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.

Australia: CSIRO Confirms Qld's H5N1 - SA Reports 2 `Suspected' Local Birds

 
Confirmed Positive H5N1 in Birds (n=18)

#19,261

While there was never much doubt, we now have confirmation from Australia's National Lab CSIRO that the seabird recovered from Moreton Island  (25 miles N.E. of Brisbane) has tested positive for H5N1.

H5 bird flu confirmed in a wild seabird found at Moreton Island

A dead petrel found by Queensland Parks and Wildlife Service (QPWS) rangers at Moreton Island (Mulgulpin) has been confirmed positive for H5 high pathogenicity avian influenza (H5 bird flu) in laboratory tests by the CSIRO Australian Centre for Disease Preparedness.

The positive test is the first confirmed detection of H5 bird flu in Queensland.

There remain no detections in poultry, nor evidence of large-scale deaths in any animals in Queensland.

The risk to human health remains low.

The community is being urged to continue to report sick or dead wild birds.

Queensland has been preparing for H5 bird flu and has strong response arrangements in place. Australia also has national arrangements to respond to emergency animal disease incidents.

Australia’s first case of H5 bird flu was reported on 20 June 2026 in Western Australia, with further detections in wild seabirds in WA, SA and NSW.

Department of Primary Industries Chief Veterinary Officer Dr Allison Crook said the confirmed positive detection was expected, and the Queensland Government was well prepared to respond.

“We have known that a H5 bird flu detection in Queensland was a matter of when, not if,” Dr Crook said.

“This confirmed positive result has not impacted our poultry sector or native wildlife.

“We have further escalated our surveillance program throughout the state, and are delivering heightened H5 bird flu awareness and engagement activities to ensure industries and communities know what to do.

“There have been more than 1,000 reports from Queensland to the hotline since the first reported case on 20 June 2026, and I thank the public for their support.

“I encourage the community and industry to continue to be vigilant,” Dr Crook said.

“If you find sick or dead wild birds or wildlife, avoid contact, keep children and pets away, record what you see, and report immediately to the Emergency Animal Disease Hotline on 1800 675 888.”

Meanwhile Australia's ABC News is reporting that South Australia has recovered 2 deceased local birds (greater crested terns), and has sent samples to CSIRO for confirmation.  While not explicitly stated in the article, this generally indicates a local test has returned a positive result.

Two additional H5 bird flu cases suspected in SA, including first on a metro beach

While I haven't found any official press releases, ABC News quotes SA Environment Minister Emily Bourke as saying one of these birds was recovered just 4 km from where Australia's first local infected seabird was detected earlier this month.

They quote the SA Environmental Minister as saying:

 "Unfortunately, we are finding that these two cases are in our local [bird] species, so we need to send these tests off now to Geelong for them to have confirmation at that centralised location that they are, or are not, the H5 high strand of this disease." 

Until now, all but one detection of H5N1 in Australian seabirds have been in migratory species.  Additional spread in local birds, while not unexpected, increases the chances of H5 establishing a permanent beachhead.

Results from CSIRO are expected early next week.

 

Preprint: Within- and between-host dynamics of highly pathogenic avian influenza in domestic birds from Pennsylvania farms and live bird markets

Graphic Generated By Gemini using USDA Data
 

#19,260

Last month 3 east coast states (RI, NJ, PA) reported outbreaks of H5N1 in Live Bird Markets (LBMs), raising the tally to 14 markets across 5 states (New York, Pennsylvania, Florida, Rhode Island, and New Jersey) reporting outbreaks in 2026.

Going back 4 years we find 78 reports across 7 states (New York, Pennsylvania, Florida, Rhode Island, New Jersey, Virginia, and California). 

As mentioned 3 weeks ago in USDA Reports Another H5N1 Outbreak at a Live Bird Market (Pennsylvania) - despite 3 June outbreaks in LBMs - the USDA had reported  zero outbreaks in commercial poultry in the region in the previous 90 days. 

Last month, in One Health: Mapping reported modes of transmission of HPAI A (H5N1) to humans: A scoping review, we looked at how live bird markets have traditionally played a large role in the spread (and evolution) of avian flu viruses.  

