Epidemiological changes of common respiratory viruses in Shanghai, during 2021–2023

Introduction:The impact of influenza on disease burden is a matter of substantial attention.The influenza viruses (types A and B) and respiratory syncytial virus (RSV) are common causes of influenza-like illness (ILI).The aim of this study was to investigate the epidemic changes of common respiratory viruses in Renji Hospital, one of the largest hospitals in Shanghai, from 2021 to 2023.Methods: A retrospective analysis was conducted on the nasopharyngeal swab samples of ILI patients from the fever clinic of Renji Hospital from January of 2021 to December of 2023.A thorough analysis was conducted on the epidemiological characteristics across various years and age demographics.Real-time fluorescent quantitative polymerase chain reaction (PCR) was used to detect three common respiratory viruses: influenza A virus (FluA), influenza B virus (FluB), and RSV.Results: From 2021 to 2023, PCR testing of 125,409 nasopharyngeal samples from patients with ILI revealed an overall positivity rate of 44.65 %, with 38.82 % positive for FluA, 4.02 % for FluB, and 1.81 % for RSV.Agespecific analysis indicated the highest viral positivity rate in the 21-30 age group, with FluA and FluB predominating, while RSV positivity was highest among those aged 0-10 and lower in older age groups.Gender differences in positivity rates were observed across all viruses (P < 0.001), although no significant gender differences were found for FluA and FluB in the 56-65 and 66+ age groups, nor for RSV in those under 18.Seasonal trends showed FluA peaking in summer and winter of 2022 and maintaining high levels throughout 2023, with a slight dip in January and summer.FluB peaked from autumn 2021 to spring 2022 and showed a rising trend in winter 2023, while RSV peaked in early summer of 2023.The interrupted time series analysis showed a significant upward trend in the positivity rate of FluA before the policy change (P < 0.05), but both the immediate impact and the trend change after the policy implementation were not significant (P > 0.05).However, no significant changes were found in the positivity rates of FluB or RSV following the policy change (P > 0.05).Conclusion: From 2021 to 2023, the influenza outbreak in Shanghai was influenced by the COVID-19 pandemic and health interventions to some extent.The influenza outbreak in Shanghai may become more pronounced after the end of the COVID-19 pandemic.Accordingly, it is essential to rapidly appraise the influenza epidemiological trends and the positivity rate of influenza virus tests.