#Bovine #H5N1 #influenza viruses have adapted to more efficiently use #receptors abundant in #cattle

 

Abstract

Sustained mammal-to-mammal transmission of high pathogenicity H5N1avian influenza viruses is reshaping the host range of these pathogens. One of the longest-running mammalian transmission chains involves the B3.13 genotype circulating in U.S. dairy cattle which was detected early in 2024. Genomic analysis revealed selection and rapid fixation of haemagglutinin mutations D104G and V147M. We demonstate, via glycomic profiling, that bovine tissues, including the mammary gland, are enriched in N- and O-linked glycans capped with N-glycolylneuraminic acid (NeuGc), a sialic acid absent in humans and birds, which instead express only N-acetylneuraminic acid (NeuAc). Early cattle H5 viruses poorly recognized NeuGc, but D104G and V147M enabled efficient engagement of both NeuAc- and NeuGc-containing receptors. These mutations enhanced replication in bovine mammary tissue without major attenuation of replication in human lung and primary nasal epithelial cells. NeuGc-driven receptor adaptation therefore promotes viral fitness in cattle while potentially limiting immediate zoonotic risk. Deep mutational scanning further identifies alternative haemagglutinin substitutions that confer NeuGc usage and represent surveillance markers for emerging cattle H5 lineages.

Competing Interest Statement

JDB and BD are inventors on Fred Hutch licensed patents related to deep mutational scanning. JDB consults for Pfizer, GSK, Apriori Bio, and Invivyd.

Funder Information Declared

Medical Research Council, MR/Y03368X/1, MR/R010757/1, MC_UU_0034/2, MC_UU_0034/3

BBSRC, BB/Y007298/1, BB/X006123/1, BB/X006166/1, BBS/E/PI/230002A, BBS/E/PI/230002B

BBSRC, BBS/E/PI/23NB0004, BBS/E/PI/23NB0003, UKRI2253, BB/V004697/1

Defra, SE2227

Royal Society, https://ror.org/03wnrjx87, RGS\R2\242118

Houghton Trust, HT/SPRG/23/04

Flanders, G005323N, G051322N, G010326N

CEIRR, 75N93021C00045

The Rockefeller Foundation, PC-2022-POP-005

Source: 

Link: https://www.biorxiv.org/content/10.64898/2026.04.02.715584v1

____

#USA, #Wastewater Data for Avian #Influenza #H5 (#CDC, April 3 '26)

 

{Excerpt}

(...)

Time Period: March 22, 2026 - March 28, 2026

-- H5 Detection: 8 site(s) (1.7%)

-- No Detection: 458 site(s) (98.3%)

-- No samples in last week: 105 site(s)

(...)

Source: 

Link: https://www.cdc.gov/nwss/rv/wwd-h5.html

____

#Chile - #Influenza A #H5N1 viruses of high pathogenicity (Inf. with) (non-poultry including wild birds) (2017-) - Immediate notification

{Magallanes y Antártica Chilena} All the birds were culled. At this time, the SAG is on the field implementing all surveillance measures in the control zone.

Source: 

Link: https://wahis.woah.org/#/in-review/7402

____

Predicting highly pathogenic avian #influenza #H5N1 #outbreak #risk using extreme #weather and bird #migration data in machine learning models

 

Abstract

Background. 

Climate change is intensifying extreme weather events (EWEs) with potentially profound consequences for zoonotic disease dynamics, yet the mechanisms linking EWEs to highly pathogenic avian influenza (HPAI) H5N1 outbreaks remain poorly characterized. The ongoing H5N1 panzootic, responsible for infection in over 500 avian and mammalian species, as well as nearly 1000 human cases and 477 deaths worldwide, provides a critical opportunity to evaluate how climate conditions shape spillover risk at landscape scales. 

Methods. 

We compiled a county-month dataset of confirmed H5N1 detections across the contiguous United States from 2022 to 2024 and integrated it with satellite-derived climate metrics, storm event data, and wild bird activity data. We trained and validated a gradient boosting machine classifier to predict outbreak risk and characterize predictor relationships. 

Results. 

Our model achieved strong discriminative performance (AUC-ROC = 0.856; AUC-PR = 0.237, representing a 7-fold improvement over chance) and high recall (0.726), supporting its utility as an early warning tool. Human population and temperature-related variables were the most influential predictors: cold temperature shocks and prolonged low temperatures were consistently associated with elevated outbreak risk, likely through enhanced environmental viral persistence, wild bird habitat compression, and allostatic stress-driven immunosuppression in reservoir hosts. Among storm variables, high wind coverage elevated risk, potentially via aerosol dispersal of contaminated particulates, while tornado activity showed an inverse relationship, consistent with documented avoidant behavior in migratory birds. Wild bird reservoir density showed a strong positive monotonic relationship with outbreak risk. 

