How CDC Uses Emergency Department Data to Watch for Unusual Patterns of Flu Amid Concerns Over H5N1 Bird Flu

What the CDC is doing

August 15, 2024 – CDC is using multiple data sources to assess and respond to emerging health threats like the avian influenza A(H5N1) virus, which has caused confirmed infections in multiple species, including U.S. poultry and dairy cattle, as well as 13 recent human infections, as of this posting. CDC's National Syndromic Surveillance Program (NSSP) captures electronic health record data reported automatically from more than 6,500 U.S. healthcare facilities, representing nearly 80% of emergency departments (ED) in all 50 states, the District of Columbia, and Guam. These data become available within 24 hours of a patient visit to a participating ED. They complement other surveillance systems, but because of their timeliness, can give public health officials an early warning about potentially concerning levels of illness, allowing further investigation and action.

CDC's NSSP helps public health departments track both symptoms and diagnoses to detect unusual levels of illness, such as increases in influenza activity. In addition to tracking respiratory illnesses, ED data can be used to monitor drug overdoses, heat-related illness, tick bites, and more. More than 1,400 state and local public health partners use these data through the NSSP Community of Practice, including to inform emergency response, track use of healthcare resources, and/or connect people with preventive care.

Using Emergency Department Data to Detect Potential Local Increases in Influenza During the Response to H5N1 Bird Flu

As part of our ongoing effort to monitor the impact of the avian influenza A(H5N1) outbreak, CDC staff monitor NSSP data and provide the weekly percentage of total ED visits associated with flu diagnoses. It is important to note that these visits are among persons with any influenza diagnosis and are not specific to H5N1 bird flu. However, monitoring all flu diagnoses helps establish baseline flu activity in a given area and provides an early indicator of change. This helps public health professionals determine where to look more closely if there are unexpected increases in flu activity.

CDC routinely uses multiple flu indicators—including syndromic surveillance—to track influenza activity. Because such a high percentage of U.S. EDs contribute to NSSP, increases in reported ED visits due to influenza infections, at a more granular geographic level, may be recognized more easily than with other surveillance systems. Local public health departments are able to view local jurisdictional data. CDC, which supports local, state, regional and national responses, looks across multiple indicators when there are increases in reported influenza visits.

Tracking the Virus

CDC monitors influenza virus and illness activity year-round. State and local public health professionals can use the surveillance data that CDC makes available to them and to the public to get a better understanding of the avian influenza A(H5N1) situation. CDC and public health departments collaboratively monitor flu activity using data 1 from laboratories, outpatient providers, hospitals and vital statistics offices.

CDC's ongoing investments in enhancing syndromic surveillance unites public health departments in tracking the spread of diseases that cross jurisdictional borders. When unusual trends or data anomalies are detected by the agency, CDC's NSSP consults with state or local health departments via the NSSP Community of Practice. Using NSSP data as well as other surveillance systems, CDC staff and state and local public health professionals are working to provide timely, accurate data so that decision-makers and the public can make informed health choices.

Information on other surveillance systems that are used to track flu is available, along with how those systems are being used to track H5N1.

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