Exposure to live birds from LBMs has often been cited as a major risk factor for human infection with avian flu - particularly in Asia -  and we've seen cases whose likely exposures were cited as simply living near, or walking past an LBM (see J. Infection: Aerosolized H5N6 At A Chinese LBM (Live Bird Market)).

All of which brings us to a new preprint, published this week, which examines how HPAI H5N1 spread and evolved inside poultry farms and live bird markets in Pennsylvania between 2023 and 2025.

Due to its length, I've only posted some excerpts. Follow the link to read it in its entirety.  I'll have more after the break.

Within- and between-host dynamics of highly pathogenic avian influenza in domestic birds from Pennsylvania farms and live bird markets

Anna S Jaeger, Elena Cruz-Adames, Stephen D Shank, Irina Chupikova, Katie Kumta, Eman Anis, Louise H Moncla
doi: https://doi.org/10.64898/2026.07.17.739193
This article is a preprint and has not been certified by peer review

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Abstract

Since late 2021, highly pathogenic avian influenza viruses (HPAI) of the H5 subtype clade 2.3.4.4b have spread across the Americas, devastating wildlife, agricultural animals, and resulting in dozens of human spillovers. National surveillance strategies generally provide only a single representative sequence per poultry outbreak, precluding fine-scale geographic transmission inference or studies of within-outbreak evolution. 

We produced high-quality deep sequence data from 46 infected Galliformes and Anseriformes sampled from commercial farm and live bird market (LBM) outbreaks in Pennsylvania from 2023-2025. We found that H5N1 viruses were introduced into Pennsylvania at least 68 independent times. 

We recover independent origins of live bird market outbreaks within the same county 3 weeks apart, and transmission between Pennsylvania LBM and New York commercial birds, suggesting high transmission risk within the Northeast live bird market distribution system. 

Analyses of within-farm variant populations show frequent variant sharing between samples from the same outbreak, suggesting that variants are propagated among epidemiologically linked infections. We identified 9 known adaptive mutations in these samples, including one instance of PB2 D701N in a LBM chicken sample, suggesting that while rare, concerning mammalian adaptive mutations can be present within these domestic outbreaks. 

Our data suggest that domestic bird outbreaks support high circulating diversity and wide transmission bottlenecks, increasing the risk of minority variants arising and propagating between infections. These data can help inform targeted biosecurity measures and better quantify the risk of viral adaptation during agricultural outbreaks.

       (SNIP)

We identified approximately 17 independent introductions into Pennsylvania that resulted in domestic infections. Most strikingly, we identify multiple distinct introductions into live bird markets in Pennsylvania among samples collected less than three weeks apart. Upon further phylogenetic analysis of transmission between Pennsylvania and New York, we present compelling evidence of transmission between live bird market settings and distributors in these states, as well as New Jersey. Live bird markets and their distribution systems are a high risk environment for transmission of HPAI given their diverse sources, mixed species, and historical lack of regulations and compliance to biosecurity measures [46,47]. 

More recently, particularly since the influx of HPAI in4 North America, stronger regulations and biosecurity measures have been implemented at the state level for live bird market systems, in particular by the Live Bird Marketing System Biosecurity Assurance Program [48]. 

Our finding of LBM transmission involving domestic birds from multiple states, along with ongoing detections in the LBMS suggest that additional interventions, improvements, or regulation may be needed.

(SNIP)

Taken together, we show here the first of its kind data from natural infections in domestic birds utilizing phylogenetic and within-host methods to quantify diversity, evolution, and transmission across domestic outbreaks. The multiple independent introductions into the state throughout the epizootic show that Pennsylvania is likely a representative model state for understanding finer-scale transmission dynamics of H5N1. 

Our results present evidence that high levels of diversity is being shared within domestic outbreaks, and particularly that continued surveillance and increased regulation of live bird market systems is important for controlling interstate spread particularly within the Northeastern United States. Future study and surveillance where samples are collected from individual domestic infections will allow for true quantification of transmission bottlenecks to further inform biosecurity and control measures in domestic settings. 