Conclusions. 

Our analyses demonstrate that routinely available environmental and infection data can be used to predict HPAI outbreak risk at fine spatiotemporal scales. These findings demonstrate the divergent roles of short- versus long-term environmental exposures in HPAI spillover dynamics, as well as the potential for machine learning-based surveillance tools to inform targeted biosecurity interventions and early warning systems.

Competing Interest Statement

The authors have declared no competing interest.

Funding Statement

This research was supported by a subaward agreement between prime award recipient Boston University (PI: Gregory Wellenius) and the subaward recipient Regents of the University of Colorado (PI: Elise Grover) under the National Institute of Environmental Health Sciences of the National Institutes of Health, Award Number U24ES035309 -01. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Source: 

Link: https://www.medrxiv.org/content/10.64898/2026.03.30.26349797v1

____

#Tropism and #Replication Competence of #Cattle #Influenza #H5N1 Genotype B3.13 Virus in #Human Bronchus and #Lung Tissue

 

Abstract

In 2024, influenza A(H5N1) genotype B3.13 viruses emerged from cattle and caused mild spillover infections in humans. Using human bronchus and lung tissue, we evaluated tropism, replication, and pathogenesis of 2 cattle influenza isolates. Those viruses showed moderate replication competence and induced robust proinflammatory responses, suggesting potential risk for human health.

Source: 

Link: https://wwwnc.cdc.gov/eid/article/32/5/25-1926_article

____

The emergence and molecular #evolution of #H5N1 #influenza viruses in #USA dairy #cattle

 

Abstract

Prior to 2024, highly pathogenic avian influenza H5N1 clade 2.3.4.4b viruses circulated predominantly in wild birds and poultry. In 2024 and 2025, 2.3.4.4b genotypes B3.13 and D1.1 were detected in United States dairy cattle. Using whole-genome and segment-specific phylodynamic inference, we estimate that B3.13 and D1.1 spilled over from wild birds into dairy cattle in late 2023 and late 2024, respectively. Spillover occurred shortly after the formation of the reassortant genotypes and was followed by months of cryptic transmission prior to detection. We found that both B3.13 and D1.1 evolved at higher rates in cattle relative to birds, primarily due to relaxed purifying selection. Site-specific analyses identified genomic sites under positive selection in cattle relative to birds, indicating adaptation and likely contributing to improved viral fitness after spillover. Intensified genomic surveillance in dairy cattle is essential as population immunity introduces additional selection pressures, with ever-changing risk for human emergence.

Competing Interest Statement

M.A.S. receives contracts from Johnson & Johnson and Gilead Sciences outside the scope of this work. M.U.G.K. received consulting fees from Takeda, Bavaria Nordic, and Google DeepMind for work unrelated to the manuscript.

Funder Information Declared

Fonds voor Wetenschappelijk Onderzoek - Vlaanderen, G051322N, G051323N

UK Medical Research Council/Department for Environment, Food and Rural Affairs (DEFRA) FluTrailMap-One Health consortium, MR/Y03368X/1

Biotechnology and Biological Sciences Research Council (BBSRC)/DEFRA ‘FluTrailMap’ consortium, BB/Y007298/1

Pirbright Institute’s Strategic Program Grants, BBS/E/PI/230002A, BBS/E/PI/230002B

EMBO Installation Grant, 5305

Academy of Medical Sciences Springboard, 1049

Centers of Excellence for Influenza Research and Response, National Institute of Allergy and Infectious Diseases, National Institutes of Health (NIH), Department of Health and Human Services, 75N93021C00015, 75N93021C00014

National Institutes of Health, AI135995, AI153044, AI192139

Rockefeller Foundation, PC-2022-POP-005

Health AI Programme from Google.org

Oxford Martin School Programmes in Pandemic Genomics & Digital Pandemic Preparedness

European Union's Horizon Europe, 874850, 101086640

Wellcome Trust, 303666/Z/23/Z, 226052/Z/22/Z, 228186/Z/23/Z

United Kingdom Research and Innovation, APP8583

Medical Research Foundation, MRF-RG-ICCH-2022-100069

UK International Development, 301542-403

Bill & Melinda Gates Foundation, INV-063472, INV-090281

Novo Nordisk Foundation, NNF24OC0094346

Source: 

