#Pathogenicity and #zoonotic potential of a novel pdm09-lineage #H1N2 swine #influenza A virus spatiotemporally linked to transboundary #pig trade

 

Abstract

International swine trade can disseminate influenza A viruses across borders, creating opportunities for reassortment with endemic swine viruses and the emergence of novel strains with zoonotic potential. Here, we characterize a novel H1N2 reassortant, A/swine/Korea/1030–3/2023 (1030–3), detected in South Korea and spatiotemporally associated with European swine lineages and international pig movement. 1030–3 harbors seven gene segments (PB2, PB1, PA, HA, NP, M and NS) closely related to a recent Danish zoonotic H1N1 virus of the 2009 pandemic (pdm09) lineage, and an N2 segment derived from an endemic Korean HxN2. In vitro assays demonstrated that 1030–3 preferentially bound human-type α2,6-linked sialic acid receptors and replicated efficiently in polarized human airway epithelial cells, although these properties were also observed in the endemic Korean H1N2 comparator 1027–1. In mice, 1030–3 caused moderate clinical signs and respiratory replication comparable to an early pdm09 reference strain NL09. In guinea pigs, 1030–3 transmitted to a subset of direct-contact animals but showed no detectable aerosol transmission under the conditions tested. In pigs, the natural host, 1030–3 replicated efficiently and transmitted to all direct-contact animals, with high respiratory shedding and a greater cumulative shedding burden in contact pigs than NL09. Contemporary human sera showed reduced neutralizing activity against 1030–3 relative to both human seasonal H1N1 and 1027–1, while antiserum against a recent H1N1 vaccine antigen also neutralized 1030–3 less efficiently. These findings identify a European-associated H1N2 swine influenza reassortant detected in South Korea that combines a distinct pdm09-derived genomic and antigenic background with efficient replication and transmission in pigs, α2,6-linked receptor preference, and reduced neutralization by human sera, underscoring the need for trade-aware surveillance of swIAVs and continued monitoring at the swine-human interface.

Source: PLoS Pathogens, https://doi.org/10.1371/journal.ppat.1014779

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#Influenza at #human - #animal #interface - #Summary and #risk #assessment, 7 Sept. - 7 Oct. '26 (WHO, Oct. 28 '26): 1 New H5N1 case in #Cambodia, 4 #H9N2 in #China, 1 #H1N1v in #Brazil

• New human cases {2}: 

        ° From 7 September to 7 October 2026, based on reporting date,  detections of influenza A(H5N1) in one human, influenza A(H9N2) in four  humans and influenza A(H1N1) variant (A(H1N1)v) in one human were  officially reported.

• Circulation of influenza viruses with zoonotic potential in animals: 

    ° High pathogenicity avian influenza (HPAI) events in poultry and non- poultry animal species continue to be reported to the World Organisation for  Animal Health (WOAH).{3} The Food and Agriculture Organization of the United  Nations (FAO) also provides a global update on avian influenza viruses with  pandemic potential.{4} Additionally, low pathogenicity avian influenza viruses as  well as swine influenza viruses continue to circulate in animal populations.

• Risk assessment {5}: 

    ° There have been no reports of sustained human-to-human  transmission associated with the above-mentioned human infection events.  Based on information available at the time of this update, the overall public health  risk from currently known influenza A viruses detected at the human-animal interface has not changed and, at present, these viruses are not thought to be capable of sustained human-to-human transmission, although  this could change as they evolve. Whilst human infections with viruses of animal  origin are infrequent, they are not unexpected at the human-animal interface.

• Risk management: 

    ° Candidate vaccine viruses (CVVs) for zoonotic influenza viruses  intended for pandemic preparedness purposes, were reviewed and updated at the  September 2026 WHO consultation on influenza vaccine composition for use in the  Southern Hemisphere 2027 influenza season. A detailed summary of zoonotic  influenza viruses characterized since February 2026 is published and updated CVVs lists are published.

• IHR compliance {6}: 

    ° This includes any influenza A virus that has demonstrated the capacity to  infect a human, and its haemagglutinin (HA) gene (or protein) is not a mutated  form of those, i.e. A(H1) or A(H3), circulating widely in the human population.  Information from these notifications is critical to inform risk assessments for  influenza at the human-animal interface. 

Avian influenza viruses in humans

    ° A(H5N1), Cambodia

        § On 27 September 2026, Cambodia notified WHO of a laboratory- confirmed human infection with an avian influenza A(H5N1) virus in an adult  male, from Oddar Meanchey Province. He developed a runny nose on 24  September, and respiratory swabs collected that day tested positive for influenza A(H5N1) at the National Institute of Public Health on 26 September.  Institut Pasteur du Cambodge, a WHO H5 Reference Laboratory, confirmed the  result on 27 September. The patient was admitted to a referral Hospital for isolation, clinical monitoring and oseltamivir treatment. Genomic sequencing of  the virus from this case could not be done due to a low viral load.

    § The national and provincial Rapid Response Teams, in collaboration  with animal health and local authorities, are conducting epidemiological,  environmental and animal health investigations. The  patient frequently visited a family member’s poultry farm. At the farm, chickens began becoming ill and  dying around 13 September. Poultry specimens collected from remaining birds on 22 September tested positive for avian influenza.

    § Additional specimens were collected for testing from five direct contacts  with the case and three persons with poultry exposure and all were negative for  influenza A(H5N1). All identified contacts are being monitored for symptoms 14 days.

    § This is the sixth case reported in Cambodia in 2026.

