Preprint: A lethal human H5N5 influenza virus isolate exhibits low pandemic risk traits - CDC NCIRD Risk Assessment (Aug 12th)

[Michael Coston]

Preprint: A lethal human H5N5 influenza virus isolate exhibits low pandemic risk traits




Credit: Multiple transatlantic incursions of HPAI
clade 2.3.4.4b A(H5N5) virus into North America
and spillover to mammals

#19,252

We've been following HPAI H5N5's spread in birds and small mammals - first in Europe, then in Canada and the United States - for several years, but last November we saw the first (and fatal) documented infection of a human with HPAI H5N5 in a elderly backyard keeper of poultry in Washington State.

Like H5N1, H5N5 isn't a single virus, but rather represents a growing array of genetically distinct genotypes which share related H5 and N5 surface proteins​

The virus detected in the Washington State patient is similar to - but not an exact match - to the H5N5 viruses that have infected cats in Iceland, raccoons in Canada, and Polar Bears on Svalbard Island.

Last March, Viral Creep: H5N5 Update, we looked at its evolution as it has spread across the United States; shifting from almost exclusively EA H5N5 prior to last fall, to largely EA/AM H5N5 since November.​

Based on USDA Data - Graph created with Gemini

The USDA defines these two strains as:

EA = Eurasian; AM = North American; the EA H5 (2.3.4.4) viruses are highly pathogenic to poultry.​

EA/AM: reassortant of H5 goose/Guangdong and North American wild bird lineage​

I mention this distinction because today's preprint focuses exclusively on the Washington State case's isolate, which they describe as having `. . . a clade 2.3.4.4b hemagglutinin and internal gene segments cluster with currently circulating Eurasian lineage clade 6 (EA6) strains'.

Since late 2025 most U.S. H5N5 isolates have been classified as EA/AM, indicating reassortment with local (North American) LPAI viruses.​

Whether this makes any practical difference in their pandemic risk is unknown, but the findings of this - or any other study - may not hold true across the full, and continually expanding range of H5N5 viruses.

Indeed, the authors cite another study from 2024 (the above mentioned Cell Reports by Erdelyan et al.) which reported A(H5N5) viruses demonstrated rapid 100% mortality and some transmission in ferrets.​

But more reassuringly - at least in the case of the isolate retrieved from the Washington State patient - that virus displayed relatively low pandemic risk traits:

• It replicated relatively poorly (compared to seasonal H1N1) in upper respiratory cell cultures,
• It has a more avian-like pH inactivation point (about 5.5) compared to human-adapted flu strains
• And - unlike in the Erdelyan et al. study- infected ferrets did not infect healthy co-housed ferrets.

While reassuring, the old adage applies. When you've seen one novel flu virus, you've only seen one novel flu virus.

Due to its length, I've only posted the abstract and some excerpts. Follow the link to read it in its entirety.

A lethal human H5N5 influenza virus isolate exhibits low pandemic risk traits
Michelle N. Vu, Grace E. Quirk, Alexis E. Smathers, Kaitlyn Busfield-Thomason, Alaina L. Dorazio, Katelyn R. Domke, G. M. Humber, John Sembrat, Anita K. McElroy, Valerie Le Sage, Seema S. Lakdawala
doi: https://doi.org/10.64898/2026.07.20.739507
This article is a preprint and has not been certified by peer review

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Abstract

In fall of 2025, a fatal infection of highly pathogenic avian influenza (HPAI) virus H5N5 occurred. To define the risk of this emerging virus to humans, we performed a comprehensive analysis based on our established triage. Serological analysis revealed that humans across all birth years had no detectable neutralizing antibodies to this H5N5 isolate.​

Further characterization revealed a lack of phenotypic signatures associated with epidemiologically successful influenza viruses in humans, including reduced replication in human airway cells and an avian-like pH of inactivation.​

Additionally, assessment of H5N5 in ferrets revealed a lack of direct contact transmission and moderate disease severity. H5N5 infection in ferrets with prior immunity against the 2009 H1N1 pandemic strain resulted in fewer clinical signs and reduced viral shedding. Together our data suggest that the current H5N5 HPAI lineage poses a low pandemic risk.