Link: https://www.biorxiv.org/content/10.64898/2026.03.30.713641v1

____

Broad #protection against #Influenza A Viruses via an adjuvant-free #mucosal microparticle #vaccine with conserved CD8/CD4 bispecific peptides

 

Abstract

Influenza A viruses (IAVs) cause substantial global morbidity and mortality and are responsible for most known viral pandemics. Their rapid antigenic evolution enables escape from natural and vaccine-induced immunity, requiring annual vaccine reformulation, which offers limited breadth and variable effectiveness. Although a universal influenza vaccine remains a critical objective, most strategies have focused on conserved viral glycoproteins to elicit broadly neutralizing antibodies, with comparatively fewer efforts targeting conserved T cell antigens to achieve cross-subtype protection. Current T cell-based approaches often rely on individual CD8+ epitopes, which are limited by peptide instability, delivery constraints, and dependence on adjuvants. Here, we demonstrate a T cell-focused vaccine strategy that uses evolutionary consensus of IAV M1 and NP from the H1N1 and H3N2 subtypes to predict, map, and screen conserved regions enriched with multiple CD8+ and CD4+ epitopes. We selected the top-performing peptides from immunogenicity screening. We encapsulated them in polylactic-co-glycolic acid microparticles (PLGA-MPs) engineered for selective uptake by APCs and pH-dependent sustained release. Intranasal delivery of this vaccine formulation targeted the primary site of infection and induced robust mucosal immunity without the need for conventional adjuvants. Both human and murine influenza-experienced T cells mounted potent recall responses to the vaccine. In mice, immunization elicited strong CD8+ and CD4+ T cell responses and conferred broad protection against homologous H1N1 and H3N2 as well as heterologous H5N1 IAV subtypes. These findings collectively establish a mucosal, T cell-based vaccine platform that is adjuvant-free and capable of providing broad protection against IAV and other viruses with pandemic potential.

Competing Interest Statement

The authors have declared no competing interest.

Funder Information Declared

DBT-ENDFLU, BT/IN/EU-INF/15/RV/19-20

Source: 

Link: https://www.biorxiv.org/content/10.64898/2026.03.29.715080v1

____

#Chile - High pathogenicity avian #influenza #H5N1 viruses (Inf. with) (#poultry) - Immediate notification

 

This incident is occurring in the Maule Region, in the municipality of San Rafael, on a recreational property where two owners are responsible for the total number of birds reported. All of the birds will be humanely culled, and the carcasses will be buried on the property under strict biosecurity measures.

Source: 

Link: https://wahis.woah.org/#/in-review/7405

____

Antiviral activities of multiple #antivirals against highly pathogenic avian #influenza A #H5N1 in vitro and in mice

 

ABSTRACT

In 2024, a bovine H5N1 strain was first isolated from dairy cows in Texas and confirmed to transmit cross-species to humans. Therefore, research on treatments for human infection should be accelerated. In our study, the antiviral effects of baloxavir acid (BXA), oseltamivir carboxylate (OSC), EIDD-1931 (NHC), and ribavirin (RBV) against five H5N1 strains were evaluated in vitro. Cell viability and viral replication were measured to assess the antiviral effects. The results showed that the EC50 of BXA treatment was the lowest. The BXA/NHC and BXA/OSC combination treatments showed more potent inhibitory effects than each monotherapy. The 15 mg/kg baloxavir marboxil (BXM) / 125 mg/kg molnupiravir (MNP) and the 15 mg/kg BXM / 10 mg/kg oseltamivir phosphate (OSP) were tested in BALB/c mice. The mice were inoculated with 10 times the 50% mouse lethal dose (10 MLD50) of bovine H5N1 virus. Treatments began 1-day post-infection (1 dpi) and were administered orally twice daily for 5 or 7 days. Changes in body weight, clinical signs, and survival were monitored; lung and brain tissues were collected for virological, immunological, and histological analyses. Most mice died from severe neurological symptoms. Compared with the 5-day treatment, the 7-day treatment effectively inhibited viral replication and increased survival rates to 50% in BXM, BXM/MNP, and BXM/OSP treatments. Mice treated with BXM/MNP or BXM/OSP combination therapy showed lower viral yields in the lungs than those treated with BXM alone. The results provide a reference for human treatment, and extending the 7-day combination treatment should be considered.