    § According to reports received by WOAH, various influenza A(H5)  subtypes continue to be detected in wild and domestic birds in Africa, the  Americas, Asia, Europe and Oceania. Infections in non-human mammals are also reported, including in marine and land mammals.{7} A list of bird and mammalian species affected by HPAI A(H5) viruses is maintained by FAO.{8}

    ° Risk assessment for avian influenza A(H5N1) viruses:

        1. What is the current global public health risk of additional human cases of  infection with avian influenza A(H5N1) viruses?

            § Most human infections so far have been reported in people exposed to A(H5N1) viruses, for example, through contact with infected  poultry or contaminated environments, including live poultry markets, and  occasionally infected mammals and contaminated environments. As long as the  viruses continue to be detected in animals and related environments humans are  exposed to, further human cases associated with such exposures are expected but  remain unusual. 

            § The impact for public health if additional sporadic cases are detected is minimal. 

            § The current overall global public health risk is low.

        2. What is the likelihood of sustained human-to-human transmission of avian  influenza A(H5N1) viruses related to the events above?

            § No sustained human-to-human transmission has been identified  associated with the recent reported human infections with avian influenza  A(H5N1) viruses. There has been no reported human-tohuman transmission of  A(H5N1) viruses since 2007, although there may be gaps in investigations. In 2007 and the years prior, small clusters of A(H5N1) virus infections in humans  were reported, including some involving health care workers, where limited  human-to-human transmission could not be excluded; however, sustained human- to-human transmission was not reported. 

            § Current evidence suggests that influenza A(H5N1) viruses  related to these events did not acquire the ability to efficiently transmit  between people.

        3. What is the likelihood of international spread of avian influenza A(H5N1) viruses by travellers?

            § Should infected individuals from affected areas travel internationally,  their infection may be detected in another country during travel or  after arrival. If this were to occur, further community-level spread is  considered  unlikely as current evidence suggests these viruses have  not acquired the ability to transmit easily among humans.  

    ° A(H9N2), China

        § Between 9 and 30 September 2026, China notified WHO of four  laboratory-confirmed human cases of A(H9N2) virus infection.

        [Age - Province - Symptom onset]

        1} < 5-year-old - Henan - 30 August 2026

        2} < 5-year-old - Guangdong - 7 September 2026

        3} Adult - Henan - 17 September 2026

        4)} < 5-year-old - Guizhou - 20 September 2026

        § Three of the cases with mild symptoms were detected in influenza-like  illness surveillance and have recovered. The adult has underlying chronic conditions and was still in hospital at the time of reporting. 

        § All cases had direct or indirect exposure to poultry and/or local live  bird markets. Samples from environments associated with the likely area of  exposure of some of the cases tested positive for A(H9) viruses. 

        § No further cases were detected among contacts of these cases.

    ° Risk assessment for avian influenza A(H9N2):

        1. What is the global public health risk of additional human cases of infection  with avian influenza A(H9N2) viruses?

            § Most human cases follow exposure to the A(H9N2) virus through  contact with infected poultry or contaminated environments. Most human  infections of A(H9N2) to date have resulted in mild clinical illness. Since the virus  is endemic in poultry in multiple countries in Africa and Asia, additional human  cases associated with exposure to infected poultry or contaminated environments  are expected but remain unusual. The impact to public health if additional  sporadic cases are detected is minimal. The overall global public health risk is low.

        2. What is the likelihood of sustained human-to-human transmission of avian  influenza A(H9N2) viruses related to these events?

            § At the present time, no sustained human-to-human transmission  has been identified associated with the recently reported human  infections with A(H9N2) viruses. Current evidence suggests that A(H9N2) viruses  from these cases did not acquire the ability of sustained transmission among humans.

        

        3. What is the likelihood of international spread of avian influenza A(H9N2)  virus by travellers? 

            § Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel  or after arrival. If this were to occur, further community level spread is considered  unlikely as current evidence suggests the A(H9N2) virus subtype has not acquired  the ability to transmit easily among humans. Swine influenza viruses in humans

    ° Influenza A(H1N1)v, Brazil

        § On 30 September 2026, Brazil notified PAHO/WHO of a laboratory- confirmed human infection with an influenza A(H1N1)v virus in an adult woman, with pre-existing chronic conditions, residing in the state of Santa  Catarina. She developed symptoms of influenza-like illness on 16 July 2026 and  sought medical attention on 20 July. Her husband and three adult children,  who live in the same household, also experienced symptoms of an influenza-like illness; at that time, all were diagnosed, based solely on clinical  examination, with a viral infection and received treatment. The woman,  however, did not recover. On 27 July, the patient was hospitalized due to worsening respiratory symptoms. She received treatment, including the  antiviral oseltamivir, and required intensive care, including ventilatory support. Despite intensive care, the patient passed away on 28 August.

        § A nasopharyngeal sample, collected on 29 July, tested positive for  influenza A on 30 July and was subsequently subtyped as influenza A(H1N1) on 31  July. The sample was sent to the reference laboratory for respiratory viruses  at the Oswaldo Cruz Institute (IOC/Fiocruz) in Rio de Janeiro for further  characterization. On 4 September, genomic analysis identified an influenza  A(H1N1)v virus. 

        § According to information gathered during an epidemiological investigation conducted by municipal and state teams between 5 and 9  September, the patient had no history of travel or direct contact with  swine. However, a possible indirect exposure to swine was identified through  a neighbour who raises these animals for personal consumption.

        § Since 2015, twelve cases of human infection with swine-origin virus  variants, including this one, have been detected within Brazil’s respiratory virus  surveillance workflow. 