        (Continue . . . )

 

In 2016's Interventions in live poultry markets for the control of avian influenza: A systematic review Vittoria Offeddu , Benjamin J. Cowling, and J.S. Malik Peiris laid out the risks of avian influenza from live bird markets, reviewed some of the possible interventions, and concluded:

 

Highlights

• Avian influenza viruses (AIVs) can infect humans. Bird-to-human transmission is particularly intense in live poultry markets.
• Periodic rest days, overnight depopulation or sale bans of certain species significantly reduce AIV-circulation in the markets.
• Market closure would lastingly reduce the risk of animal and human infection.

While there have been numerous attempts to phase out LBMs (see 2009's China Announced Plans To Shut Down Live Poultry Markets In Many Cities), most have encountered stiff public resistance. 

Larger, more modern cities like Hong Kong have gone to great lengths to regulate and restrict live bird markets, and last year, Shanghai Banned Live Poultry Sales until at least 2027.

Currently the CDC considers the risk of human infection (at least, for the general public) from our current clade 2.3.4.4b strain of H5N1 to be low. LBMs are treated as a `manageable' risk; where rest days, mandatory testing, enhanced cleaning, and occasional short‑term shutdowns are employed rather than permanent closures.

But HPAI H5 is always evolving, as the recent introductions of the B3.13 and D1.1 H5N1 genotypes attest.  How long the risk from LBMs will remain manageable is anyone's guess. 

In the meantime, the author's of today's report suggest that - in view of the ongoing detection of avian flu in LBMs - `. . . additional interventions, improvements, or regulation may be needed'.

Queensland Reports `Presumed Positive' H5N1 Test in Migratory Seabird

 

#19,259

After going a week without any new reports of H5N1 in Australia or New Zealand, overnight Queensland's Department of Agriculture announced that samples from a deceased petrel found on Moreton Island (25 miles N.E. of Brisbane) have (preliminarily) tested positive for H5N1.

Initial testing was by the Queensland Government’s Biosecurity Sciences Laboratory (BSL). Samples have been forwarded to CSIRO’s Australian Centre for Disease Preparedness for confirmatory testing.

Typically, even when CSIRO has been unable to fully sequence a sample, Australia has been treating local test results as `likely positive'.  Two announcements from Queensland's government follows:

Queensland detects first case of H5 bird flu


The Department of Primary Industries is responding to a suspect positive case of H5 high pathogenicity avian influenza (H5 bird flu) in Queensland.

This is Queensland’s first suspect positive case of H5 bird flu in a wild seabird.

Samples from a deceased petrel found on Moreton Island have tested positive for H5 bird flu during initial testing at the Queensland Government’s Biosecurity Sciences Laboratory (BSL).

Samples have been sent to CSIRO’s Australian Centre for Disease Preparedness for confirmatory testing.

Australia’s first case of H5 bird flu was reported on 20 June in Western Australia, with further detections in wild seabirds in WA, SA and NSW.

There remain no detections in poultry, nor evidence of large-scale deaths in any animals in Queensland.

The community is being urged to continue to report sick or dead wild birds.

Department of Primary Industries Chief Veterinary Officer Allison Crook said the suspect positive detection was expected and Queensland is well prepared for the next steps.

“We have known that a H5 bird flu detection in Queensland was a matter of when, not if,” Dr Crook said.

“The Queensland Government is well prepared to respond. This suspect positive result has not impacted our poultry sector or domestic wildlife,” she said.

“We are continuing our enhanced passive surveillance program throughout the state and would encourage the community and industry to continue to be vigilant,” Dr Crook said.

“If you find sick or dead wild birds or wildlife, avoid contact, keep children and pets away, record what you see, and report immediately to the Emergency Animal Disease Hotline on 1800 675 888.”

Suspect positive case of H5 bird flu

Biosecurity Queensland has detected suspect positive H5 bird flu in a deceased wild migratory seabird found on Moreton Island (Mulgulpin).

This result is not unexpected following the recent detections of H5 bird flu in Western Australia, South Australia and New South Wales.
On this page

What we are doing
What you need to do
Actions for bird and livestock owners
Signs of bird flu in birds
More information

What we are doing

We are:

• continuing to carry out surveillance
• testing birds and animals
• working with industry, government agencies and response partners
• supporting strong biosecurity practices to help reduce the risk of spread
• keeping Queenslanders informed as the situation develops.

Queensland has been preparing for H5 bird flu and has strong response arrangements in place. Australia also has national arrangements to respond to animal disease incidents.