Source: Emerging Microbes and Infections, https://www.tandfonline.com/journals/temi20

Link: https://www.tandfonline.com/doi/full/10.1080/22221751.2026.2645843

____

#Cambodia - #Influenza A #H5N1 viruses of high pathogenicity (Inf. with) (non-poultry including wild birds & one #human case) (2017-) - Immediate notification

 

On 26 March 2026, an outbreak investigation team visited a backyard farm following reports of illness and mortality suspected to be caused by Avian Influenza (AI). A total of five samples (three chickens and two ducks) were collected and submitted to NAHPRI/GDAHP for testing of Avian Influenza (H5N1). On 27 March 2026, laboratory results confirmed that three out of five samples (two chickens and one duck) tested positive for Avian Influenza (H5N1). Additionally, in the same area, one human case of Avian Influenza (H5N1) was confirmed by the Ministry of Health on 31 March 2026.

Source: 

Link: https://wahis.woah.org/#/in-review/7409

____

A Live Attenuated #Vaccine Candidate against Emerging Highly Pathogenic #Cattle-Origin 2.3.4.4b #H5N1 [#Influenza] Viruses

 

Abstract

Influenza viruses present a significant public health risk, causing substantial illness and death in humans each year. Seasonal flu vaccines must be updated regularly, and their effectiveness often decreases due to mismatches with circulating strains. Furthermore, inactivated vaccines do not provide protection against shifted influenza viruses that have the potential to cause a pandemic. The highly pathogenic avian influenza H5N1 clade 2.3.4.4b is prevalent among wild birds worldwide and is causing a multi-state outbreak affecting poultry and dairy cows in the United States (US) since March 2024. In this study, we have generated a NS1 deficient mutant of a low pathogenic version of the cattle-origin human influenza A/Texas/37/2024 H5N1, namely LPhTXdNS1, and validated its safety, immunogenicity, and protection efficacy in a prime vaccination regimen against wild-type (WT) A/Texas/37/2024 H5N1. The attenuation of LPhTXdNS1 in vitro was confirmed by its reduced replication in cultured cells and inability to control IFNβ promoter activation. In C57BL/6J mice, LPhTXdNS1 has reduced viral replication and pathogenicity compared to WT A/Texas/37/2024 H5N1. Notably, LPhTXdNS1 vaccinated mice exhibited high immunogenicity that reach its peak at weeks 3 and 4 post-immunization, leading to robust protection against subsequent lethal challenge with WT A/Texas/37/2024 H5N1. Altogether, we demonstrate that a single dose vaccination with LPhTXdNS1 is safe and able to induce protective immune responses against H5N1. Both safety profile and protection immunity suggest that LPhTXdNS1 holds promise as a potential solution to address the urgent need for an effective vaccine in the event of a pandemic for the treatment of infected animals and humans.

Competing Interest Statement

The A.G.-S. laboratory has received research support from GSK, Pfizer, Senhwa Biosciences, Kenall Manufacturing, Blade Therapeutics, Avimex, Johnson & Johnson, Dynavax, 7Hills Pharma, Pharmamar, ImmunityBio, Accurius, Nanocomposix, Hexamer, N-fold LLC, Model Medicines, Atea Pharma, Applied Biological Laboratories and Merck. A.G.-S. has consulting agreements for the following companies involving cash and/or stock: Castlevax, Amovir, Vivaldi Biosciences, Contrafect, 7Hills Pharma, Avimex, Pagoda, Accurius, Esperovax, Applied Biological Laboratories, Pharmamar, CureLab Oncology, CureLab Veterinary, Synairgen, Paratus, Pfizer and Prosetta. A.G.-S. has been an invited speaker in meeting events organized by Seqirus, Janssen, Abbott, Astrazeneca and NovavaxA.G.-S. is inventor on patents and patent applications on the use of antivirals and vaccines for the treatment and prevention of virus infections and cancer, owned by the Icahn School of Medicine at Mount Sinai, New York. All other authors declare no commercial or financial conflict of interest.

Source: 

Link: https://www.biorxiv.org/content/10.1101/2025.03.28.646033v2

____

#USA, #Wastewater Data for Avian #Influenza #H5 (#CDC, March 27 '26)

 

{Excerpt}

(...)

Time Period: March 15, 2026 - March 21, 2026

-- H5 Detection: 9 site(s) (2.0%)

-- No Detection: 436 site(s) (98.0%)

-- No samples in last week: 130 site(s)

(...)

Source: 

Link: https://www.cdc.gov/nwss/rv/wwd-h5.html

_____

#Finland - #Influenza A #H5N1 viruses of high pathogenicity (Inf. with) (non-poultry including wild birds) (2017-) - Immediate notification

 

Two wild Canada Geese in the Lounais-Suomen aluehallintovirasto Region.

