This report provides an update to the April 26, 2024, report to include three additional sporadic human cases (1 in Australia and 2 in the United States) and recent activity in wild birds, poultry, and other animals, including the multi-state outbreak in U.S. dairy cattle, and updated information on monitoring for human infections with highly pathogenic avian influenza A(H5N1) virus infections in the United States. CDC continues to believe that the overall risk to human health associated with the ongoing outbreaks of highly pathogenic avian influenza A(H5N1) viruses has not changed and remains low to the U.S. general public at this time.

Executive summary
A small number of sporadic human cases of highly pathogenic avian influenza (HPAI) A(H5N1) have been identified worldwide since 2022, amidst a panzootic of these viruses in wild birds and poultry. Nearly all human cases reported globally since 2022 were associated with poultry exposures, and no cases of human-to-human transmission of HPAI A(H5N1) virus have been identified. Three human cases of HPAI A(H5N1) virus infection in dairy farm workers were reported during April and May 2024 in the United States and were attributed to exposures to dairy cattle. One previous human case was detected in the United States in 2022 during poultry culling work. In a few cases, the source of exposure to HPAI A(H5N1) virus was unknown. To date, HPAI A(H5N1) viruses currently circulating most commonly in birds and poultry, with spillover to mammals and humans, do not have the ability to efficiently bind to receptors that predominate in the human upper respiratory tract. This is a major reason why the current risk to the public from HPAI A(H5N1) viruses remains low. However, because of the potential for influenza viruses to rapidly evolve and the wide global prevalence of HPAI A(H5N1) viruses in wild birds and poultry outbreaks and following the identification and spread among dairy cattle in the United States, additional sporadic human infections are anticipated. Continued comprehensive surveillance of these viruses in wild birds, poultry, mammals, and people worldwide, and frequent reassessments are critical to determine the public health risk, along with ongoing preparedness efforts.

Key Points
CDC is actively working on the domestic outbreak of clade 2.3.4.4b HPAI A(H5N1) viruses in wild birds, with outbreaks in poultry and backyard flocks, and infections of other animals, including dairy cattle. Response activities include conducting surveillance among people with relevant exposures and preparing for the possibility that contemporary HPAI A(H5N1) viruses gain the ability for increased transmissibility to and among people.
CDC, along with state and local public health partners, continues to monitor people in the United States who have been exposed to infected birds, poultry, dairy cattle, or other animals for 10 days after their last exposure. To date, more than 9,000 people in 52 jurisdictions have been monitored since 2022.
H5 candidate vaccine viruses (CVVs) produced by CDC are expected to provide good protection against current clade 2.3.4.4b HPAI A(H5N1) viruses detected in birds and mammals, including dairy cattle. These H5 CVVs are available and have been shared with vaccine manufacturers.
Because influenza viruses are constantly changing, CDC performs ongoing analyses of HPAI A(H5N1) viruses to identify genetic changes that might allow for: spread more easily to and between people, more serious illness in people, reduced susceptibility to antivirals, changes in the sensitivity of diagnostic assays, or reduced neutralization of the virus by vaccine induced antibodies. To date, few genetic changes of public health concern have been identified in HPAI A(H5N1) viruses circulating in wild birds and poultry worldwide and detected in dairy cattle in the United States.
Currently, HPAI A(H5N1) viruses circulating in birds and U.S. dairy cattle are believed to pose a low risk to the general public in the United States; however, people who have job-related or recreational exposures to infected birds or mammals are at higher risk of infection and should take appropriate precautions outlined in CDC guidance.
Comprehensive surveillance and readiness efforts are ongoing, and CDC continually takes preparedness measures to be ready in case the risk to people from HPAI A(H5N1) virus or from other novel influenza A viruses changes.