Wastewater Surveillance Data as a Complement to Emergency Department Visit Data for Tracking Incidence of Influenza A and Respiratory Syncytial Virus - Wisconsin, August 2022-March 2023

Abstract

Wastewater surveillance has been used to assist public health authorities in tracking local transmission of SARS-CoV-2. The usefulness of wastewater surveillance to track community spread of other respiratory pathogens, including influenza virus and respiratory syncytial virus (RSV), is less clear. During the 2022-23 respiratory diseases season, concentrations of influenza A virus and RSV in wastewater samples in three major Wisconsin cities were compared with emergency department (ED) visits associated with these pathogens. In all three cities, higher concentrations of influenza A virus and RSV in wastewater were associated with higher numbers of associated ED visits (Kendall's tau range = 0.50-0.63 for influenza-associated illness and 0.30-0.49 for RSV-associated illness). Detections of both influenza A virus and RSV in wastewater often preceded a rise in associated ED visits for each pathogen, and virus material remained detectable in wastewater for up to 3 months after pathogen-specific ED visits declined. These results demonstrate that wastewater surveillance has the potential to complement conventional methods of influenza and RSV surveillance, detecting viral signals earlier and for a longer duration than do clinical data. Continued use of wastewater surveillance as a supplement to established surveillance systems such as ED visits might improve local understanding and response to seasonal respiratory virus outbreaks.

FIGURE 1

Respiratory syncytial virus–associated and influenza-associated emergency department visits and wastewater concentrations for respiratory syncytial virus and influenza A virus — three Wisconsin cities, August 2022–March 2023 Abbreviations: ED = emergency department; LOESS = locally estimated scatterplot smoothing; RSV = respiratory syncytial virus. * Collected from the Electronic Surveillance System for the Early Notification of Community-Based Epidemics (ESSENCE) and based on CDC-provided definitions. ^† Wastewater concentration values were log(10) and denoted in gene copies per L (gc/L); a value of 1 gc/L was added to all wastewater concentrations to allow for log(10) transformation of previously zero values. ^§ LOESS lines are overlaid to display general trend in wastewater concentration data.

FIGURE 2

Correlation between weekly emergency department visits and wastewater surveillance for influenza and respiratory syncytial virus — three Wisconsin cities, August 2022–March 2023 * For visual ease of comparison, data from both surveillance systems were transformed to the log(10) scale, and a value of 1 was added to all datapoints in both datasets (e.g., 1 case or 1 gc/L) to allow for log(10) transformation of previously zero values. Values along the 0 gc/L, log(10) line are jittered slightly to display the density of points. Linear regression lines are overlaid to display general trend in relationships between paired data. ^† Kendall’s tau values = 0.50 (Green Bay), 0.52 (Madison), and 0.63 (Milwaukee). ^§ Kendall’s tau values = 0.37 (Green Bay), 0.49 (Madison), and 0.30 (Milwaukee).