Review 2: "United States Influenza 2022-2023 Season Characteristics as Inferred from Wastewater Solids, Influenza Hospitalization and Syndromic Data"

RR:C19 Evidence Scale rating by reviewer:

• Strong. The main study claims are very well-justified by the data and analytic methods used. There is little room for doubt that the study produced has very similar results and conclusions as compared with the hypothetical ideal study. The study’s main claims should be considered conclusive and actionable without reservation.

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Review:

Wastewater surveillance is a valuable epidemiological tool to enhance influenza surveillance, particularly in situations where multiple respiratory diseases, such as COVID-19, Influenza, and RSV, are concurrently circulating in communities. The authors of this study concluded that the concentration of Influenza A virus RNA in wastewater-settled solids can complement clinical surveillance, providing insights into the timing and intensity of the seasonal influenza epidemic in the US. 

The reviewer strongly supports the conclusion made by the authors in the present study. The authors analyzed Influenza A virus (IAV) RNA concentration in 18,590 wastewater-settled solids, and their methodology and interpretation are robust. There are several strengths to this study. Firstly, the data on IAV RNA concentration is highly reliable. The sample size is likely the largest among previous studies on wastewater surveillance, and the samples were collected across the US. Their wastewater sample processing protocol is well-documented in multiple peer-reviewed journal articles, and thorough quality assurance and quality control (QA/QC) measures were conducted. Secondly, the approach to defining duration and intensity using wastewater surveillance is reasonable. The authors used public health authorities as a benchmark to establish a baseline and create aggregated data for wastewater surveillance, facilitating comparisons between IAV RNA concentrations and clinical indicators such as hospitalization rates or cases of influenza-like illness (ILI). Thirdly, the authors explicitly acknowledge the limitations of their findings. They point out that wastewater samples were not collected from some states including Florida (FL) and Texas (TX), which might exhibit early-season onsets. Additionally, their data covers only one year of the influenza epidemic season. However, these limitations are expected to be addressed as more samples are collected from additional wastewater treatment plants over a longer period. Given that determining the correct baseline is critical for using wastewater-based epidemiology in influenza surveillance, this manuscript would have a more practical implication if the authors explored more multiplication factors of minimum value (3x or 4x the lowest detectable concentration) to determine the baseline since the current baseline with 2x seems to provide too conservative onset time.