How CDC is monitoring influenza data to better understand the current avian influenza A (H5N1) situation in people
Updated May 14, 2024
CDC influenza (flu) surveillance systems show no indicators of unusual influenza activity in people, including avian influenza A(H5N1).
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 and state and local health departments monitor people exposed to infected birds, poultry or other animals for 10 days after exposure. Between February 2022 and now, there have been
- At least 9,200 people monitored and
- At least 200 people tested for novel influenza A
CDC and state and local health departments monitor people exposed to infected cattle for 10 days after exposure. Between March 2024 and now, there have been
- At least 260 people monitored
- At least 33 persons tested for novel influenza A
- One case of avian influenza A(H5N1) was identified
*CDC numbers are based on state reports and CDC defers to states for updated information on people being monitored and tested.
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 May 10, 2024, 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(H5N1)
Rapid detection and reporting of human infections with novel influenza A viruses, including influenza A(H5N1), 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(H5N1) cases reported in 2024 are summarized below.
Data presented through: 05/04/2024; Data as of: 05/09/2024
Additional novel influenza case surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: Case Characteristics
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(H5N1). 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.
Data presented through: 05/04/2024; Data as of: 05/09/2024
Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data
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. WHO 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(H5N1) 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 H5N1.
Data presented through: 05/04/2024; Data as of: 05/09/2024
Additional clinical laboratory surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data
Monitoring for Changes in Emergency Department Visits for Influenza
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.
Additional emergency department surveillance information for current and past seasons:
Surveillance Methods |Data.CDC.gov: NSSP Emergency Department Visit Trajectories
Monitoring for Influenza in Wastewater
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.
