NonPharmaceutical Interventions and PostPandemic Resurgence: The Shifting Epidemiology of Influenza, Respiratory Syncytial Virus, and Adenovirus in Hangzhou, Zhejiang Province (2020-2024)

Abstract Background Acute respiratory infections (ARIs) caused by influenza A/B (Flu A/Flu B), respiratory syncytial virus (RSV), and adenovirus (ADV) remain a major global health burden. The COVID-19 pandemic profoundly altered respiratory virus transmission through nonpharmaceutical interventions (NPIs). Methods Samples from patients with respiratory tract infections at Hangzhou Hospital of Traditional Chinese Medicine from January 2020 to October 2024 were collected. ADV, Flu A, Flu B, and RSV were detected to evaluate the impact of NPIs on viral epidemiology. Statistical analyses were performed using SPSS software and GraphPad Prism. Multivariable binary logistic regression models, adjusting for age group and sex, were constructed to evaluate the independent effect of NPI relaxation (strict NPI period 2020-2022 vs. post-relaxation period 2023-2024) on virus positivity. Stratified analyses by age group and demographic comparability assessments between periods were also performed. Results Overall virus positivity declined from 27.4% (2020) to 14.5% (2021) during strict NPIs, then rebounded to 35.1% in 2023 after relaxation. Flu A resurged atypically in summer 2022 and dominated the 2023 winter peak, while Flu B increased annually and peaked in 2024. ADV regained its prepandemic summer seasonality by 2024, whereas RSV showed limited seasonal variation. Higher ADV positivity was observed in males, and higher Flu B positivity in females. By 2024, influenza burden shifted toward adults and geriatric populations, while children under five remained most vulnerable to RSV and ADV. However, after adjusting for demographic shifts—including a significant increase in adult patients (18-60 years: 22.1% to 40.0%) and altered sex ratios between periods (both P < 0.001)-the post-relaxation period showed lower adjusted odds of positivity for all four pathogens (Flu A: aOR = 0.64, 95% CI: 0.61-0.67; Flu B: aOR = 0.67, 95% CI: 0.63-0.70; ADV: aOR = 0.72, 95% CI: 0.68-0.76; RSV: aOR = 0.62, 95% CI: 0.59-0.65; all P < 0.001), whereas no significant sex differences were observed after multivariable adjustment (all P > 0.05). Stratified analyses further revealed that the magnitude of period-related risk reduction increased with age for all pathogens. Conclusion This study demonstrates that the postpandemic resurgence of respiratory viruses in Hangzhou, Zhejiang Province, was not a monolithic recrudescence following NPIs relaxation, but a multifaceted shifting epidemiology driven by three convergent forces: expanded school-based and community screening that broadened the tested denominator, demographic case-mix shifts toward higher-risk adult populations, and pathogen-specific alterations in seasonal and age-specific transmission patterns. The consistently lower adjusted odds of positivity during the post-relaxation period indicate that crude resurgence signals must be interpreted through the lens of surveillance structure.