Tracking resurgence of SARS-CoV-2 in Shenzhen, China, 2023–2025: evidence from wastewater, clinical, and climatic data

Wastewater-based epidemiology has emerged as a crucial tool for monitoring SARS-CoV-2 circulation beyond clinical testing. Despite the official end of the COVID-19 public health emergency, intermittent resurgences continue to challenge preparedness and early response. This study investigated SARS-CoV-2 epidemic dynamics in Shenzhen, China (April 2023-June 2025), integrating 5,868 wastewater measurements, 277,869 reported cases, 4,249 clinical genomes, 1,897 wastewater genomes, and corresponding meteorological data. Correlation analyses, subgroup analyses, and generalized additive models were applied to examine the relationships between temperature, humidity, and epidemic activity. Epidemic baselines were empirically defined based on candidate points showing the largest sustained changes over at least two consecutive weeks in reported infections and wastewater SARS-CoV-2 RNA concentrations, using both manual assessment and a median-based threshold, enabling identification of epidemic onset and characterization of recurrent infection activity. Four distinct epidemic waves were identified, with resurgences predominantly occurring between February and April. Epidemic activity occurred under diverse temperature-humidity conditions, rather than being confined to low-temperature or low-humidity periods. Derived baselines (wastewater: 2.75 × 10⁴ copies/L; clinical: 1,318 cases/week) and median baselines (wastewater: 1.75 × 10⁴ copies/L; clinical: 1,156 cases/week) provide practical benchmarks for signaling periods warranting public health attention. Across the major epidemic waves included in the analysis, wastewater-based surveillance detected epidemic onsets that were temporally concordant with, or modestly earlier than, those indicated by clinical reporting. These findings highlight the potential of wastewater surveillance with empirically defined baselines as a framework for early-warning and informed public health response in post-pandemic settings.