August 16, 2024 – CDC continues to respond to the public health challenge posed by a multistate outbreak of avian influenza A(H5N1) virus, or "H5N1 bird flu," in dairy cows, poultry and other animals in the United States. CDC is working in collaboration with the U.S. Department of Agriculture (USDA), the Food and Drug Administration (FDA), Administration for Strategic Preparedness and Response (ASPR), state public health and animal health officials, and other partners using a One Health approach. Since April 2024, 13 human cases of avian influenza A(H5) virus infection have been reported in the United States. Four of these cases were associated with exposure to sick dairy cows and nine were associated with exposure to avian influenza A(H5N1) virus-infected poultry. 1 2 Based on the information available at this time, CDC´s current assessment is that the immediate risk to the general public from H5 bird flu remains low. On the animal health side, USDA is reporting that 191 dairy cow herds in 13 U.S. states have confirmed cases of avian influenza A(H5N1) virus infections in dairy cows as the number of infected herds continues to grow. USDA reports that since April 2024, there have been A(H5) detections in 35 commercial flocks and 21 backyard flocks, for a total of 18.68 million birds affected.

Among other activities previously reported in past spotlights and ongoing, recent highlights of CDC´s response to this include:

Serology Update
CDC continues to work with Michigan and Colorado to conduct avian influenza A(H5N1) seroprevalence investigations among dairy farm workers with potential exposure to infected cattle. Specimens from both states will be tested by CDC for the presence of neutralizing antibodies to these viruses, a sign of prior infection. In preliminary findings from the Michigan-led study released on July 19 (CDC A(H5N1) Bird Flu Response Update, July 19, 2024), no antibodies specific to avian influenza A(H5N1) viruses were detected in any of the blood samples collected in June 2024 from 35 people who work on dairy farms in Michigan with herds that were confirmed positive for HPAI A(H5N1) virus. Michigan has since completed enrollment of additional farms in the state for a second phase of the study. CDC is working with both states to interpret and share results of these studies as they become available.

Laboratory Update
Antigenic characterization of the viruses isolated from five of the nine human cases in Colorado poultry workers has shown that the viruses remain antigenically similar to two previously developed candidate vaccine viruses (CVVs) developed by CDC and available to manufacturers for vaccine production. Virus was not recoverable from the four remaining Colorado cases, so antigenic characterization could not be performed. Testing has also shown that the viruses are antigenically similar to previous clade 2.3.4.4b viruses isolated from dairy farm workers. Results of antiviral susceptibility testing of the viruses are pending.

CDC also continues to meet with commercial laboratories to discuss H5 assay licensing agreements and interest in development of commercial H5 tests. CDC´s Technology Transfer Office and the Influenza Division are actively pursuing licensing agreements with multiple companies, and several have been completed.

Outreach Update
CDC continues to support outreach to farm workers. This includes specific outreach to farm workers in affected counties through Meta (Facebook and Instagram) and digital display and audio (Pandora). CDC also is running advertisements on local radio stations about 30 times each week to reach people who may not be on social media platforms. These resources provide information in English and Spanish about potential risks of avian influenza A(H5N1) infection, recommended preventive actions, symptoms to be on the lookout for, and what to do if they develop symptoms. Outreach metrics are summarized in Table 1 (below).

Surveillance Update
CDC has posted a spotlight describing the National Syndromic Surveillance Program (NSSP) and its role in supporting CDC´s response to avian influenza A(H5N1). One of multiple surveillance tools CDC uses year-round, the NSSP collects, analyzes, and shares electronic health record data received 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. CDC uses syndromic surveillance in partnership with participating state and local health departments to capture data quickly, identify unusual trends in flu activity, improve situational awareness, and inform decision-making. 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. 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 continues to support states that are monitoring people with exposure to cows, birds, or other domestic or wild animals infected, or potentially infected, with avian influenza A(H5N1) viruses. To date, more than 4,500 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 230 people who have developed flu-like symptoms have been tested as part of this targeted, situation-specific testing. Testing of exposed people who develop symptoms is happening at the state or local level, and CDC conducts confirmatory testing. Since February 25, 2024, more than 40,900 specimens have been tested at public health labs that would have detected avian influenza A(H5) or other novel influenza viruses. More information on monitoring can be found at Symptom Monitoring Among Persons Exposed to HPAI.

CDC also continues to monitor flu surveillance data using CDC´s enhanced, nationwide summer surveillance strategy, especially in areas where avian influenza A(H5N1) viruses have been detected in dairy cows or other animals, for any unusual trends, including in flu-like illness, conjunctivitis, or influenza virus activity.

Overall, for the most recent week of data, CDC flu surveillance systems show no indicators of unusual flu activity in people, including avian influenza A(H5N1) viruses.