On 15 November 2025, WHO was notified of the 71st confirmed human case with influenza A(H5) since early 2024 in the United States of America— the first human case reported in the United States of America since February 2025. On 20 November, U.S. Centers for Disease Control and Prevention (CDC) laboratory sequencing verified the virus as influenza A(H5N5), representing the first globally reported human case caused by an influenza A(H5N5) virus. The investigation by health authorities in the United States of America is ongoing.
Contact tracing identified no further cases amongst contacts, and there is currently no evidence of human-to-human transmission. Due to the constantly evolving nature of influenza viruses, WHO continues to stress the importance of global surveillance to detect and monitor virological (including genomics), epidemiological and clinical changes associated with emerging or circulating influenza viruses that may affect human health and timely virus sharing for risk assessment. Based on available information, the WHO currently assesses the overall public health risk posed by A(H5) viruses as low. However, for individuals with occupational risk of exposure, the risk of infection is considered low to moderate.
The patient was an adult with underlying medical conditions residing in Washington State. The patient developed symptoms including fever during the week ending 25 October 2025. During the week ending 8 November 2025, the patient was hospitalized with a serious illness and subsequently died on 21 November.
Respiratory specimens collected at the healthcare facility tested positive for influenza A virus by RT-PCR and were presumptive positive for influenza A(H5) at the University of Washington. The specimens were sent to the Washington State Public Health Laboratory, where influenza A (H5) was confirmed using the CDC influenza A(H5) assay. The sample was received at the CDC on 19 November. Sequencing conducted at the University of Washington and at the CDC indicated this was an influenza A (H5N5) virus belonging to the H5 haemagglutinin (HA) clade 2.3.4.4b[1].
Public health investigation revealed that the patient kept backyard poultry and domestic birds. Additional epidemiological investigations are under way and include active monitoring of anyone who was in close contact with the patient. Animal influenza viruses typically circulate within animal populations, but some have the potential to infect humans. Human infections are predominantly acquired through direct contact with infected animals or exposure to contaminated environments. Based on the original host species, influenza A viruses can be categorized such as avian influenza, swine influenza, and other animal-origin influenza subtypes.
Human infection with avian influenza viruses may result in a spectrum of illness, ranging from mild upper respiratory tract symptoms to severe, life-threatening conditions. Clinical manifestations may include conjunctivitis, respiratory, gastrointestinal symptoms, encephalitis (brain swelling), and encephalopathy (brain damage). In some cases, asymptomatic infections with the virus have been reported in individuals with known exposure to infected animals and environments.
A definitive diagnosis of human avian influenza infection requires laboratory confirmation. WHO regularly updates its technical guidance on the detection of zoonotic influenza, utilizing molecular diagnostic methods such as RT-PCR. Clinical evidence indicates that certain antiviral agents, particularly neuraminidase inhibitors (e.g., oseltamivir, zanamivir), have been shown to shorten the duration of viral replication and improve patient outcomes in some cases. This antiviral agent should be administered within 48 hours of symptom onset.
High pathogenicity avian influenza A(H5) clade 2.3.4.4b A (H5N5) viruses have been detected in North America in wild birds and wild mammals since at least 2023.[2] This is the first laboratory-confirmed human infection with an influenza A(H5N5) virus in the United States of America and reported globally.
WHO risk assessment
Human infections with avian influenza A(H5) viruses are considered unusual, as A(H5) viruses remain primarily avian influenza viruses. However, in rare cases, individuals exposed to infected animals or contaminated environments can become infected with A(H5) viruses. Influenza A(H5N5) viruses are detected in birds, including wild birds and domestic poultry, and sometimes in non-human mammals. When avian influenza viruses circulate in poultry populations, there is an inherent risk of human infection through exposure to infected birds or contaminated environments. As such, sporadic human cases are expected. The case had underlying conditions and subsequently died. The investigation by health authorities in the United States of America is ongoing and included contact tracing which identified no further cases amongst contacts, and there is currently no evidence of human-to-human transmission.
This is the 71st confirmed human case of A(H5) in the United States of America since early 2024, and the first since February 2025. To date, no human-to-human transmission has been identified in any of the A(H5) cases reported in the United States of America. From a global perspective, while a few events with limited human-to-human transmission of zoonotic influenza A(H5) have been described between 1997 and 2007, sustained human-to-human transmission has not been detected to date. Based on available information, the WHO currently assesses the overall public health risk posed by A(H5) viruses as low. However, for individuals with occupational risk of exposure, the risk of infection is considered low to moderate. The risk assessment will be updated as needed, based on any new epidemiological or virological information related to this event.
- Centers for Disease Control and Prevention (CDC). H5 Bird Flu: Current Situation. Available from: https://www.cdc.gov/bird-flu/situation-summary/index.html
- Centers for Disease Control and Prevention (CDC). Weekly US Influenza Surveillance Report: Key Updates for Week 46, ending November 15, 2025. Available from: https://www.cdc.gov/fluview/surveillance/2025-week-46.html