Wastewater epidemiology of respiratory viruses: implementation of a regional surveillance system

Abstract Wastewater surveillance is an innovative epidemiological tool that detects pathogens in sewage, providing population-wide indicators of infectious disease circulation. It offers a cost-effective approach to gathering epidemiological data from large populations, including asymptomatic individuals. In Lombardy (Italy), the Regional Government activated a surveillance system based on wastewater analysis for three respiratory viruses: SARS-CoV-2, Influenza virus and Respiratory Syncytial Virus (RSV). Since September 2024, over 400 influent wastewater samples have been collected from up to 14 wastewater treatment plants (WWTPs) across Lombardy. Samples were processed in three laboratories using standardised methodologies, and viral nucleic acid quantification was performed via real-time PCR. All data were compiled at the regional level and shared weekly with public health authorities and the general public. SARS-CoV-2 RNA was detected in 92.2% of the samples, while Influenza A and Influenza B RNA were identified in 36% and 34.4% of total samples, respectively. RSV RNA was detected in 43.5% of samples. Preliminary findings show a positive correlation between viral concentrations in wastewater and the number of positive clinical specimens. Wastewater surveillance is a relatively recent addition to the public health monitoring toolkit, supporting long-term monitoring of various pathogens. The establishment of a structured, regionally based wastewater surveillance system strengthens the integration of wastewater data with traditional epidemiological surveillance, providing a comprehensive overview of the circulation of respiratory viruses to inform the implementation of appropriate preventive measures. Finalised data from the 2024-2025 winter season will also serve as a baseline to evaluate the impact of the 2026 Winter Olympics, taking place in February 2026 in Lombardy, on the circulation of respiratory viruses.