    ° Risk Assessment:

        1. What is the public health risk of additional human cases of infection with  swine influenza viruses?

            § Swine influenza viruses circulate in swine populations in many  regions of the world. Depending on geographic location, the genetic characteristics  of these viruses differ. Most human cases are exposed to swine  influenza viruses through contact with infected animals or contaminated environments. Human infection tends to result in mild clinical illness  in most cases. Since these viruses continue to be detected in swine  populations, further human cases are expected but remain unusual. The impact  for public health if additional cases are detected is minimal. The overall risk of additional human cases is low.

        2. What is the likelihood of sustained human-to-human transmission of  swine influenza viruses?  

            § No sustained human-to-human transmission was identified  associated with the event described above. Current evidence suggests that  contemporary swine influenza viruses have not acquired the ability of sustained  transmission among humans, therefore sustained human-to-human transmission  is thus currently considered unlikely.

        3. What is the likelihood of international spread of swine influenza viruses by  travelers?  

            § Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel  or after arrival. If this were to occur, further community level spread is considered  unlikely as current evidence suggest that these viruses have not acquired the ability to transmit easily among humans.

    For more information on zoonotic influenza viruses, see the report from  the WHO Consultation on the Composition of Influenza Virus Vaccines for Use in  the Southern Hemisphere Influenza Season that was held on 21-24 September 2026 at this link. 

Overall risk management recommendations:

    ° Surveillance and investigations

        • Due to the constantly evolving nature of influenza viruses, WHO  continues to stress the importance of global strategic surveillance in animals and  humans to detect virologic, epidemiologic and clinical changes associated with  circulating influenza viruses that may affect human (or animal) health. Continued  vigilance is needed within affected and neighbouring areas to detect infections in  animals and humans. Close collaboration with the animal health and environment  sectors is essential to understand the extent of the risk of human exposure and to prevent and control the spread of animal influenza. WHO has published  guidance on surveillance for human infections with avian influenza A(H5) viruses.

        • Epidemiologic and virologic surveillance and the follow-up of  suspected human cases should continue systematically. Guidance on investigation  of non-seasonal influenza and other emerging acute respiratory diseases has been  published on the WHO website.

        • Countries should increase avian influenza surveillance in domestic  and wild birds, enhance surveillance for early detection in cattle  populations in countries where HPAI is known to be circulating, include HPAI as a  differential diagnosis in non-avian species, including cattle and other livestock  populations, with high risk of exposure to HPAI viruses; monitor and investigate cases in non-avian species, including livestock, report cases of HPAI in  all animal species, including unusual hosts, to WOAH and other international  organizations, share genetic sequences of avian influenza viruses in publicly  available databases, implement preventive and early response measures to break  the HPAI transmission cycle among animals through movement restrictions of  infected livestock holdings and strict biosecurity measures in all holdings, employ  good production and hygiene practices when handing animal products, and protect persons in contact with suspected/infected animals. {7} More guidance can be  found from WOAH and FAO.

        • When there has been human exposure to a known outbreak of an  influenza A virus in domestic poultry, wild birds or other animals – or when there  has been an identified human case of infection with such a virus – enhanced  surveillance in potentially exposed human populations becomes necessary.  Enhanced surveillance should consider the health care seeking behaviour of the  population, and could include a range of active and passive health care and/or  communitybased approaches, including: enhanced surveillance in local influenza- like illness (ILI)/SARI systems, active screening in hospitals and of groups that  may be at higher occupational risk of exposure, and inclusion of other sources  such as traditional healers, private practitioners and private diagnostic laboratories.

        • Vigilance for the emergence of novel influenza viruses with  pandemic potential must be maintained at all times, including during a non- influenza emergency. To maximize the value of influenza surveillance platform for  other respiratory threats, WHO has updated and published practical guidance for integrated surveillance.

    

    ° Notifying WHO

        • All human infections caused by a new subtype of influenza virus are  notifiable under the International Health Regulations (IHR, 2005).{8,9} State  Parties to the IHR (2005) are required to  immediately notify WHO of  any laboratory-confirmed {10}  case of a recent human infection caused by an  influenza A virus with the potential to cause a pandemic {11}. Evidence of  illness is not required for this report. Evidence of illness is not required for this report.

        • WHO published the case definition for human infections with avian  influenza A(H5) virus requiring notification under IHR (2005): https://www.who.int/teams/global-influenzaprogramme/avian-influenza/case-definitions.

    ° Virus sharing and risk assessment

        • It is critical that these influenza viruses from animals or from humans are fully characterized in appropriate animal or human health influenza  reference laboratories. Under WHO’s Pandemic Influenza Preparedness (PIP)  Framework, Member States are expected to share influenza viruses with pandemic potential on a timely basis {12} with a WHO Collaborating Centre for influenza of GISRS. The viruses are used by the public health laboratories to assess the risk  of pandemic influenza and to develop candidate vaccine viruses.

        • The Tool for Influenza Pandemic Risk Assessment (TIPRA) provides  an in-depth assessment of risk associated with some zoonotic influenza viruses –  notably the likelihood of the virus gaining human-to-human transmissibility, and  the impact should the virus gain such transmissibility. TIPRA maps relative risk  amongst viruses assessed using multiple risk elements. The results of TIPRA  complement those of the risk assessment provided here, and those of prior TIPRA  risk assessments are published at http://www.who.int/teams/global-influenza-programme/avianinfluenza/tool-for-influenza-pandemic-risk-assessment-(tipra).

    ° Risk reduction

        • Given the observed extent and frequency of avian influenza in poultry, wild birds and some wild and domestic mammals, the public should  avoid contact with animals that are sick or dead from unknown causes, including  wild animals, and should report dead birds and mammals or request their removal by contacting local wildlife or veterinary authorities.