What you need to do

If you see sick or dead wild birds or wildlife:

• Avoid contact with sick or dead birds, wildlife and their environment. Do not touch or collect them.
• Record what you see, if it is safe. Record the number of dead or sick birds or animals, the species, location, date and time. Take a photo if you can.
• Report sick or dead birds or wildlife to the Emergency Animal Disease Hotline on 1800 675 888.

Your report will be reviewed, and we will contact you if more information is needed. Not all reports will require follow-up, sampling, or further action.

Actions for bird and livestock owners

If you own, care for or work with poultry, review your biosecurity practices now. A strong biosecurity plan helps protect your birds and your property.

Bird and livestock owners should report unusual signs of disease or sudden deaths immediately to the Emergency Animal Disease Hotline on 1800 675 888.

Poultry owners should:

• keep poultry away from wild birds
• keep feed and water where wild birds cannot access it
• keep sheds, yards, aviaries and equipment clean
• limit visitors to areas where birds are kept
• ask essential visitors if they have recently been on other properties with poultry
• wash hands before and after handling birds and eggs
• change into clean footwear before entering poultry areas
• keep good records of bird movements or sales
• report unusual signs of disease or increased deaths in birds.
• Signs of bird flu in birds

Signs can include:

• sudden death
• lack of coordination, tremors or swimming in circles
• twisted necks or other unusual posture
• inability to stand or fly
• diarrhoea
• difficulty breathing, coughing or sneezing
• swelling around the head, neck and eyes
• cloudiness or change in colour of the eyes.

More information

Stay up to date with the H5 bird flu current situation in Queensland.
Visit birdflu.gov.au for more information.

Probably Nothing . . . But It feels a Bit Like Deja Flu

 
Credit China Daily - March 2006

#19,258

Readers with extremely long memories will remember the great shuttlecock shortage of 2006, which was blamed - at least partially - on China's reported culling of roughly 200 million birds between 2003-2006 due to HPAI H5N1. 

This was back when China actually acknowledged avian flu outbreaks in their poultry or wild birds, something which rarely happens today.  The most recent FAO avian flu activity map (since Oct 1st 2025) shows almost no HPAI H5 in Mainland China. 

Of course, South Korea and Japan both saw major avian flu outbreaks, and in South Korea they are still dealing with a serious egg shortage due to the culling of 11 million birds last winter. 

While it seems reasonable to assume that China (and Russia) both saw some poultry losses from avian flu last winter, you wouldn't know it from their official reports. 

All of which brings us to a Yonhap News report overnight which reads almost beat-for-beat like the 2006 article posted above.  So much so, I had to double check to make sure it wasn't a phantom news report from the past.

Badminton Shuttlecock Prices in Japan Soar... Shortage of Chinese Duck and Goose Feathers

Posted July 23, 2026, 15:30

They report a `stagnant demand for duck meat', which they attribute to `changing Chinese diary habits', which has resulted in a growing shortage of duck and goose feathers, and is forcing a move towards using `synthetic shuttlecocks'. 

While this article provides few details, it does state: 

`However, as avian influenza (AI) has recently been rampant, disease control costs have increased due to concerns about infection, worsening the management of poultry farms and leading to a shortage of feathers, a raw material.' 

Perhaps the most important caveat is, although this article mentions avian influenza as a potential factor, that doesn't necessarily mean HPAI H5.  China continues to battle multiple avian viruses, including LPAI H9N2, H3Nx, H6Nx, and H10Nx viruses. Some recent blogs include:

Virulence: Surveillance and biological characterization of H3 subtype avian influenza viruses in Eastern China

J. Inf.: Zoonotic Threat of Novel H6N2 Avian Influenza Virus with Internal Genes Exclusively Derived from H9N2, China, 2025

Vet. Microbiology: The novel H10N3 Avian Influenza Virus Acquired Airborne Transmission Among Chickens: An Increasing Threat to Public Health

Admittedly, this reported shortage of duck feathers is a weak signal at best, but the paucity of official reports from China makes it worth our notice, since we've seen other similarly vague news items prove to be significant in the past.

