Wastewater surveillance complements other existing human influenza surveillance systems to monitor influenza trends. CDC’s National Wastewater Surveillance System (NWSS) has more than 600 sites with a variety of partners reporting influenza A virus data to CDC. Current wastewater monitoring methods detect influenza A viruses but do not distinguish the subtype. This means that avian influenza A(H5N1) viruses are detected but cannot be distinguished from other influenza A virus subtypes. Wastewater data also cannot determine the source of the influenza A virus. It could come from a human or from an animal (like a bird) or an animal product (like milk from an infected cow). Efforts to monitor influenza A virus activity using wastewater data are likely to evolve as the methodologies and interpretation are evaluated and refined.

For monitoring influenza A virus in wastewater, CDC compares the most recent weeks of influenza A virus levels recorded at a wastewater site to levels reported between October 1, 2023 and March 2, 2024 for that same wastewater site, and those at ≥80th percentile are categorized as high (see Data Methods).

For the week ending May 4, 2024, 189 wastewater sampling sites reported data meeting criteria for analysis for influenza A viruses, and 1 (<1%) site in one state was at the high influenza A virus level.
For the week ending April 27, 2024, 229 wastewater sampling sites reported data meeting criteria for analysis for influenza A viruses, and 3 (1%) sites in three states were at the high influenza A virus level.
Across these two most recent weeks, a total of 230 sites from 34 states reported data meeting criteria for analysis for influenza A viruses in both weeks or in either week and 3 (1%) sites in three states were at the high influenza A virus level.
The data from these sites are being closely monitored by CDC and its partners to identify potential contributing factors, including assessing whether any of the high levels are related to any human illness, and looking more closely at available state or local level data from other human seasonal surveillance systems.

This interactive map shows current site-level data for influenza A virus levels in wastewater. Each dot on the map represents a wastewater sampling site. Sites are categorized based on current influenza A levels compared to past levels at the same site during the 2023-2024 influenza season. When influenza A virus levels are at the 80th percentile or higher, CDC will work with relevant partners to better understand the factors that could be contributing to these levels.