Importance

HPAI H5N5 viruses have caused widespread infection and death in avian species, and characterizing their pandemic risk traits is critical to understanding the threat posed to humans. In this work we analyzed an isolate that resulted in a human fatality in 2025.​

We found that this strain lacks many key features of influenza viruses with epidemiological success in humans including reduced growth in human lung cultures, a pH of inactivation less than 5.0, and lack of transmission to cohoused recipient ferrets. Prior immunity with seasonal H1N1 strain also reduced the viral load and disease burden of the virus.​

Taken together, these data suggest that currently circulating H5N5 poses a low risk to humans but highlights the importance of phenotypic characterizations for future risk assessments as the virus evolves in wild birds.​

(SNIP)

Discussion

The H5N5 (A/Washington/2148/2025) virus that caused a lethal human infection in 2025 was characterized for its pandemic potential with in vitro characterization of H5N5 revealing no neutralizing antibodies across a panel of human sera, indicating widespread susceptibility in the population.​

However, in vivo assessment indicated that prior H1N1pdm09 immunity reduced viral burden and disease signs during H5N5 infection, suggesting that there is some protection in the human population from prior IAV infection. This observation is of particular interest given evidence that cross-reactive NA immunity to N1 from human seasonal H1N1 strains was protective against H5N1 infections (19–21), suggesting that there is likely another mechanism of action since the NA proteins are distinct subtypes.

In contrast to our data, characterization of two closely related (>99% nucleotide identity) H5N5 strains by Erdelyan et al. demonstrated high mortality in ferrets after infection (6). These two representative strains were isolated from a bird (crow) and a mammal (raccoon) and displayed a preference for binding avian influenza virus receptor, 3’SLN-linked sialic acid (6). Sequence comparison of the crow and racoon strains as well as the lethal human H5N5 isolate characterized herein indicates that there are no differences at canonical mammalian adaptive sites (Supplemental Table 1). The raccoon strain had modest transmission to two of three cohoused recipient ferrets, with one displaying clinical disease and another one only seroconverting, whereas we did not observe transmission to our cohoused recipients (Figure 2B).​

Differences between the study outcomes could be due to the dose of the crow and racoon inoculums, which was 10x higher, differences in virus strains, and the duration of the cohoused exposure. A limitation of our study includes the assessment of only one H5N5 isolate, but there are currently no other human H5N5 isolate to compare to. Further characterization of circulating H5N5 strains is warranted to identify potential strains of higher pandemic potential.​

H5N5 continues to circulate in wild bird populations in the US and Europe (22, 23) and thus will continue to evolve and adapt to new hosts. Ongoing surveillance and characterization studies are therefore warranted given its prevalence. Our in vitro studies provide guidance for targeted risk assessment studies in the future. We observed that H5N5 was stable in the environment at mid-range humidity levels and had NA activity similar to the H1N1pdm09 strain. However, H5N5 pH of inactivation was higher than 5.0, and this virus had poor replication capacity in HBE cultures.

Rather than full characterizations of evolving H5N5 strains, future studies could focus on pH of inactivation and HBE growth. Emerging isolates with higher replication fitness in HBE cultures and pH of inactivation ≤5.0 will be of higher risk and could then be tested for ferret transmission.
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https://afludiary.blogspot.com/2026/07/preprint-lethal-human-h5n5-influenza.html
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[Michael Coston]

J. Virology: Risk assessment of avian influenza A(H5N5) virus from the first human case using the ferret model


Credit: Multiple transatlantic incursions of HPAI
clade 2.3.4.4b A(H5N5) virus into North America
and spillover to mammals


#19,290

Last November we saw the first, and as far as we know - only - human infection with emerging H5N5 subtype of avian flu in a resident of Washington State who had contact with backyard poultry and/or wild birds.

We've been following HPAI H5N5's spread in birds and small mammals - first in Europe, then in Canada and the United States - for several years, including a report last month; Preprint: HPAI H5N5 in a Polar Bear and Atlantic Walrus, Svalbard, 2026, with Widespread Seroconversion in Polar Bears.​

In late July we saw a Preprint: A lethal human H5N5 influenza virus isolate exhibits low pandemic risk traits, which somewhat reassuringly found the virus displayed relatively low pandemic risk traits:

• It replicated relatively poorly (compared to seasonal H1N1) in upper respiratory cell cultures,
• It has a more avian-like pH inactivation point (about 5.5) compared to human-adapted flu strains
• And - unlike in the Erdelyan et al. study (which reported rapid 100% mortality and some transmission in ferrets) - they reported that infected ferrets did not infect healthy co-housed ferrets.

Today we've a study published in the Journal of Virology by researchers at the CDC's Influenza Division, NCIRD, which is slightly less reassuring.

• While they still found no evidence of airborne transmission in the ferret model, they report limited direct-contact transmission in 2 of 3 co-housed ferrets.

• And while the preprint reported moderate disease severity in ferrets, today's study reports uniformly severe/fatal disease, with rapid systemic spread.

• And where the preprint characterized WA/2148 as having reduced replication in human bronchial epithelial cultures, the CDC study instead found robust replication in polarized Calu-3 airway cells at both 33°C and 37°C.

There were differences in both the methods and materials used in these two studies, which could account for these disparities. Today's study even notes that primary human airway cultures or organoids would provide a more physiologically relevant comparison than the proxies used by these two studies.