In 2008 a report by Helen Branswell (then of the Canadian Press) described an early signal of the first SARS-CoV outbreak in 2002. She reported:

Surging vinegar sales in China grabbed the attention of the folks who regularly scour the globe for what might be budding disease outbreaks, like those who work for the Canadian-led Global Public Health Intelligence Network. 

"We were getting lots of rumours, like a lot of sales of vinegar," explains Dick Thompson, who was the spokesperson for the World Health Organization's communicable diseases section at the time.

“Vinegar in southern China is used as a disinfectant. And so if there’s a run on vinegar, there’d be a suspicion that there was some kind of infectious disease or a widespread belief that there is an infectious disease outbreak. So at these meetings we’d been hearing this kind of drip, drip, drip come in about that.”

While our visibility of avian flu in China is limited, it is worth noting that last November - in China MOA Announces New Guidelines to Expedite Animal Vaccine Strain Approvals - we saw a rare acknowledgement that their agricultural vaccines program (including avian flu) was in need of an overhaul. 

Whether today's report turns out to be significant remains to be seen. Like I said, it is probably nothing. 

But, it does seem curious enough to warrant a mention.

China CDC Weekly: First Human Infection with Influenza A(H1N2)v Virus — Yunnan Province, China, 2026

 

#19,257

While avian flu continues to sit at - or near the top of - most people's pandemic threat lists, swine influenza viruses (which often spread stealthily in pigs) may pose similar risks.  

Unlike many avian flu, swine variant viruses are already well adapted to a mammalian host, and in many ways porcine physiology is remarkably similar to that of humans (see The pig: a model for human infectious diseases).

Swine flu viruses are primarily H1 and H3 subtypes; both of which have a long track record of sparking human pandemics (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?).

Since 2010 we've seen more than 500 scattered reports of human infection with swine variant influenza viruses (H1N1v, H1N2v & H3N2v) in the United States, often associated with agricultural exhibits at county and state fairs.   

Many of the viruses circulating in pigs today are descendants of human flu viruses which spilled over into swine over the years. This sharing of viruses is a two-way street. 

The CDC's IRAT (Influenza Risk Assessment Tool) lists 3 North American swine viruses as having at least some pandemic potential (2 added in 2019).

H1N2 variant [A/California/62/2018]  Jul   2019   5.8  5.7 Moderate

H3N2 variant [A/Ohio/13/2017]          Jul   2019   6.6  5.8 Moderate

H3N2 variant [A/Indiana/08/2011]      Dec 2012   6.0  4.5 Moderate

In addition to the 3 North American swine-variant viruses on the CDC's IRAT list, we continue to watch the evolution of China's EA H1N1 `G4' virus, Brazil's H1N2v virus, and emerging variants (and spillovers) in Europe (see ANSES Reports A `New' Swine Flu Virus Has Taken Over Other Genotypes in France).

But most of the world only rarely bothers to test, or to share reports on, swine influenza.

Last April, in WHO Influenza at the human-animal interface (March 31st): 13 Novel Flu Infections Detailed, we saw a very busy WHO report which included two brief accounts from Yunnan Province, China on an H1N1v and an H1N2v human infection.

Swine influenza viruses in humans

Influenza A(H1N1)v, China

On 20 March 2026, China notified WHO of a laboratory-confirmed case of A(H1N1)v influenza virus infection in a child from Yunnan province. The patient had onset of illness on 30 January 2026, was hospitalized on 2 February with pneumonia, and recovered in a few days. The patient had reported exposure to domestic pigs prior to illness onset.

Influenza A(H1N2)v, China 

On 3 February 2026, China notified WHO of a laboratory-confirmed case of A(H1N2)v influenza virus infection in a child from Yunnan province. The patient had onset of mild illness on 20 January 2026, and the infection was laboratory-confirmed on 2 February 2026. The patient had reported exposure to domestic pigs prior to illness onset. This case and the one above are not epidemiologically linked.

While both follow the typical pattern of recent exposure to pigs, last week the CCDC published a more detailed review of the H1N2v case which walks back that initial assertion. Instead, they now report:

Notably, unlike many previously reported A(H1N2)v cases in which direct exposure to swine was identified as a key risk factor (8), no direct contact with poultry or livestock was reported in this patient. This finding suggests that indirect or environmental exposure may play a role in the development of infection. 

