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

Abstract

Influenza A virus (IAV) causes significant morbidity and mortality in the United States and has pandemic potential. Identifying IAV epidemic patterns is essential to inform the timing of vaccinations and nonpharmaceutical interventions. In a prospective, longitudinal study design, we measured IAV RNA in wastewater settled solids at 163 wastewater treatment plants across 33 states to characterize the 2022-2023 influenza season at the state, health and human services (HHS) regional, and national scales. Influenza season onset, offset, duration, peak, and intensity using IAV RNA in wastewater were compared with those determined using laboratory-confirmed influenza hospitalization rates and outpatient visits for influenza-like illness (ILI). The onset for HHS regions as determined by IAV RNA in wastewater roughly corresponded with those determined using ILI when the annual geometric mean of IAV RNA concentration was used as a baseline (i.e., the threshold that triggers onset), although offsets between the two differed. IAV RNA in wastewater provided early warning of onset, compared to the ILI estimate, when the baseline was set at twice the limit of IAV RNA detection in wastewater. Peak when determined by IAV RNA in wastewater generally preceded peak determined by IAV hospitalization rate by 2 weeks or less. IAV RNA in wastewater settled solids is an IAV-specific indicator that can be used to augment clinical surveillance for seasonal influenza epidemic timing and intensity.

Keywords: baseline; epidemic pattern; influenza A; onset; wastewater-based epidemiology.

Figure 1

Comparison of alternative baseline Avg_IAV_Region values (y-axis) and corresponding influenza season onset, offset, and duration for HHS regions (abbreviated R in title) for 2022–2023 influenza season (x-axis). Baseline values were calculated using Avg_IAV_Region annual mean (Mean), annual geometric mean (GeoMean), and twice the minimum observation (2 × Min). Shading indicates ILI onset, offset, and duration. Lines or shading that extend beyond the plotted time period indicate that onset or offset did not occur during the period of analysis using the selected baseline. Missing duration lines and onset/offset points indicate that data was not available for calculation.

Figure 2

Comparison of alternative baseline Avg_IAV_State values (y-axis) and corresponding influenza season onset, offset, and duration for states with at least 11 months of data for 2022–2023 influenza season (x-axis). Baseline values were calculated using Avg_IAV_State annual mean (Mean), annual geometric mean (GeoMean), and twice the minimum observation (2 × Min). Lines that extend beyond the plotted time period indicate that onset or offset did not occur during the period of analysis using the selected baseline.

Figure 3

Cumulative curves of the proportion of states with onset (y-axis) over time (x-axis) during the 2022–2023 influenza season using Avg_IAV_State baselines of the annual mean (N = 5), geometric mean (N = 5), or twice the minimum observation (2 × Min) (N = 9).

Figure 4

2022–2023 Overall FluServe-Net hospitalization rate (hospitalizations per 100,000 population) and Avg_IAV_US: (a) linear relationship using log₁₀ transformed values and weekly median Avg_IAV_US and (b) comparison of peak and intensity with vertical lines presenting time of peak.