        • Eggs, poultry meat and other poultry food products should be  properly cooked and properly handled during food preparation. Due to the  potential health risks to consumers, raw milk should be avoided. WHO advises  consuming pasteurized milk. If pasteurized milk isn’t available, heating raw milk  until it boils makes it safer for consumption.

        • WHO has published practical interim guidance to reduce the risk of  infection in people exposed to avian influenza viruses.

    ° Trade and travellers

        • WHO advises that travellers to countries with known outbreaks of  animal influenza should avoid farms, contact with animals in live animal markets,  entering areas where animals may be slaughtered, or contact with any surfaces  that appear to be contaminated with animal excreta. Travelers should also wash  their hands often with soap and water. All individuals should follow good food  safety and hygiene practices.

        • WHO does not advise special traveller screening at points of entry or  restrictions with regards to the current situation of influenza viruses at the  human-animal interface. For recommendations on safe trade in animals and  related products from countries affected by these influenza viruses, refer to WOAH  guidance. 

Links:

    ° WHO Human-Animal Interface web page https://www.who.int/teams/global-influenza-programme/avian-influenza

    ° WHO Influenza (Avian and other zoonotic) fact sheet https://www.who.int/news-room/fact-sheets/detail/influenza-(avian-and-other-zoonotic)

    ° Global framework to accelerate action on avian  influenza https://iris.who.int/handle/10665/387613

    ° WHO Protocol to investigate non-seasonal influenza and other emerging acute  respiratory diseases https://www.who.int/publications/i/item/WHO-WHE-IHM-GIP-2018.2

    ° WHO Public health resource pack for countries experiencing outbreaks of  influenza in animals: https://www.who.int/publications/i/item/9789240076884

    ° Cumulative Number of Confirmed Human Cases of Avian Influenza A(H5N1)  Reported to WHO https://www.who.int/teams/global-influenza-programme/avian-influenza/avian-a-h5n1-virus

    ° Avian Influenza A(H7N9) Information https://www.who.int/teams/global-influenza-programme/avian-influenza/avian-influenza-a-(h7n9)-virus

    ° World Organisation of Animal Health (WOAH) web page: Avian Influenza https://www.woah.org/en/home/

    ° Food and Agriculture Organization of the United Nations (FAO) webpage:   Avian Influenza https://www.fao.org/animal-health/avian-flu-qa/en/

    ° WOAH/FAO Network of Expertise on Animal Influenza (OFFLU) http://www.offlu.org/

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{1} This summary and assessment covers information confirmed during this  period and may include information received outside of this period.

{2} For epidemiological and virological features of human infections with animal  influenza viruses not reported in this assessment, see the reports on human cases  of influenza at the human-animal interface published in the Weekly  Epidemiological Record here.

{3} Avian influenza. Global situation. World Organisation for Animal Health  (WOAH) (https://www.woah.org/en/disease/avian-influenza/#ui-id-2).

{4} Global Avian Influenza Viruses with Zoonotic Potential situation update. Food  and Agriculture Organization of the United Nations (FAO)  (https://www.fao.org/animal-health/situation-updates/global-aiv-with-zoonoticpotential).

{5} Rapid risk assessment of acute public health events. World Health Organization; 2012 (https://iris.who.int/handle/10665/70810).

{6} Case definitions for the four diseases requiring notification in all  circumstances under the International Health Regulations (2005). World Health  Organization; 2009 (https://www.who.int/publications/m/item/casedefinitions-for-the-four-diseases-requiring-notification-to-who-in-all-circumstances-under-the-ihr-(2005)).

{7} World Organisation for Animal Health. Statement on High Pathogenicity Avian  Influenza in Cattle, 6 December 2024 (https://www.woah.org/en/high-pathogenicity-avian-influenza-hpai-in-cattle/).

{8} World Health Organization. International Health Regulations (2005), as  amended through resolutions WHA67.13 (2014), WHA75.12 (2022), and  WHA77.17 (2024) (https://apps.who.int/gb/bd/pdf_files/IHR_2014-2022-2024-en.pdf).

{9} World Health Organization. Case definitions for the four diseases requiring  notification in all circumstances under the International Health Regulations (2005)  (https://www.who.int/publications/m/item/casedefinitions-for-the-four-diseases-requiring-notification-to-who-in-all-circumstances-under-the-ihr-(2005)).

{10} World Health Organization. Manual for the laboratory diagnosis and  virological surveillance of influenza (2011)  (https://apps.who.int/iris/handle/10665/44518).

{11} World Health Organization. Pandemic influenza preparedness framework for  the sharing of influenza viruses and access to vaccines and other benefits, 2nd  edition (https://iris.who.int/handle/10665/341850).

{12} World Health Organization. Operational guidance on sharing influenza  viruses with human pandemic potential (IVPP) under the Pandemic Influenza  Preparedness (PIP) Framework (2017) (https://apps.who.int/iris/handle/10665/259402)

Source: 

Link: https://www.who.int/publications/m/item/influenza-at-the-human-animal-interface-summary-and-assessment--7-october-2026

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Substitutions affecting the HA–NA– #receptor #balance preceded the emergence of the #pandemic 2009 #H1N1 virus

 

