Wastewater-based epidemiology of influenza viruses: a systematic review

Introduction: Wastewater-based epidemiology (WBE) has emerged as a valuable public health tool for monitoring the circulation of many pathogens, including influenza viruses (IVs). The general aim of this study is to systematically retrieve and summarize evidence on the use of WBE for supporting influenza surveillance. Specific objectives are: (i) to map influenza monitoring activities using WBE; (ii) to assess the performance of viral recovery methods; (iii) to explore association with clinical data; (iv) to evaluate the feasibility of typing/subtyping IVs directly from wastewater. Methods: We conducted a systematic review following the PRISMA guidelines, focusing on original data from peer-reviewed studies identified through PubMed/Medline, Scopus, and Web of Science. Results: Of 882 identified citations, 42 studies were included in the review. IVs detection was reported in all but one study, although typically at lower concentration than SARS-CoV-2. Thirteen studies (38.09 %) performed comparative analysis of different protocols, with mostly inconclusive results. Detection of IVs in the solid fraction of wastewater samples generally outperformed detection in the supernatant/liquid. Additionally, we describe the findings from 22 studies (52.38 %) that examined the link between environmental viral concentrations and clinical data, and 14 studies (33.33 %) that described IVs subtyping in wastewater. Conclusion: WBE has the potential to monitor influenza circulation in humans and animals, offering insights into outbreak size and circulating IVs subtypes. However, several key areas remain unexplored. Further research is needed to refine experimental techniques and standardize protocols, and to understand how to successfully integrate WBE data into public health strategies for influenza control.