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

Overview

This page provides information on how CDC systems that monitor national, state, and local level influenza data are being used during the current avian influenza A(H5N1) situation.

• Influenza virus and illness activity are monitored year-round through a collaborative effort between CDC and many partners, including state, local, and territorial health departments; public health and clinical laboratories; clinics; and emergency departments.
• Human cases of novel influenza, which are human infections with non-human influenza A viruses that are different from currently spreading seasonal human influenza viruses, are nationally notifiable. Every identified case is investigated and reported to CDC.
• CDC is actively looking at multiple flu indicators during the current situation to monitor for influenza A(H5N1) viruses, including looking for spread of the virus to, or among people, in jurisdictions where the virus has been identified in people or animals.

Monitoring of Persons Exposed to Infected Animals*

*CDC numbers are based on weekly state aggregate reports since 2022. CDC defers to states for updated information on people being monitored and tested. March 24, 2024, was the date of the first reported dairy cow infections in the United States.

Main Findings from Surveillance Systems

CDC has multiple surveillance systems that are used year-round to monitor key flu indicators. These data are reviewed comprehensively each week. Taken together, as of March 21, 2025, these systems currently show no indicators of unusual flu activity in people, including avian influenza A(H5N1) viruses.

Monitoring for Novel Influenza A virus infections among people, including Influenza A (H5)

Rapid detection and reporting of human infections with novel influenza A viruses, including influenza A(H5), is important to facilitate prompt awareness and an effective public health response. For confirmed cases, the reporting jurisdiction completes a case report form, which is submitted to CDC. The information includes patient demographics, symptoms, the clinical course of illness, and exposure history. The reporting jurisdiction for influenza A(H5) cases reported in 2024 are summarized below.

Data presented through: 3/22/2025; Data as of: 3/26/2025

Additional cases meeting the Council of State and Territorial Epidemiologists (CSTE) probable case definition have been reported by states: CA (1), WA (3), AZ (2), DE (1). Confirmatory testing at CDC for these cases was negative.

Public health laboratory reporting

Public health laboratories use CDC's diagnostic tools to detect both seasonal influenza viruses and novel influenza A viruses including influenza A(H5). These diagnostic tools are used at more than 100 public health laboratories in all 50 U.S. states. The results of tests performed by these public health laboratories nationwide are summarized below.

*This graph reflects the number of specimens tested and the number determined to be positive for influenza viruses at the public health lab (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. Specimens tested as part of routine influenza surveillance as well as those tested as part of targeted testing for people exposed to influenza A(H5) are included.

Data presented through: 3/22/2025; Data as of: 3/26/2025

Systems used to monitor influenza activity

Influenza activity is monitored year-round using multiple systems. These systems are used for monitoring seasonal influenza and, because influenza viruses are constantly changing in small, and occasionally more significant ways, these systems are also useful for monitoring signals and trends from novel influenza virus infections. Some examples are provided below.

Monitoring for changes in tests positive for influenza in clinical settings

Approximately 300 clinical laboratories located throughout all 50 states, Puerto Rico, Guam, and the District of Columbia report the results of clinical testing for influenza through either the U.S. Influenza Collaborating Laboratories System or the National Respiratory and Enteric Virus Surveillance System (NREVSS). The results of tests performed by clinical laboratories nationwide are summarized below. While these laboratories don't test specifically for influenza A(H5) virus, by tracking the percentage of specimens tested that are positive for influenza A viruses, we can monitor for unusual increases in influenza activity that may be an early sign of spread of novel influenza A viruses, including H5.

Data presented through: 3/22/2025; Data as of: 3/26/2025

The National Syndromic Surveillance Program (NSSP) collects, analyzes, and shares electronic data received from multiple health care settings, including emergency departments (ED). CDC uses syndromic surveillance in partnership with participating state and local health departments to capture data quickly, monitor for unusual trends, improve situational awareness, and inform decision making.

Data from NSSP on the weekly percentage of total emergency department visits associated with influenza-related diagnoses are summarized below and are closely monitored by the NSSP team. It's important to note that these visits are among persons with any influenza diagnosis and are not specific to avian influenza A(H5N1) viruses. However, by tracking all influenza diagnoses, as well as symptoms potentially related to influenza virus infections, among patients in EDs, the chance of detecting unusual levels of influenza is improved, including in jurisdictions where A(H5N1) viruses have been identified in animals or in the one person.

Monitoring for Avian Influenza A(H5) Virus In Wastewater

Wastewater surveillance may complement other existing human influenza surveillance systems to monitor influenza. Data on wastewater testing for influenza A viruses can be found through CDC's National Wastewater Surveillance System (NWSS). CDC receives weekly avian influenza A(H5) virus data from a CDC contract partner, an academic partner group, and some state health departments. In each site, wastewater sampling and testing occurs one or more times during the week.

Wastewater data cannot determine the source of avian influenza A(H5) viruses. They could come from a human or from an animal (like a bird) or an animal product (like milk from an infected cow), and detections of avian influenza A(H5) virus in wastewater do not necessarily indicate human cases. During the spring and summer of 2024, investigations in many states with avian influenza A(H5) virus detections in wastewater revealed likely animal-related inputs including milk-processing inputs. CDC, state and local public health departments, and other partners are closely monitoring the data from these wastewater sites to identify potential contributing factors. Wastewater results for avian influenza A(H5) virus provide an additional source of data to complement traditional influenza surveillance systems and inform public health planning and response efforts. Efforts to monitor avian influenza A(H5) virus activity using wastewater data are likely to evolve as the methodologies and interpretation are evaluated and refined.

This interactive map shows current site-level data for avian influenza A(H5) viral RNA detections in wastewater. Each dot on the map represents a wastewater sampling site. For each site, results are reported as "H5 Detection" when any of the samples were positive during the week reported. When avian influenza A(H5) virus is not detected in any of the samples, then the site is classified as "No Detection." All data are preliminary and may change as more reports are received. Data will be updated on this site every Friday, based on data submitted to CDC by the end of the day on the previous Wednesday.

Click on the legend below to see detections.

H5 detection in wastewater in the past six weeks

To filter for sites with a recent H5 detection, type + in the search bar.

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