Abstract

Swine are a crucial species in influenza A virus (IAV) ecology, capable of supporting replication and consequently evolution of human, avian, and swine strains. They are also the source of the 2009 pandemic H1N1 virus (H1N1pdm09). Understanding how swine IAV breach the species barrier is essential for detection of potentially pandemic viruses, but is often overlooked compared to avian IAV. We hypothesize that the HA–NA balance of (pre)pandemic swine viruses is a key feature that needs adjustment as swine and humans differ in their sialoglycome. We focused on a set of (pre)pandemic swine viruses collected in Mexico following the H1N1pdm09 emergence. Phylogenetic analysis showed that the emergence was preceded by substitutions in HA and NA predicted to affect their function. Thus, HA acquired substitution A227E, previously shown to reduce receptor binding in H1N1pdm09, while NA obtained substitution S369N in the 2nd sialic acid binding site (2SBS) predicted to reduce NA cleavage. The substitutions’ effect on the HA–NA balance, was confirmed with biolayer interferometry using recombinant proteins attached to nanoparticles. Engineering recombinant attenuated influenza viruses with swine HA and NA with/without the mentioned substitutions, showed that the 2SBS substitution negatively affected replication in primary human and swine cultures. Interestingly, replication was restored by the HA substitution. Overall, we show that the emergence of H1N1pdm09 in humans was preceded by substitutions in HA and NA that were acquired in swine and adjusted the HA–NA balance. We hypothesize that adjustment of the HA–NA balance was a critical step for crossing the host–species barrier.

Source: 

Link: https://doi.org/10.1073/pnas.2609870123

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Molecular #Epidemiology and #Evolution of Swine #Influenza A Viruses, #Vietnam, 2020–2024

 

Abstract

Swine influenza A viruses (IAV-S) caused the 2009 H1N1 pandemic and pose a future zoonotic and pandemic threat. Vietnam represents a critical hotspot for IAV-S emergence within East and Southeast Asia, with dense swine and human populations and intensive livestock trade. We conducted genomic surveillance of IAV-S in Vietnam during 2020–2024, extending previous surveillance from 2013–2019. We identified multiple co-circulating H1 and H3 clades, including pandemic H1N1, Eurasian avian-like, and European lineages, by conducting phylogenetic analysis of 56 IAV-S isolates (21 H1N1, 31 H1N2, and 4 H3N2). Three H1 clades persisted exclusively in Vietnam, circulating up to 12 years. Phylogeographic analysis revealed multiple independent introduction events from North America, Europe, China, Thailand, and Cambodia. We detected extensive reassortment that frequently involved pandemic H1N1 virus internal genes. We identified several lineage-specific mutations associated with mammalian adaptation. Our findings underscore the ongoing IAV-S evolution and need for sustained surveillance in Vietnam.

Source: 

Link: https://doi.org/10.3201/eid3210.260593

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#Host type governs #influenza evolutionary #strategy across reservoir and #spillover hosts

 

Abstract

Despite its high propensity for host switching, the evolutionary mechanisms underlying influenza host adaptation remain unclear. H3Nx influenza viruses are uniquely generalist, with long-term lineages that circulate in avian, human, swine, equine, and canine hosts. Using 13,295 H3Nx sequences, we quantified host-specific adaptive evolution and developed a pipeline to map reassortment events onto trees with measures of statistical uncertainty. We find that while H3Nx viruses in mammals undergo adaptive evolution in HA and NA, viruses in birds experience very little directional selection. Instead, avian lineages exhibit high rates of reassortment, frequently generating novel reassortant lineages that persist transiently and turn over rapidly. 29.8-47.4% of all avian reassortant lineages are purged within the first year of circulation, and reassortment shows no fitness benefit in birds. In contrast, reassorted lineages in swine are more likely to persist long-term, suggesting that reassortment in swine may be broadly beneficial. Segment-specific reassortment patterns were also distinct between avian and mammalian viruses, with NA reassorting more frequently than expected in birds, but less frequently than expected in swine. Reassortment events are enriched between mammalian, but not avian, host switches, suggesting that reassortment may be most beneficial for mediating host switches among mammalian species. Together, our data suggest that host differences drive fundamentally different evolutionary outcomes for influenza viruses, transitioning from reassortment-dominant evolution in their avian reservoir, to varying degrees of adaptation upon establishment in mammals.

Competing Interest Statement

The authors have declared no competing interest.

Funder Information Declared

Pew Charitable Trusts

Margaret Q. Landenberger Research Foundation

National Institute of Allergy and Infectious Diseases

National Institutes of Health

Department of Health and Human Services

United States Department of Agriculture

Agricultural Research Service

Source: 

Link: https://www.biorxiv.org/content/10.64898/2026.09.15.751820v1

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#Influenza at #human - #animal #interface - #Summary & #risk assessment, 8 Aug. - 6 Sept. '26 (WHO): 1 #H5N1 case in #Bangladesh, 3 #H9N2 cases in #China, 2 #H1N2v cases in #US

 

Influenza at the human-animal interface Summary and risk assessment, from 8 August to 6 September 2026 {1}

    • New human cases {2}: 

        ° From 8 August to 6 September 2026, based on reporting date,  detections of influenza A(H5N1) in one human and influenza A(H9N2) in  three humans were officially reported. Additionally, two human cases of infection with influenza A(H1N2) variant (A(H1N2)v) viruses were detected.

    • Circulation of influenza viruses with zoonotic potential in animals: 

        ° High pathogenicity avian influenza (HPAI) events in poultry and  non-poultry animal species continue to be reported to the World Organisation for  Animal Health (WOAH).{3} The Food and Agriculture Organization of the United  Nations (FAO) also provides a global update on avian influenza viruses with  pandemic potential.{4} Additionally, low pathogenicity avian influenza viruses as  well as swine influenza viruses continue to circulate in animal populations.

