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 Switzerlands 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.