Analysis on incidence trend of influenza in an area of Shenzhen, Guangdong in 2023 based on urban wastewater surveillance system

Objective To analyze the wastewater surveillance data of influenza virus in an area in Shenzhen, Guangdong province, evaluate its early warning effect and provide reference for the application of wastewater surveillance for influenza virus.

Methods Based on the wastewater surveillance system for severe acute respiratory syndrome coronavirus 2 in Shenzhen, influenza virus detection was conducted weekly at four wastewater surveillance sites in an area in Shenzhen from the 10^th week of 2023 to the 9^th week of 2024. Kendall's tau and time-lagged cross-correlation methods were used to analyze the correlation between the influenza virus concentration in wastewater and the reported influenza cases.

Results During the surveillance period, a total of 5 066 influenza cases were reported in this area, with an incidence rate of 4.61%. The detection rate of influenza virus in wastewater samples was 42.31% (88/208), higher than influenza-like illness detection rate (38.48%, 227/575), the difference was not significant (χ²=0.509, P=0.476). The concentration of influenza virus in wastewater ranged from 0 to 4.11×10⁶ copies/mL. The detection rate was high in spring, autumn, and winter. H1N1 subtype was predominated in spring (34.62%), while H3N2 subtype was predominated in autumn (23.08%) and in winter (36.54%). Kendall's tau correlation analysis showed a significant correlation coefficient of 0.63 between the concentration of influenza virus in wastewater and the reported influenza case count (Z=6.507, P<0.001). Time-lagged cross-correlation analysis indicated that when the reported influenza case count shifted to the right (future direction) by one week, the concentration of influenza virus in wastewater showed the highest correlation (offset=1).

Conclusion The influenza virus detection rate and virus subtype composition reflected by wastewater surveillance in this area were significantly correlated with the hospital-based influenza surveillance data, and early warning of influenza epidemic by one week can be made based on wastewater surveillance results. It is demonstrated that the wastewater surveillance could be used as a supplement to the traditional influenza surveillance system.