And as always, the caveat remains that we can only draw limited conclusions from the evaluation and characterization of a single isolate, since even minor differences between strains or genotypes can make significant changes to its behavior.​

Due to it length, and technical nature, I've only reproduced the abstract and some excerpts from the study below. Follow the link to read it in its entirety. I'll have a brief postscript after you return.

Risk assessment of avian influenza A(H5N5) virus from the first human case using the ferret model
Authors: Joanna Pulit-Penaloza , Jessica A. Belser, Nicole Brock, Troy J. Kieran, Claudia Pappas, Hui Zeng, Xiangjie Sun, Juan A. De La Cruz, Yasuko Hatta, Han Di, C. Todd Davis , Taronna R. Maines
https://doi.org/10.1128/jvi.00856-26

PDF/EPUB

ABSTRACT

The incursion of Eurasian-origin genotype A6 A(H5N5) virus into North America expanded the genetic diversity among North American highly pathogenic avian influenza viruses and heightened concern about zoonotic risk. Following a fatal human infection with the A(H5N5) virus A/Washington/2148/2025, viral replication was assessed in polarized human bronchial epithelial cells, and pathogenicity, transmissibility in direct contact and respiratory droplet models, and airborne virus shedding were evaluated in ferrets to inform pandemic risk assessment.

A(H5N5) displayed robust replication in Calu-3 cells at 33°C and 37°C, showing kinetics and peak titers comparable to those of contemporary genotype B3.13 and D1.1 A(H5N1) viruses. In ferrets, A(H5N5) replicated efficiently in the respiratory tract, disseminated to extrapulmonary tissues, and caused fatal disease in all inoculated animals. Airborne transmission was not observed, and infrequent, low-level detection of virus in air samples paralleled that of A(H5) viruses that are not transmissible via air in ferrets.

In a direct contact model, limited transmission was detected within 4 days of exposure, with evidence of lower respiratory tract replication in contact animals. These findings indicate that the A(H5N5) virus has the capacity for robust replication in an airway epithelial cell line and can cause severe systemic infection and mortality in ferrets but has not acquired adaptations for airborne spread in mammals. Collectively, these results underscore heterogeneity among clade 2.3.4.4b A(H5Nx) viruses in North America and the need for genotype-by-genotype evaluation of newly emerged viruses to understand public health risk.

IMPORTANCE
The emergence of Eurasian-origin genotype A6 highly pathogenic avian influenza A(H5N5) virus in North America has increased viral diversity and raised concerns about zoonotic and pandemic risk. In this study, we evaluated the replication kinetics, pathogenesis, and transmission of A/Washington/2148/2025 A(H5N5) virus, which was isolated from the first reported human infection with this influenza virus subtype, using polarized human bronchial epithelial cells and the ferret model.

The A(H5N5) virus replicated efficiently in vitro at temperatures representative of the upper and lower respiratory tracts and caused fatal systemic disease in inoculated ferrets. Limited transmission was observed during 4 days of direct contact. Airborne virus detection was infrequent and did not result in airborne transmission. These findings show that A(H5N5) virus can replicate robustly in mammalian cells and cause severe disease but lacks adaptations supporting efficient airborne spread, informing assessment of the pandemic risk posed by genotype A6 influenza viruses.​

(SNIP)

DISCUSSION​

The rapid expansion of clade 2.3.4.4b A(H5N1) influenza viruses into novel mammalian hosts has necessitated comprehensive risk assessments of representative viral isolates from different genotypes associated with human infection and detections in mammalian hosts (19, 20).​

Collectively, these studies have revealed that clade 2.3.4.4b A(H5N1) viruses can replicate efficiently in a diversity of mammalian epithelial cell types derived from the human respiratory tract (21–23), cause severe and fatal disease in animal models, and have varied capacity for airborne transmission to susceptible contacts (4–6, 18, 24).​

The detection of A(H5N5) virus associated with fatal human disease in 2025 underscores the confounding unpredictability of A(H5) viruses, and their continued threat to human health.​

(SNIP)

Overall, the WA/2148 A(H5N5) virus showed the capacity to cause severe, systemic mammalian infection and robust replication in polarized human airway epithelial cells, characteristics similar to those of other contemporary A(H5Nx) viruses. However, the virus lacked clear adaptation for efficient airborne spread, indicating limited adaptation to a mammalian host. Our findings underscore substantial heterogeneity in pathogenesis and transmission among clade 2.3.4.4b A(H5Nx) viruses, supporting continued genotype-by-genotype evaluations.​

(Continue . . .)

As a pandemic virus, H5N5 may not be ready for prime time, but as a zoonotic threat it already appears to be capable of robust replication in human airway cells and causing severe systemic disease in mammals.

All of which makes it a virus very much worth our continued attention.


https://afludiary.blogspot.com/2026/08/j-virology-risk-assessment-of-avian.html
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