Source: 

Link: https://wahis.woah.org/#/in-review/7401

____

Deciphering #HPAI #Influenza A Virus #H5N1: Molecular Basis of #Pathogenicity, Zoonotic Potential, and Advances in #Vaccination Strategies

 

Abstract

The ongoing panzootic of the highly pathogenic avian influenza (HPAI) H5N1 virus, dominated by clade 2.3.4.4b, constitutes a significant global threat to wildlife, animal health, and public health. Once characterized by sporadic outbreaks, H5N1 has evolved into a sustained, year-round infection with an expanded host range that now includes numerous mammalian species. Its high pathogenicity is primarily driven by the acquisition of a polybasic haemagglutinin cleavage site, enabling systemic viral spread, alongside emerging endothelial and neurotropic properties that contribute to severe disease and high mortality in mammals. Although zoonotic transmission remains limited, H5N1 continues to accumulate mutations associated with mammalian adaptation, particularly within the haemagglutinin and polymerase complex. Notably, recent outbreaks in U.S. dairy cattle highlight the emergence of novel mammalian reservoirs with increased human exposure risk. Concurrently, vaccination strategies are advancing beyond traditional adjuvanted inactivated vaccines toward next-generation platforms, including mRNA and virus-like particle vaccines, designed for rapid deployment and broader immune protection. However, ongoing viral evolution, constrained vaccine availability, and gaps in coordinated surveillance underscore the urgent need for an integrated One Health approach to reduce panzootic risk.

Source: 

Link: https://www.mdpi.com/1999-4915/18/4/410

____

On the brink of emergence: an evolutionary approach to #Influenza A virus #H5N1 isolated from #humans

 

Highlights

• A detailed phylogenetic analysis of H5N1 HPAIV isolated from humans was performed.

• All strains infecting humans have acquired substitutions in several key proteins.

• All strains have gained specific substitutions to better adapt to the human host.

• Substitutions in key proteins involved in replication and immune evasion were found.

• A significant degree of polymorphic sites was found in the polymerase complex.

• Substitutions in hemagglutinins and neuraminidases from different clades were found.

Abstract

Avian Influenza Viruses (AIVs) pose today a very significant risk to global health given the widespread circulation of H5N1 highly pathogenic avian influenza viruses (HPAIV). After decimating the avian population all over the world, these viruses spill over to many different mammal species, causing also fatal outbreaks. As the virus continues to evolve increasing human cases of H5N1 HPAIV have been reported, causing concern that these viruses may adapt to the human host and became a pandemic new virus. In order to gain insight into this matter, a detailed phylogenetic analysis of H5N1 HPAIV isolated from humans was performed. A significant number of substitutions have been found in the hemagglutinins (HA) and neuraminidases (NA) among the three H5N1 clades already detected in human cases. Some of these substitutions were found to produce changes in the 3D structure of these proteins. Substitutions providing an evolutionary advantage to replicate or evade the immune response in mammals have been found in several non-structural proteins of strains infecting humans, including regulatory proteins, like PA-X or PB1-F2. A significant degree of polymorphic sites was observed in the proteins of the polymerase complex. The results of these studies are discussed in terms of the evolution of H5N1 HPAIV infecting humans and future work to be done to address the pandemic potential of these viruses.

Source: 

Link: https://www.sciencedirect.com/science/article/pii/S0168170226000353?via%3Dihub

____

#Nepal - #Influenza A #H5N1 viruses of high pathogenicity (Inf. with) (non-poultry including wild birds) (2017-) - Immediate notification

 

There is an observation of death of several numbers (30) of crows in the jungle of Tribhuvan University premises.

Source: 

Link: https://wahis.woah.org/#/in-review/7387

____

#Nepal - High pathogenicity avian #influenza #H5N1 viruses (Inf. with) (#poultry) - Immediate notification

Frequent sightings of wild birds were reported in the vicinity of the farm. The farm comprises multiple poultry sheds having birds of different age (4-56 weeks) groups. On 15 March, a sudden mortality event occurred in one shed, where approximately 100 commercial layer birds died acutely. Since that incident, mortality has been observed across all sheds on the farm.

Commercial Layers of various age (4-56 weeks) group affected since 15 March, 2026 and a large number of chicken appear slightly droopy or depressed, and die suddenly.

