Temporal association between influenza A and B positivity and fever-clinic visit burden in Shanghai, China, 2022-2025: a retrospective time-series study.

BACKGROUND: Temporal changes in respiratory virus circulation after the COVID-19 pandemic have created new challenges for fever-clinic preparedness. Outpatient influenza laboratory activity may provide useful operational information, but its type-specific temporal relationship with subsequent fever-clinic burden has not been well characterized.

METHODS: We conducted a retrospective time-series study using aggregated fever-clinic visit records and outpatient influenza testing data from Renji Hospital, Shanghai, China, from 2022 to 2025. Lead-lag correlations were evaluated over a primary -28 to +28-day window using 7-day centered moving averages, with trailing moving averages and an extended -56 to +56-day window as sensitivity analyses. High-burden days were defined as days with visits at or above the 90th percentile. Adjusted quasi-Poisson models accounted for day of week, calendar month, calendar year, and testing volume. Additional analyses excluded 2022, evaluated adjacent 0-21-day influenza A leads, and used Newey-West autocorrelation-robust inference.

RESULTS: We included 254,346 fever-clinic visits and 303,597 outpatient influenza testing episodes. Overall influenza positivity was 33.96%, with 84,819 influenza A-positive and 18,504 influenza B-positive episodes. Influenza A was the main contributor to increased overall activity, particularly in 2023 and 2025. Peak correlations occurred at -12 days for overall influenza positivity and -15 days for influenza A positivity, whereas influenza B peaked at the positive boundary of the primary window. In the extended analysis, the influenza B peak shifted to +50 days, indicating no stable leading relationship. High-burden days were preceded by higher overall influenza and influenza A positivity than reference days. In the conventional testing-volume-adjusted model, influenza A positivity at a 15-day lead was associated with daily fever-clinic visits, but this association was attenuated after exclusion of 2022 and was no longer statistically significant with Newey-West inference. Adjacent-lag analyses did not identify a robust positive day-specific association after false discovery rate correction.

CONCLUSION: Outpatient influenza A positivity showed clearer temporal alignment with subsequent fever-clinic burden than influenza B. Although the fixed 15-day estimate was sensitive to period restriction and residual serial correlation, influenza A positivity may still serve as a complementary hospital-level surveillance indicator rather than a stand-alone forecasting tool.