Post-pandemic immunity debt and environmental decoupling reshape urban influenza transmission

Relaxation of COVID-19 non-pharmaceutical interventions was followed by a global resurgence of respiratory infections, yet how accumulated population susceptibility interacts with environmental forcing and human mobility remains unclear. Here we analyse ~1.88 million clinical influenza tests from Macau (2010-2025) alongside meteorology, air quality, digital information-seeking, and independent surveillance and time-resolved viral genomes from Hong Kong, benchmarked against global surveillance data. Counterfactual inference indicates a post-pandemic excess in influenza activity consistent with immunity debt, mirroring global resurgence patterns, and an independently parameterised susceptible–exposed–infectious–recovered–susceptible model reproduces this excess (best-fit debt fraction 42%; approximate Bayesian computation posterior 58%, 95% uncertainty interval 32-77%). Susceptibility rises disproportionately in adults rather than children, whereas vaccination coverage in high-risk groups does not decline, arguing against reduced uptake as the driver. Time-varying models and convergent cross mapping reveal environmental decoupling, whereby high susceptibility weakens meteorological constraints and sustains influenza A transmission, and asymmetric suppression of influenza B. Spatiotemporal and phylogenetic evidence supports travel-linked reintroduction from ports of entry into dense neighbourhoods, and digital information-seeking shifts from lagging to leading clinical burden. These findings indicate a post-pandemic regime shift in influenza dynamics rather than a return to historical baselines, motivating adaptive, multi-source surveillance in high-density cities. Using 1.88 million clinical records from Macau and regional and global surveillance, in this study, the authors show how reduced population immunity and renewed travel reshaped post-pandemic influenza transmission and weakened its usual links with climate.