    • Risk assessment {5}: 

        ° There have been no reports of sustained human-to-human transmission associated with the above-mentioned human infection events.  Based on information available at the time of this update, the overall public health  risk from currently known influenza A viruses detected at the  human-animal interface has not changed and, at present, these viruses are  not thought to be capable of sustained human-to-human transmission, although  this could change as they evolve. Whilst human infections with viruses of animal  origin are infrequent, they are not unexpected at the human-animal interface.

    • IHR compliance {6}: 

        ° This includes any influenza A virus that has demonstrated the  capacity to infect a human, and its haemagglutinin (HA) gene (or protein) is not a  mutated form of those, i.e. A(H1) or A(H3), circulating widely in the human  population. Information from these notifications is critical to inform risk  assessments for influenza at the human-animal interface.

Avian influenza viruses in humans

A(H5N1), Bangladesh

    On 30 August, Bangladesh, through the national IHR focal point, notified WHO of one laboratory-confirmed human infection with an A(H5) virus in a  child from Rangpur division. 

    On 15 August 2026, the child developed a fever, cough and rhinitis and was  admitted to hospital on 17 August. 

    On 18 August, a nasopharyngeal swab and throat swab was collected as  part of hospital-based influenza surveillance and tested positive for influenza A(H5) by real-time RT-PCR at the icddr,b laboratory on the same day. 

    The sample was subsequently confirmed positive for influenza A(H5N1)  at the Molecular and Genomic Laboratory Department of Institute of Epidemiology,  Disease Control and Research (IEDCR) and National Influenza  Centre (NIC) of Bangladesh. 

    Genetic sequencing is underway. 

    The child remained hospitalized and was improving clinically at the time  of reporting.

    There was no reported history of travel outside the area of residence. The  child had a history of exposure to duck and chickens, including some that  were sick, in the household and adjacent households. 

    Poultry meat and oropharyngeal swab specimens collected from several  backyard chickens on 26 August tested negative for influenza A(H5).

    Close contacts were identified and placed under monitoring and all  contacts remained asymptomatic during the observation period, except for two heath care workers. 

    Respiratory specimens collected from these two individuals tested negative for influenza A viruses.

    This is the 4th laboratory-confirmed human case of avian influenza A(H5)  reported in Bangladesh in 2026.

    According to reports received by WOAH, various influenza A(H5) subtypes  continue to be detected in wild and domestic birds in Africa, the Americas, Asia,  Europe and Oceania. Infections in non-human mammals are also reported,  including in marine and land mammals.{7} A list of bird and mammalian species  affected by HPAI A(H5) viruses is maintained by FAO.{8}

Risk assessment for avian influenza A(H5N1) viruses:

    1. What is the current global public health risk of additional human cases of  infection with avian influenza A(H5N1) viruses?

        ° Most human infections so far have been reported in people exposed  to A(H5N1) viruses, for example, through contact with infected poultry or  contaminated environments, including live poultry markets, and occasionally  infected mammals and contaminated environments. As long as the viruses  continue to be detected in animals and related environments humans are exposed  to, further human cases associated with such exposures are expected  but remain unusual. The impact for public health if additional sporadic cases are detected is minimal. The current overall global public health risk is low.

    2. What is the likelihood of sustained human-to-human transmission of avian  influenza A(H5N1) viruses related to the events above?

        ° No sustained human-to-human transmission has been identified  associated with the recent reported human infections with avian  influenza A(H5N1) viruses. There has been no reported human-to-human transmission of A(H5N1) viruses since 2007, although there may be gaps in  investigations. In 2007 and the years prior, small clusters of A(H5N1) virus  infections in humans were reported, including some involving health care workers,  where limited human-to-human transmission could not be excluded;  however, sustained human-to-human transmission was not reported.  Current evidence suggests that influenza A(H5N1) viruses related to these events did not acquire the ability to efficiently transmit between people.

      3. What is the likelihood of international spread of avian influenza A(H5N1) viruses by travellers?

        ° Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel  or after arrival. If this were to occur, further community- level spread is  considered unlikely as current evidence suggests these viruses have  not acquired the ability to transmit easily among humans.  

A(H9N2), China

    Between 13 August and 1 September 2026, China notified WHO of three laboratory-confirmed human cases of A(H9N2) virus infection. 

(...)

    All cases had direct or indirect exposure to poultry and/or local live bird  markets. Samples from environments associated with the likely area of exposure  of some of the cases tested positive for A(H9) viruses. 

    No further cases were detected among contacts of these cases.

Risk assessment for avian influenza A(H9N2):

    1. What is the global public health risk of additional human cases of infection  with avian influenza A(H9N2) viruses?

        ° Most human cases follow exposure to the A(H9N2) virus through  contact with infected poultry or contaminated environments. Most human  infections of A(H9N2) to date have resulted in mild clinical illness. Since the  virus is endemic in poultry in multiple countries in Africa and Asia, additional human cases associated with exposure to infected poultry or  contaminated environments are expected but remain unusual. The impact to  public health if additional sporadic cases are detected is minimal. The  overall global public health risk is low.

    2. What is the likelihood of sustained human-to-human transmission of avian  influenza A(H9N2) viruses related to these events?

        ° At the present time, no sustained human-to-human transmission  has been identified associated with the recently reported human  infections with A(H9N2) viruses. Current evidence suggests that A(H9N2) viruses  from these cases did not acquire the ability of sustained transmission among humans.

    3. What is the likelihood of international spread of avian influenza A(H9N2)  virus by travellers?

        ° Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel  or after arrival. If this were to occur, further community level spread is considered  unlikely as current evidence suggests the A(H9N2) virus subtype has not acquired the ability to transmit easily among humans.

