Influenza transmission dynamics quantified from wastewater

Influenza infections are challenging to monitor at the population level due to a high proportion of mild and asymptomatic cases and confounding of symptoms with other common circulating respiratory diseases, including COVID-19. Alternate methods capable of tracking cases outside of clinical reporting infrastructure could improve monitoring of influenza transmission dynamics. Influenza shedding into wastewater represents a promising source of information where quantification is unbiased by testing or treatment-seeking behaviors. We quantified influenza A and B virus loads from influent at Switzerland’s three largest wastewater treatment plants, serving about 12% of the Swiss population. We estimated trends in infection incidence and the effective reproductive number Re in these catchments during a 2021/22 epidemic and compared our estimates to clinical influenza surveillance data. We showed that wastewater-based incidence is better aligned with catchment-level confirmed cases than national ILI, and that only the wastewater data capture a peak in incidence in December 2021. We further estimated Re to have been below 1.05 after introduction of work from home measures in December 2021 and above 0.97 after these measures were relaxed in two out of three catchments based on wastewater data. The third catchment yielded qualitatively the same results, although with wider confidence intervals. The confirmed-case data yielded comparatively less precise estimates that include 1 before and during the period of measures. On the basis of this research we developed an online dashboard for wastewater-based influenza surveillance in Switzerland where we will continue to monitor the onset and dynamics of the 2022/23 flu season. ### Competing Interest Statement Tanja Stadler is a member of the scientific advisory board COVID-19 to the Swiss government. The other authors declare no competing interests. ### Funding Statement We gratefully acknowledge funding support from the Swiss Federal Office of Public Health and a Swiss National Science Foundation Sinergia grant number CRSII5_205933. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes The catchment-level wastewater load and confirmed case data used in this manuscript is available along with the code at the project repository at .