What the CDC is doing

15 August 2024 – CDC uses multiple data sources to assess and respond to emerging health threats such as avian influenza A(H5N1) virus. This virus has caused confirmed infections in several animal species, including poultry and dairy cattle in the United States, as well as 13 recent human infections. CDC’s National Syndromic Surveillance Program (NSSP) collects electronic health data automatically reported by more than 6,500 U.S. health care facilities, representing nearly 80% of emergency department (ED) visits in all 50 states, the District of Columbia, and Guam. These data are available within 24 hours of a patient’s visit to a participating ED. They complement other surveillance systems, but their timeliness can provide public health authorities with early warning of potentially concerning levels of illness, enabling further investigation and action.

CDC’s NSSP helps public health departments track both symptoms and diagnoses to identify unusual disease manifestations, such as an increase in flu activity. In addition to tracking respiratory illnesses, emergency department data can also be used to monitor drug overdoses, heat-related illnesses, tick bites, and more. More than 1,400 state and local public health partners use this data through the NSSP Community of Practice to inform emergency response, track health resource utilization, and/or connect people with preventive measures, among other things.

Using emergency department data to detect potential local flu surges during the response to H5N1 avian influenza

As part of our ongoing efforts to monitor the impact of the avian influenza A (H5N1) outbreak, CDC staff are monitoring NSSP data and reporting the weekly percentage of all emergency department visits that are related to flu diagnoses. It is important to note that these visits include people with any flu diagnosis and are not specific to avian influenza H5N1. However, monitoring all flu diagnoses helps determine flu activity in a given area and provides an early indicator of changes. This helps public health staff determine where to look more closely when unexpected increases in flu activity occur.

CDC routinely uses several influenza indicators – including syndromic surveillance – to track influenza activity. Because such a high percentage of U.S. emergency department patients contribute to the NSSP, increases in reported emergency department visits due to influenza infection can be more easily detected at a more granular geographic level than with other surveillance systems. Local health departments can view local jurisdiction data. CDC, which supports local, state, regional, and national response, considers several indicators when there is an increase in reported visits due to influenza.

Tracking the virus

CDC monitors influenza virus activity and cases year-round. State and local health professionals can use the surveillance data CDC provides to them and the public to better understand the avian influenza A(H5N1) situation. CDC and health departments work together to monitor influenza activity using data 1 from laboratories, outpatient facilities, hospitals and registry offices.

CDC’s ongoing investment in improving syndromic surveillance unites health departments in tracking the spread of disease that crosses jurisdictional boundaries. When the agency identifies unusual trends or data anomalies, CDC’s NSSP consults with state or local health departments through the NSSP Community of Practice. Using NSSP data as well as other surveillance systems, CDC staff and state and local health experts work to provide timely and accurate data so decision makers and the public can make informed health decisions.

Information is available on other surveillance systems used to track flu, and how these systems are being used to track H5N1.

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