Source: 

Link: https://wahis.woah.org/#/in-review/7386

____

Prophylactic and therapeutic efficacy of #monoclonal #antibodies against #H5N1 #influenza virus

 

Highlights

• mAbs could enhance our armamentarium against H5N1 in support of pandemic preparedness

• Several mAbs have shown prophylactic and therapeutic efficacy against H5N1 in animal models

• Anti-IAV mAbs that have advanced in clinical trials could be evaluated against H5N1

• Resistance emergence during mAb treatment was infrequent in pre- and clinical studies

Abstract

Highly pathogenic avian influenza H5N1 continues to pose a serious zoonotic and pandemic threat due to its increasing cross-species transmission and high virulence in humans. Despite the availability of vaccines and antivirals for seasonal influenza, effective prophylactic and treatment options for H5N1 remain limited. Herein we explore the potential action of monoclonal antibodies (mAbs) against H5N1, focusing on those with demonstrated efficacy in animal models. Most of these mAbs target conserved hemagglutinin epitopes and function as broad neutralizing fusion/entry inhibitors; notably, CR9114 targets both groups 1 and 2 influenza A strains as well as B lineages. Other mAbs prevent viral release by targeting neuraminidase, and those directed against the M2 ectodomain and nucleoprotein function through Fc receptor-mediated pathways. These mAbs have shown robust protection against lethal H5N1 challenge in mice, ferrets, and/or non-human primates. Compounds such as CR6261, MEDI8852, and TCN-032 have been evaluated in clinical trials for seasonal influenza, yielding encouraging safety and pharmacokinetics results and notably, no reported emergence of resistance. Despite these positive results their clinical development was prematurely discontinued. Integrating these highly effective mAbs into our H5N1 pandemic preparedness arsenal is a logical next step to provide a robust prophylactic and therapeutic option at the early stages of an outbreak. Future efforts must address regulatory and logistical barriers, invest in stockpiling and emergency use protocols, and support adaptive clinical trial frameworks to ensure rapid deployment when needed.

Source: 

Link: https://www.sciencedirect.com/science/article/pii/S0924857926000737?via%3Dihub

____

#Dispersal, #adaptation and #persistence of #H5N1 in the sub-Antarctic and #Antarctica

 

Abstract

High pathogenicity avian influenza virus (HPAIV) H5N1 reached the sub-Antarctic and Antarctica in 2023, subsequently spreading to remote locations within this region where it had devastating impacts on seal, penguin and albatross populations. The threat to marine wildlife over this broad area exemplifies the need to understand H5N1 long-distance dispersal and evolution. We obtained 104 novel viral genomic sequences from samples that we collected at South Georgia, Kerguelen, Crozet, Prince Edward, Falklands/Malvinas Islands and the Antarctic Peninsula in a region spanning 8,000 kilometers. Using recent phylogeographic modeling advances we show that H5N1 spread encompassed numerous transmission events between distant locations, accumulating mammalian-adaptive mutations in the process. Seals are the most affected species, but we reveal that the long-distance eastward virus dispersal better aligns with the long-distance movements of large petrels and albatrosses. The risk of H5N1 endemisation, dispersal to other locations and ongoing evolution are highly concerning.

Competing Interest Statement

The authors have declared no competing interest.

Source: 

Link: https://www.biorxiv.org/content/10.64898/2026.03.20.713283v1

____

14th Meeting of #WHO #Expert Working Group of the Global #Influenza #Surveillance and Response System (GISRS) for Surveillance of #Antiviral Susceptibility (March 20 '26)

Weekly epidemiological record 

20 MARCH 2026, 101th YEAR, No 12, 2026, 101, 53–56

http://www.who.int/wer 

Executive Summary 

The WHO Expert Working Group on Surveillance of Influenza Antiviral Susceptibility (AVWG) supports the WHO GISRS by providing practical guidance for monitoring antiviral susceptibility of seasonal and emerging influenza viruses through global surveillance efforts. 

The 14th WHO-AVWG meeting was held in virtual format on 10-12 June 2025. 

Update on susceptibility of seasonal influenza viruses to approved antiviral agents 

From approximately May 2024 to May 2025, five WHO Collaborating Centres (CCs) and two National Influenza Centres (NICs) reported co-circulation of influenza A(H1N1) pdm09, A(H3N2), and B/Victoria viruses. 

A(H1N1)pdm09 dominated in Eastern Asia{1}. Elevated frequency of influenza neuraminidase (NA) inhibitor (NAI) reduced inhibition/ highly reduced inhibition (RI/HRI) was identified among A(H1N1)pdm09 viruses, largely conferred by the NA-H275Y substitution. 

Reporting frequency was 3.8% in China, lower (≤1%) in other reporting regions, but still measurable and were in some cases a result of prior antiviral use or specific local outbreaks (e.g., a hospital in Iceland with a NA-H275Y+S247N cluster, a primary school classroom outbreak in Japan{2}. The NA-S247N substitution (≤3.3%) was also noted by three centres, but these viruses exhibited normal inhibition (NI) by NAIs when available isolates were tested. 

