October 11, 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, 20 human cases of avian influenza A(H5) virus infection have been reported in the United States; 21 in total since 2022. Ten of these cases were associated with exposure to H5N1 bird flu-infected poultry and nine were associated with exposure to sick or infected dairy cows 12. This includes six cases in California, two of which were confirmed by CDC on Thursday, October 3, two on Wednesday, October 9, and two on Thursday, October 10. All California cases occurred in dairy workers on affected farms. All of the California cases are from different farms, except for one case reported this week, which was from a farm that had a case reported previously. The two cases from the same affected farm worked on different parts of the farm and are not close contacts of each other. The epidemiology of the situation continues to suggest sporadic instances of animal-to-human spread. All six California cases are reported to have experienced mild symptoms, including eye redness or discharge (conjunctivitis), and none were hospitalized. Additional testing is ongoing in California, and presumptive positives are being routinely forwarded to CDC for confirmatory testing. Going forward, CDC will track and report confirmed cases, by state and source of exposure, in a table on its website. The source of the exposure in one case, which was reported by Missouri on September 6, could not be determined. Serological tests of the contacts of the Missouri case are pending. CDC believes the immediate risk to the general public from H5N1 bird flu remains low, but people with exposure to infected animals are at higher risk of infection.

On the animal health side, USDA is reporting that 299 dairy herds in 14 U.S. states have confirmed cases of H5 bird flu virus infections in dairy cows. The number of affected herds continues to grow nationally, fueled by increases in California. Last week, USDA reported 56 affected dairy herds in California as of October 3. That number increased to 100 by October 11. USDA reports that since April 2024, there have been H5 detections in 36 commercial flocks and 26 backyard flocks, for a total of 18.75 million birds affected.

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

Laboratory Update
CDC has performed genetic sequencing on samples from the two human cases of H5 bird flu in California confirmed on Thursday, October 3, and one of two human cases in California confirmed on Wednesday, October 9. Efforts to sequence additional cases are in progress. The results confirm that all three viruses sequenced to date are clade 2.3.4.4b H5N1 viruses, closely related to those detected in dairy cattle. The whole genome was sequenced from one of the first two cases (A/California/135/2024) and was confirmed to be a B3.13 genotype virus.

CDC has publicly posted in GISAID and submitted to GenBank the HA, NA, and NS gene segments for A/California/134/2024 (GISAID EPI_ISL_19463619; NCBI_PQ435213- PQ435215) and the whole genome sequence for A/California/135/2024 (GISAID EPI_ISL_19463618; NCBI_PQ435216-PQ435223). Additional sequencing data will be posted as it becomes available.

While the hemagglutinins (HAs) of the three sequenced viruses contained new amino acid changes compared to closely-related candidate vaccine viruses (CVVs) – two in A/California/135/2024, three in A/California/134/2024, and three in A/California/146/2024 – none contained changes associated with increased infectivity or transmissibility among humans. CDC identified no changes associated with reduced susceptibility to neuraminidase inhibitors or polymerase acidic inhibitors, nor were there any changes associated with mammalian adaptation in other gene segments in samples from any of the cases based on currently available sequence data. CDC successfully isolated virus from samples from the first two confirmed cases in California. Attempts to isolate virus from subsequent specimens are pending. Antigenic characterization and antiviral susceptibility testing of the two viruses that have been isolated to date are underway. Antigenic characterization will inform whether existing H5 bird flu CVVs are still well-matched to these viruses.

CDC also has completed development and quality control testing of a reverse genetics-generated virus with the HA mutations (HA P136S and A156T) identified in the virus isolated from the H5N1 bird flu case in Missouri reported on September 6, 2024. Initial testing showed that the virus had reduced cross-reactivity to ferret antisera raised to viruses without these changes. To ensure optimal match with the Missouri H5N1 bird flu virus, this reverse genetics-generated virus will be used for serology testing of serum samples from the confirmed case in Missouri and close contacts.

Seasonal Flu Vaccination Initiative
CDC has allocated more than 100,000 doses of seasonal influenza vaccine to 12 states with dairy herds that have tested positive for H5 bird flu. This is part of a CDC initiative announced this summer to provide a number of supplemental free seasonal influenza vaccines to farm workers across states affected by H5 to prevent the spread of seasonal flu in these communities and safeguard public health. The 12 participating states include: California, Colorado, Idaho, Iowa, Kansas, Michigan, Minnesota, New Mexico, North Carolina, Ohio, South Dakota, and Texas. All states with dairy herds that have tested positive for H5 bird flu were invited to apply to receive additional doses of seasonal influenza vaccine – allocated specifically for farm workers – beyond what the states were already planning to provide. States will conduct screening to ensure these doses are going to farm workers and will track the number of doses given monthly by age. CDC also has awarded approximately $2 million in funding to support the operational costs of the various activities jurisdictions are implementing to encourage and facilitate the uptake of seasonal flu vaccination among farm workers. Examples include the use of mobile vaccine clinics, promotion via virtual town halls and vaccine education workshops, and partnering with local public health agencies and community-based organizations to organize mass vaccination events. While a seasonal flu vaccine does not protect against H5 bird flu, expanding access to seasonal flu vaccines among farm workers can prevent severe illness and help reduce the strain of the flu season on rural health care systems. Reducing the prevalence of seasonal flu also can help public health agencies better detect cases of H5 bird flu, should they occur. And fewer people infected with seasonal flu means fewer opportunities for the very rare possibility of co-infection with both a human seasonal virus and an avian virus.

Surveillance Update
As part of CDC´s Farmworker Enhanced Surveillance Program (FWESP), CDC is working with pharmacy networks eTrueNorth and Walgreens on a pilot program to provide free testing of symptomatic persons in California and one other state initially that have confirmed H5N1 bird flu infections in people, poultry, or livestock. The program aims to increase testing for seasonal flu and triage for H5 testing if needed, raise awareness about the symptoms and risk factors for infection with H5N1 bird flu among higher-risk populations, and determine whether certain rapid point-of-care (POC) tests identify influenza A-positive specimens when farm or dairy workers are infected with H5N1 bird flu.

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 H5N1 bird flu. To date, more than 5,000 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 240 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 53,000 specimens have been tested at public health labs that would have detected avian influenza A(H5) or other novel influenza viruses. One of the specimens tested at a public health laboratory as a part of routine surveillance was identified as presumptive A(H5) positive and was confirmed as H5N1 positive by CDC. 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 2024-2025 influenza surveillance strategy, especially in areas where H5N1 bird flu 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 H5N1 bird flu viruses.

Outreach
CDC continues to support outreach to farm workers around the topic of H5N1 bird flu. This includes targeted 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 H5N1 bird flu infection, recommended preventive actions, symptoms to be on the lookout for, and what to do if people develop symptoms. Outreach metrics are summarized in Table 1 (below). CDC also is developing an outreach strategy to raise awareness about the free seasonal flu vaccinations being offered to farm workers in affected states this season.