Wastewater-based surveillance of respiratory viruses in Northern Tuscany (Italy): Challenges and added value for public health purposes.

During the COVID-19 pandemic, wastewater-based surveillance (WBS) showed great potential as an early warning system and could complement human clinical surveillance. This study aimed to highlight the added value of WBS for respiratory infections alongside different clinical surveillance systems. Sewage collected at the entry of four Wastewater Treatment Plants in Northern Tuscany (Italy) were analyzed for SARS-CoV-2, Human Adenovirus (HAdV), Respiratory Syncytial Virus (RSV) and Influenza Virus (IV), over two years. Clinical data for COVID-19 were available for the study area, while data for other viruses came from national virological surveillance. For SARS-CoV-2, the correlation was highly significant between clinical and hospitalization data (ρ = 0.8460), but not significant between wastewater and clinical or hospitalization data (ρ = 0.1682 and ρ = 0.0569, respectively). SARS-CoV-2 RNA was found in wastewater even in period when clinical cases were not reported, indicating a continuous community circulation. HAdVs were detected in 74.3 % of samples, but most of the sequences identified belonged to enteric species (HAdV-F41), indicating the need of distinguishing the species causing respiratory diseases for the surveillance. RSV were found only in winter 2022-2023, while IV had not been detected in wastewater, probably due to poor test sensitivity. In conclusion, although there may be various challenges in testing different targets, WBS can provide pathogen-specific situational assessment which complements existing surveillance systems.