Incidence of RI/HRI or NA-associated markers were less frequently reported for A(H3N2) and B/Victoria viruses than A(H1N1)pdm09 viruses. 

Markers and incidence of reduced susceptibility to baloxavir was detected at low frequencies of 0.07 to 2.2%, where the latter value represented a small sample set of only 2 of 89 viruses in Japan. 

Reduced susceptibility or amino acid markers indicative of reduced susceptibility were observed only in influenza A viruses and not influenza B. 

Update on susceptibility of zoonotic and animal influenza viruses  to approved antiviral agents 

From approximately May 2024 to May 2025, global surveillance data from WHO CCs, NICs, and associated partners including WHO Essential Regulatory Laboratories and the OFFLU (WOAH/FAO Network of Expertise on Animal Influenza) network reported that most zoonotic and avian influenza viruses, particularly circulating A(H5N1/x) HA clade 2.3.4.4b and 2.3.2.1a/e viruses, were broadly susceptible to NAIs and baloxavir. 

A(H5N1) 2.3.4.4b virus oseltamivir inhibitory concentrations remain elevated vs. seasonal N1 viruses. 

Small and isolated incidence of NAI associated RI/HRI or markers included: NA-D199G mediated oseltamivir/zanamivir RI detected in A(H5N1) 2.3.4.4b poultry in the Russian Federation (February 2024, reported June 2025), NA-N295S in poultry in India A(H5N1) 2.3.2.1a isolates, and 8 poultry farms in British Columbia, Canada exhibiting A(H5N1) 2.3.4.4b with NA-H275Y. 

Only two viruses with reduced baloxavir susceptibility were identified, 1 human virus with PA-I38M (California, USA) and 1 environmental virus isolate with PA-V100I (China, Hong Kong Special Administrative Region). 

Beyond A(H5N1/x), nearly 30 avian influenza subtypes including A(H9N2), A(H7N2), A(H7N7), and A(H7N9), and A(H10N7) were analysed across surveillance sites in the Bangladesh, Egypt, the Netherlands and the United States of America (USA). 

They generally lacked NA or PA genotypic markers of reduced drug susceptibility and when available for phenotypic testing, were susceptible to both NAIs and baloxavir. 

A(H7N2) and A(H7N7) viruses from the Netherlands displayed oseltamivir RI compared to human seasonal references, but this may be due to foldchange comparison to a mismatched NA subtype. 

Swine-origin variant viruses (A(H1N1)v, A(H1N2)v, A(H3N2)v) tested across the USA and Europe were largely free of genotypic or phenotypic indicators of reduced susceptibility/inhibition to NAIs or baloxavir. 

Some viruses (the  Netherlands) showed slightly higher NAI median inhibitory concentrations to historical or human seasonal baselines, but all remained below NAI RI thresholds. 

Update of protocols and guidance for GISRS laboratories 

Both genotypic and phenotypic assays may be used as tools to monitor susceptibility of influenza viruses to NAIs and baloxavir. 

The WHO-AVWG routinely reviews and updates influenza NA and PA amino acid substitutions associated with reduced susceptibility to NAIs and baloxavir; updated tables for the previous reporting period were included on the WHO website{3–5}. 

The US CDC continues to update and ship reference virus panels that can be used for NAI and baloxavir susceptibility testing, available via the International Reagent Resource{6} 

Further guidance on baloxavir and other PA inhibitor testing included the Influenza Replication Inhibition Neuraminidase-based Assay (IRINA), published by the Centers for Disease Control and Prevention, USA{7} and included on the WHO website{8}. 

The WHO AVWG continues to develop algorithms for NICs to aid in influenza response planning (zoonotic, pandemic, and antiviral resistance-specific events), guidance to aid in decisions making for testing strategies (genotypic vs. phenotypic), and guidance for consideration of baloxavir and PA inhibitor specific amino acid substitutions associated with reduced drug susceptibility{9}. 

Additionally, the WHO-AVWG has worked with GISAID to continue to refine and implement modifications to existing tools to facilitate identification of NA and PA substitutions upon sequence submission. 

Outbreak and pandemic preparedness with clinicians’ perspectives 

Two physicians, Profs. Prof. David Hui and Bin Cao, were invited to present recently updated WHO guidance on clinical practice guidelines for influenza{10}. 

Significant updates and discussion surrounded inclusion of baloxavir, which was conditionally recommended for non-severe disease high-risk patients and post-virus exposure prophylaxis (PEP) including influenza viruses associated with high mortality. 

