The first human H5N5 case ever recorded was fatal, and it came from a backyard flock

Avian influenza subtypes are named for two surface proteins, and the pairings matter less to the public than they do to virologists — until one of them turns up in a person for the first time.

On 15 November, the World Health Organization was notified of a confirmed human infection with influenza A(H5) in the United States, the first reported in that country since February 2025 and the 71st since early 2024 [s1]. On 20 November, sequencing at the US Centers for Disease Control and Prevention identified the virus as influenza A(H5N5) — the first human case of that subtype reported anywhere in the world [s1].

The patient was an adult with underlying medical conditions living in Washington State [s1]. Symptoms including fever began during the week ending 25 October. The patient was hospitalised with serious illness during the week ending 8 November, and died on 21 November [s1].

How it was found

Respiratory specimens collected at the healthcare facility tested positive for influenza A by RT-PCR and were presumptive positive for influenza A(H5) at the University of Washington [s1]. Confirmation followed at the Washington State Public Health Laboratory using the CDC influenza A(H5) assay, with the sample reaching the CDC on 19 November [s1]. Sequencing at both the University of Washington and the CDC identified an influenza A(H5N5) virus belonging to H5 haemagglutinin clade 2.3.4.4b [s1].

Clade 2.3.4.4b is the lineage behind the global panzootic in wild birds and poultry, and the one behind the dairy cattle outbreak in the United States. A(H5N5) viruses within it have been detected in North America in wild birds and wild mammals since at least 2023 [s1]. What had not happened before is a human case.

Public health investigation found that the patient kept backyard poultry and domestic birds [s1]. Contact tracing identified no further cases among contacts, and WHO reports no evidence of human-to-human transmission [s1].

The scale of the monitoring behind one case

The response detail in WHO's notice is the part worth sitting with. Since March 2024, at least 30,100 people have been monitored in the United States after exposure to infected animals, and at least 1,260 have been tested [s1].

The CDC recommends that state and local health departments monitor people exposed to birds or other animals suspected of avian influenza infection for signs and symptoms for up to 10 days after last exposure, and that anyone developing respiratory illness or conjunctivitis be tested for influenza [s1].

That is the machinery that turns a fatal pneumonia in one adult into a globally notifiable finding about a novel subtype. All human infections with a novel influenza A subtype are notifiable under the International Health Regulations, and states parties must notify WHO within 24 hours of a laboratory-confirmed case — evidence of illness is not required for the report [s1].

What is happening in birds

Ten days after WHO's notice, a joint assessment from the European Food Safety Authority, the European Centre for Disease Prevention and Control and the EU reference laboratory described a bird season that is not normal.

Between 6 September and 28 November 2025, there were 2,896 detections of highly pathogenic avian influenza A(H5) virus in birds across 29 European countries — 442 in domestic birds and 2,454 in wild birds [s2]. The assessment describes the magnitude and geographical extent as unprecedented for that time of year, particularly in wild birds, with large numbers of waterfowl affected and mass mortality of common cranes along their migratory routes [s2].

Because virus levels in wild birds were high and environmental contamination correspondingly widespread, most poultry outbreaks were primary — indirect contact with wild birds was the most likely source rather than spread between farms [s2]. Turkeys were proportionally the most affected poultry species, and outbreaks were again reported in vaccinated ducks [s2]. Detections in mammals rose slightly among foxes and domestic cats [s2].

Over roughly the same period, 9 September to 28 November, 19 human infections with avian influenza viruses including two deaths were reported in four countries: three A(H5N1) cases with one death in Cambodia, 14 A(H9N2) cases in China, one A(H5N2) case in Mexico, and the fatal A(H5N5) case in the United States [s2]. All five of the A(H5) human cases reported exposure to poultry or a poultry environment before illness [s2].

The risk assessment, stated as WHO states it

WHO assesses the overall public health risk posed by A(H5) viruses as low, and the risk of infection for people with occupational exposure as low to moderate [s1]. The European assessment reaches a matching conclusion for A(H5N1) clade 2.3.4.4b in Europe: low for the general public in the EU and European Economic Area, low to moderate for those occupationally or otherwise exposed [s2].

No human-to-human transmission was documented in the European reporting period [s2]. WHO notes that while a few events of limited human-to-human transmission of zoonotic A(H5) were described between 1997 and 2007, sustained transmission between people has not been detected to date [s1].

There is a gap in the preparedness picture that both documents leave visible. There are no readily available human vaccines against influenza A(H5); candidate vaccine viruses have been selected against several A(H5) clades for pandemic preparedness, and existing seasonal influenza vaccines are unlikely to offer protection against avian A(H5) viruses on current data [s1].

What this does not mean

A first-ever human case of a subtype is a surveillance milestone, not an epidemiological turn. One case with a known animal exposure, no secondary cases, and no evidence of transmission between people is what sporadic zoonotic spillover looks like, and WHO says sporadic cases are expected where these viruses circulate in poultry [s1].

The subtype designation also does not by itself carry information about severity. This case was fatal in a person with underlying conditions; a single case cannot establish a case fatality ratio, and mild and asymptomatic A(H5) infections have been reported in people with known exposure [s1].

WHO does not recommend special traveller screening at points of entry, and recommends no restrictions on travel to or trade with the United States on the basis of this event [s1].

What to watch

Whether any further A(H5N5) human cases are reported, whether European detections in wild birds continue at the current pace into the winter, and whether the rise in detections among foxes and domestic cats extends further into mammals [s1][s2].

This article is informational and is not medical advice.

Sources

• [s1] World Health Organization. "Avian Influenza A(H5N5) - United States of America." Disease Outbreak News, 5 December 2025. https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON590
• [s2] European Food Safety Authority, European Centre for Disease Prevention and Control, European Union Reference Laboratory for Avian Influenza, et al. "Avian influenza overview September–November 2025." EFSA Journal, 23(12), published online 18 December 2025. https://doi.org/10.2903/j.efsa.2025.9834