Swine influenza viruses in humans

Influenza A(H1N2)v, United States of America

    Since the risk assessment of 8 August 2026, two human infections with influenza A(H1N2v) viruses were detected in the state of Michigan.{7} 

    The patients, both <18 years of age, developed illness in the week of 15  August. They were not hospitalized and have recovered. Both attended the same agricultural fair where sick swine were present. The cases did not have  contact with each other and  no additional human infections with A(H1N2)v  viruses have been detected around this event. 

    Including these two cases, there have been a total of three human  infections with variant influenza A viruses reported in the United States in 2026.

Risk Assessment:

    1. What is the public health risk of additional human cases of infection with  swine influenza viruses?

        ° Swine influenza viruses circulate in swine populations in many  regions of the world. Depending on geographic location, the genetic characteristics  of these viruses differ. Most human cases are exposed to swine  influenza viruses through contact with infected animals or contaminated environments. Human infection tends to result in mild clinical illness  in most cases. Since these viruses continue to be detected in swine  populations, further human cases are expected but remain unusual. The impact  for public health if additional cases are detected is minimal. The overall risk of additional human cases is low.

    2. What is the likelihood of sustained human-to-human transmission of swine influenza viruses? 

        ° No sustained human-to-human transmission was identified  associated with the event described above. Current evidence suggests  that contemporary swine influenza viruses have not acquired the ability  of sustained transmission among humans, therefore sustained human-to-human  transmission is thus currently considered unlikely.

    3. What is the likelihood of international spread of swine influenza viruses by travelers? 

        ° Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel  or after arrival. If this were to occur, further community level spread is considered  unlikely as current evidence suggest that these viruses have not acquired the ability to transmit easily among humans.

Overall risk management recommendations:

Surveillance and investigations

    • Due to the constantly evolving nature of influenza viruses, WHO  continues to stress the importance of global strategic surveillance in animals and  humans to detect virologic, epidemiologic and clinical changes associated with  circulating influenza viruses that may affect human (or animal) health. Continued  vigilance is needed within affected and neighbouring areas to detect infections in  animals and humans. Close collaboration with the animal health and environment  sectors is essential to understand the extent of the risk of human exposure and to prevent and control the spread of animal influenza. WHO has published  guidance on surveillance for human infections with avian influenza A(H5) viruses.

    • Epidemiologic and virologic surveillance and the follow-up of suspected  human cases should continue systematically. Guidance on investigation of non- seasonal influenza and other emerging acute respiratory diseases has been  published on the WHO website.

    • Countries should increase avian influenza surveillance in domestic  and wild birds, enhance surveillance for early detection in cattle  populations in countries where HPAI is known to be circulating, include HPAI as a  differential diagnosis in non-avian species, including cattle and other livestock  populations, with high risk of exposure to HPAI viruses; monitor and investigate cases in non-avian species, including livestock, report cases of HPAI in  all animal species, including unusual hosts, to WOAH and other international  organizations, share genetic sequences of avian influenza viruses in publicly  available databases, implement preventive and early response measures to break  the HPAI transmission cycle among animals through movement restrictions of  infected livestock holdings and strict biosecurity measures in all holdings, employ  good production and hygiene practices when handing animal products, and protect  persons in contact with suspected/infected animals.{8} More  guidance can be found from WOAH and FAO.

    • When there has been human exposure to a known outbreak of an  influenza A virus in domestic poultry, wild birds or other animals – or when there  has been an identified human case of infection with such a virus – enhanced  surveillance in potentially exposed human populations becomes necessary.  Enhanced surveillance should consider the health care seeking behaviour of the  population, and could include a range of active and passive health care and/or  communitybased approaches, including: enhanced surveillance in local influenza- like illness (ILI)/SARI systems, active screening in hospitals and of groups that  may be at higher occupational risk of exposure, and inclusion of other sources  such as traditional healers, private practitioners and private diagnostic laboratories.

    • Vigilance for the emergence of novel influenza viruses with pandemic  potential must be maintained at all times, including during a non-influenza  emergency. To maximize the value of influenza surveillance platform for other  respiratory threats, WHO has updated and published practical guidance for integrated surveillance.

Notifying WHO

    • All human infections caused by a new subtype of influenza virus are  notifiable under the International Health Regulations (IHR, 2005).{9,10} State  Parties to the IHR (2005) are required to immediately notify WHO of any  laboratory-confirmed {11} case of a recent human infection caused by an influenza A virus with the potential to cause a pandemic {12}. Evidence of illness  is not required for this report. Evidence of illness is not required for this report.

    • WHO published the case definition for human infections with avian  influenza A(H5) virus requiring notification under IHR (2005):  https://www.who.int/teams/global-influenzaprogramme/avian-influenza/case-definitions.

Virus sharing and risk assessment

    • It is critical that these influenza viruses from animals or from humans are  fully characterized in appropriate animal or human health influenza reference  laboratories. Under WHO’s Pandemic Influenza Preparedness (PIP) Framework,  Member States are expected to share influenza viruses with pandemic potential on a timely basis {13} with a WHO Collaborating Centre for influenza of GISRS. The  viruses are used by the public health laboratories to assess the risk of pandemic influenza and to develop candidate vaccine viruses.

    • The Tool for Influenza Pandemic Risk Assessment (TIPRA) provides an  in-depth assessment of risk associated with some zoonotic influenza viruses –  notably the likelihood of the virus gaining human-to-human transmissibility, and  the impact should the virus gain such transmissibility. TIPRA maps relative risk  amongst viruses assessed using multiple risk elements. The results of  TIPRA  complement those of the risk assessment provided here, and those of prior TIPRA  risk assessments are published at http://www.who.int/teams/global-influenza-programme/avianinfluenza/tool-for-influenza-pandemic-risk-assessment-(tipra).

