes were most strongly correlated when Northern China led Southern China by one week (r = 0.781). Among 253,104 explicitly reported subtype/lineage detections, A(H3N2) was largest (45.6%), followed by A(H1N1)pdm09 (33.8%) and B(Victoria) (20.6%). Predominance followed A(H3N2) in 2024/W01, B(Victoria), A(H1N1)pdm09, A(H3N2) and B(Victoria) again in early 2026. Reported outbreaks peaked in 2025/W48 at 1,541, during A(H3N2) predominance. Conclusions Recent influenza activity in China was multi-wave and regionally heterogeneous, with measurable north-south temporal differences, sequential subtype replacement and a large A(H3N2)-associated outbreak surge. The analysis also demonstrates the importance of explicit source-level quality control when reusing aggregated public surveillance reports.