How CDC is monitoring influenza data to better understand the current avian influenza A (H5N1) situation in people

The percentile represents the position of the current level within the historical range. A value of 0 indicates that the current level is the lowest recorded at the site, while a value of 100 indicates that it is the highest.

The wastewater level for influenza A is categorized as follows:

• Minimal = The site's current influenza A virus level is minimal compared to data collected from the last influenza season. The current level is within the lowest 0-<20th percentile of influenza A virus levels recorded at that site or influenza A viruses were not detected in the most recent sample.
• Low = The site's current influenza A virus level is low compared to data collected from the last influenza season. Its current level is within the 20-<40th percentile of influenza A virus levels recorded at that site.
• Moderate = The site's current influenza A virus level is moderate compared to data collected from the last influenza season. Its current level is within the 40-<60th percentile of influenza A virus levels recorded at that site.
• Above Average = The site's current influenza A virus level is above average compared to data collected from the last influenza season. Its current level is within the 60-<80th percentile of influenza A virus levels recorded at that site.
• High = The site's  influenza A virus level is high compared to data collected from the last influenza season. Its current level is at 80th percentile or higher for influenza A virus levels recorded at that site.
• Insufficient Data = Site is testing for influenza A but does not have sufficient data for a comparison with the 2023-2024 influenza season or a site that has not submitted data in the last two weeks. For more information on these criteria, see Data Methods.

Wastewater sampling sites can encompass populations of varying sizes (also known as a sewershed population) that may extend across county or state boundaries.

What CDC is Doing with Influenza A Wastewater Data: Wastewater surveillance complements other existing influenza virus surveillance systems to monitor influenza trends. Sites with high influenza A virus levels (80-100th percentile) detected in wastewater are being closely monitored by CDC and its partners to identify potential contributing factors. This involves analyzing routine influenza virus and syndromic surveillance data to understand human influenza A infections and following up with the relevant jurisdiction to better understand the factors that could be contributing to these levels. Additionally, this could include reviewing other potential contributors of virus into wastewater. For instance, some states have identified non-human sources such as milk processing waste that contribute to wastewater sites.

Wastewater surveillance is an evolving science. Efforts to monitor influenza A virus activity using wastewater data are likely to evolve as the methodologies and interpretation are evaluated and refined.

Data Limitations:

• Current wastewater monitoring methods detect influenza A viruses but do not distinguish the subtype. This means that avian influenza A(H5N1) viruses can be detected but cannot be distinguished from other influenza A virus subtypes.
• Wastewater testing 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).

Data Source: CDC's National Wastewater Surveillance System (NWSS) has over 600 sites from a variety of partners reporting influenza A virus data to CDC.

For more information on how CDC conducts Wastewater Surveillance, see Data Methods.

For more information on Influenza A Virus wastewater data, see here.