Risk reduction

    • Given the observed extent and frequency of avian influenza in poultry, wild birds and some wild and domestic mammals, the public should  avoid contact with animals that are sick or dead from unknown causes, including  wild animals, and should report dead birds and mammals or request their removal by contacting local wildlife or veterinary authorities.

    • Eggs, poultry meat and other poultry food products should be properly  cooked and properly handled during food preparation. Due to the potential health  risks to consumers, raw milk should be avoided. WHO advises consuming  pasteurized milk. If pasteurized milk isn’t available, heating raw milk until it boils makes it safer for consumption.

    • WHO has published practical interim guidance to reduce the risk of  infection in people exposed to avian influenza viruses.

Trade and travellers

    • WHO advises that travellers to countries with known outbreaks of  animal influenza should avoid farms, contact with animals in live animal markets,  entering areas where animals may be slaughtered, or contact with any surfaces  that appear to be contaminated with animal excreta. Travelers should also wash  their hands often with soap and water. All individuals should follow good food safety and hygiene practices.

    • WHO does not advise special traveller screening at points of entry or  restrictions with regards to the current situation of influenza viruses at the  human-animal interface. For recommendations on safe trade in animals and  related products from countries affected by these influenza viruses, refer to WOAH guidance.

Links:

    ° WHO Human-Animal Interface web page https://www.who.int/teams/global-influenza-programme/avian-influenza

    ° WHO Influenza (Avian and other zoonotic) fact sheet https://www.who.int/news-room/fact-sheets/detail/influenza-(avian-and-other-zoonotic)

    ° WHO Protocol to investigate non-seasonal influenza and other emerging acute respiratory diseases https://www.who.int/publications/i/item/WHO-WHE-IHM-GIP-2018.2

    ° WHO Public health resource pack for countries experiencing outbreaks of  influenza in animals: https://www.who.int/publications/i/item/9789240076884

    ° Cumulative Number of Confirmed Human Cases of Avian Influenza A(H5N1) Reported to WHO https://www.who.int/teams/global-influenza-programme/avian-influenza/avian-a-h5n1-virus

    ° Avian Influenza A(H7N9) Information https://www.who.int/teams/global-influenza-programme/avian-influenza/avian-influenza-a-(h7n9)-virus

    ° World Organisation of Animal Health (WOAH) web page: Avian Influenza https://www.woah.org/en/home/

    ° Food and Agriculture Organization of the United Nations (FAO) webpage:  Avian Influenza https://www.fao.org/animal-health/avian-flu-qa/en/

    ° WOAH/FAO Network of Expertise on Animal Influenza (OFFLU) http://www.offlu.org/

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{1} This summary and assessment covers information confirmed during this  period and may include information received outside of this period.

{2} For epidemiological and virological features of human infections with animal  influenza viruses not reported in this assessment, see the reports on human cases  of influenza at the human-animal interface published in the Weekly Epidemiological Record here.

{3} Avian influenza. Global situation. World Organisation for Animal Health  (WOAH) (https://www.woah.org/en/disease/avian-influenza/#ui-id-2).

{4} Global Avian Influenza Viruses with Zoonotic Potential situation update. Food  and Agriculture Organization of the United Nations (FAO)  (https://www.fao.org/animal-health/situation-updates/global-aiv-with-zoonoticpotential).

{5} Rapid risk assessment of acute public health events. World Health  Organization; 2012 (https://iris.who.int/handle/10665/70810).

{6} Case definitions for the four diseases requiring notification in all  circumstances under the International Health Regulations (2005). World Health Organization; 2009 (https://www.who.int/publications/m/item/casedefinitions-for-the-four-diseases-requiring-notification-to-who-in-all-circumstances-under-the-ihr-(2005)).

{7} Weekly US Influenza Surveillance Report: Key Updates for Week 32, ending  August 15, 2026. US Centers for Disease Control and Prevention; 2026  (https://www.cdc.gov/fluview/surveillance/2026-week-32.html).

{8} World Organisation for Animal Health. Statement on High Pathogenicity Avian  Influenza in Cattle, 6 December 2024 (https://www.woah.org/en/high-pathogenicity-avian-influenza-hpai-in-cattle/).

{9} World Health Organization. International Health Regulations (2005), as  amended through resolutions WHA67.13 (2014), WHA75.12 (2022), and  WHA77.17 (2024) (https://apps.who.int/gb/bd/pdf_files/IHR_2014-2022-2024-en.pdf).

{10} World Health Organization. Case definitions for the four diseases requiring  notification in all circumstances under the International Health Regulations (2005)  (https://www.who.int/publications/m/item/casedefinitions-for-the-four-diseases-requiring-notification-to-who-in-all-circumstances-under-the-ihr-(2005)).

{11} World Health Organization. Manual for the laboratory diagnosis and  virological surveillance of influenza (2011) (https://apps.who.int/iris/handle/10665/44518).

{12} World Health Organization. Pandemic influenza preparedness framework for  the sharing of influenza viruses and access to vaccines and other benefits, 2nd  edition (https://iris.who.int/handle/10665/341850).

{13} World Health Organization. Operational guidance on sharing influenza  viruses with human pandemic potential (IVPP) under the Pandemic Influenza  Preparedness (PIP) Framework (2017) (https://apps.who.int/iris/handle/10665/259402).

Source: 

Link: https://www.who.int/publications/m/item/influenza-at-the-human-animal-interface-summary-and-assessment--6-september-2026

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