This isn't the first time we've seen swine variant influenza infection in humans without recent contact with pigs (see here, here, and here), but it is a noteworthy finding. 

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

Outbreak Reports
First Human Infection with Influenza A(H1N2)v Virus — Yunnan Province, China, 2026
Lihua Chen1, 2, Yaoyao Chen3, Xiaoyu Han3, Ruize Ni3, Ming Zeng4, Jie Deng4, Chunyuan Deng5, Guijing Chen5, Xiaoqing Fu3, Jibo He1, Chunrui Luo3,  

Summary

What is already known about this topic? 
Human infections with influenza A(H1N2)v viruses of swine origin have been sporadically reported in several countries, usually following direct or indirect exposure to pigs or contaminated environments. However, sustained human-to-human transmission has not yet been documented.

What is added by this report? 
This report describes the first laboratory-confirmed human infection with the influenza A(H1N2)v virus in China. Whole-genome sequencing showed that all eight gene segments were closely related to influenza viruses of swine origin circulating in China. No secondary human cases were identified among the close contacts, and no evidence of sustained human-to-human transmission was detected.

What are the implications for public health practice? 
This highlights the importance of routine influenza-like illness surveillance, timely whole-genome sequencing, and systematic investigation of unusual influenza A infections. Strengthened surveillance at the human-animal interface and cross-sector collaboration under the One Health framework are essential for early detection and risk assessment of variant influenza viruses.

(SNIP) 

DISCUSSION

This investigation documented the first confirmed human infection with the influenza A(H1N2)v virus in China. Similar to previous reports from the United States and Europe, human infections with A(H1N2)v have remained rare and have primarily been associated with influenza A viruses of swine origin that cross the species barrier (6). Most reported cases present with mild-to-moderate influenza-like illness, and sustained human-to-human transmission has not been documented (7). The clinical presentation in the present case was consistent with these observations, further supporting the characterization of A(H1N2)v as a sporadic zoonotic pathogen with limited transmissibility. The key finding of this investigation is that a swine-origin influenza A(H1N2)v virus was detected in a young child through routine ILI surveillance, with whole-genome sequencing supporting a swine-origin reassortant virus and no evidence of secondary human transmission.

Notably, unlike many previously reported A(H1N2)v cases in which direct exposure to swine was identified as a key risk factor (8), no direct contact with poultry or livestock was reported in this patient. This finding suggests that indirect or environmental exposure may play a role in the development of infection. Environmental contamination, shared living spaces, and short-distance exposure to animal housing have been proposed as alternative transmission pathways for various influenza viruses, particularly in rural settings where humans and animals coexist in close proximity (9).

These findings underscore the complexity of exposure assessment in zoonotic influenza investigations. In the present case, pigs and poultry were raised near the residence, and influenza A virus was detected at low viral loads in several pig and environmental samples. These findings support the possibility of indirect environmental exposure, but do not prove the exact source of infection.

(SNIP)

Human infection with the influenza A(H1N2)v virus described in this investigation represents a sporadic zoonotic event with no evidence of onward transmission and a limited public health impact. This case is comparable to previously reported A(H1N2)v infections globally, which are characterized by mild illness and the absence of sustained human-to-human spread.

Nevertheless, this finding highlights the ongoing risk posed by variant influenza viruses at the human–animal interface. Strengthened sentinel surveillance, timely epidemiological investigation, enhanced laboratory capacity, and cross-sector collaboration under the One Health framework remain critical for the early detection and mitigation of potential future outbreaks, confirming that human A(H1N2)v infection in China provides important evidence for national influenza surveillance and emphasizes the need to strengthen the genomic monitoring of swine-origin influenza viruses.

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It is worth noting that this patient was seen by local clinics 3 times before finally being admitted to a hospital where lab tests confirmed H1N2v 10 days after the child's illness began. 

Mild-to-moderate influenza rarely results in testing for novel flu subtypes, which often only occurs if a patient is hospitalized.  As a result, we've no idea how many cases fly under the surveillance radar. 

While most of the emerging novel viruses we look at in this blog will probably never pose a genuine global health threat - there are a lot of contenders out there - and it only takes one overachiever to make an indelible mark on the world.