Conditional recommendation against any NAI or baloxavir intervention remains for non-severe disease low-risk patients or seasonal virus PEP. 

Data was presented on multiple PA inhibitors rapidly moving through late-stage clinical trials in China which may have implications on expanded usage of this newer class of influenza drugs. 

Review of External Quality Assessment Programme (EQAP) panels 

EQAP was initiated in 2007 to monitor the quality of GISRS, NICs, other national influenza reference laboratories’ capacity for influenza diagnosis and detection. 

An optional antiviral phenotypic NAI panel was introduced in 2013, and genotypic baloxavir susceptibility was introduced in 2020. 

Results for the 2024 Global EQAP panel were reported during the 14th WHO-AVWG meeting. 

Of the 194 participating laboratories, 26.3% participated in NAI susceptibility testing. 

Results and subsequent discussion from this year’s panel were used by members of WHO-AVWG to assess the training needs of NICs. 

Way forward 

The 2020–2023 Annual Global Update on the Susceptibility of Influenza Viruses (Global AVS) manuscript was published{11} and drafting of a 2023–2025 publication is underway. The next WHO-AVWG meeting will be held in June 2026.

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{1} World Health Organization. Influenza Transmission Zones. 2026. https://cdn.who.int/media/docs/ default-source/influenza/influenzaupdates/2025_09_24_influenza-transmission-zones. pdf?sfvrsn=22361408_3&download=true. 

{2} Takashita E, Shimizu K, Usuku S, Senda R, Okubo I, Morita H, et al. An outbreak of influenza A(H1N1) pdm09 antigenic variants exhibiting cross-resistance to oseltamivir and peramivir in an elementary school in Japan, September 2024. Euro Surveill. 2024;29(50).

{3} World Health Organization. Summary of neuraminidase (NA) amino acid substitutions assessed for their effects on inhibition by neuraminidase inhibitors (NAIs). 2025. https://cdn.who.int/media/docs/default-source/ influenza/laboratory---network/quality-assurance/human-nai-marker-table_ for-publication_final_20240918.pdf. 

{4} World Health Organization. Summary of neuraminidase (NA) amino acid substitutions assessed for their effects on inhibition by NA inhibitors (NAIs) among avian influenza viruses of Group 1 (N1, N4, N5, N8 subtypes) and Group 2 (N2, N3, N6, N7, N9 subtypes) NAs. 2025. https://cdn.who.int/media/ docs/default-source/influenza/avwg/avian-nai-marker-whotable__10-10-2025.pdf?sfvrsn=bc0d1e9a_10 

{5} World Health Organization. Summary of polymerase acidic protein (PA) amino acid substitutions assessed for their effects on PA inhibitor (PAI) baloxavir susceptibility. 2025. https://cdn.who.int/media/docs/default-source/influenza/ laboratory---network/quality-assurance/antiviral-susceptibility-influenza/ pa-marker-who-table_28-11-2025_updated.pdf?sfvrsn=5307d6fe_4. 

{6} International Reagent Resource. 2026. https://www. internationalreagentresource.org/. 

{7} Patel MC, Flanigan D, Feng C, Chesnokov A, Nguyen HT, Elal AA, et al. An optimized cell-based assay to assess influenza virus replication by measuring neuraminidase activity and its applications for virological surveillance. Antiviral Res. 2022;208:105457. 

{8} World Health Organization. Baloxavir Susceptibility Assessment using Influenza Replication Inhibition Neuraminidase-based Assay (IRINA). https:// cdn.who.int/media/docs/default-source/influenza/avwg/cdc-phenotypic-lp492rev01d---baloxavir-susceptibility-assessment-using-irina.pdf? 

{9} Patel MC, Nguyen HT, Mishin VP, Pascua PNQ, Champion C, Lopez-Esteva M, et al. Antiviral susceptibility monitoring: testing algorithm, methods, and f indings for influenza season, 2023-2024. Antiviral Res. 2025;244:106299. 

{10} World Health Organization. Clinical practice guidelines for influenza 2024. https://www.who.int/publications/i/item/9789240097759.

{11} Hussain S, Meijer A, Govorkova EA, Dapat C, Gubareva LV, Barr I, et al. Global update on the susceptibilities of influenza viruses to neuraminidase inhibitors and the cap-dependent endonuclease inhibitor baloxavir, 2020-2023. Antiviral Res. 2025:106217.

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Source: 

Link: https://iris.who.int/server/api/core/bitstreams/1ea408da-cd90-438b-b80c-b00aaf4e7315/content

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