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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 vir...

  Abstract While influenza A virus undergoes rapid antigenic drift in humans, at least some subtypes, such as H3, have relatively stable antigenicity in natural waterfowl reservoirs, despite the presence of immune pressure . However, the underlying mechanisms remain poorly understood. This study identified and characterized 187 antibodies to H3 hemagglutinin from experimentally infected mallard ducks , 18 of which were further analyzed by cryo-EM . Compared with human H3 antibodies , duck H3 antibodies exhibited higher glycan-binding propensity , more balanced immunodominance hierarchy , and targeted distinct epitopes . Other unique features of duck H3 antibodies included a convergent CDR H3-independent heavy chain-only binding mode and an N-glycosylated CDR H3 as decoy receptor . By annotating duck immunoglobulin germline genes , we also demonstrated the importance of gene conversion in duck H3 antibodies. Overall, our findings provide insights into how millennia of coevolution ha...

  Abstract In the United States, annual influenza vaccination has been recommended for all persons aged ≥6 months , including during the 2025–26 season. Interim influenza vaccine effectiveness (VE) estimates were calculated for patients with acute respiratory illness–associated outpatient visits and hospitalizations from three U.S. respiratory virus VE networks during the 2025–26 influenza season, using a test-negative case-control design . Among children and adolescents aged <18 years, VE was 38%–41% against influenza outpatient visits and 41% against influenza-associated hospitalization . Among adults aged ≥18 years, VE was 22%–34% against influenza outpatient visits and 30% against influenza-associated hospitalization . Among children and adolescents , VE against influenza A ranged from 37% (against outpatient visits) to 42% (against hospitalization) across settings; among adults , VE against influenza A ranged from 30% (against hospitalization) to 34% (against outpatient vis...

  ABSTRACT Ferrets are widely used to model airborne transmission of influenza viruses in humans . Airborne transmission is evaluated by infecting donor ferrets with a high virus dose and monitoring transmission to contact animals sharing the same airspace . Humans can be infected with a broad range of influenza virus doses . Therefore, we evaluated the relationship between inoculation dose and transmission for two pandemic influenza viruses in ferrets . Donor ferrets were inoculated with 100 to 106 tissue culture infectious dose 50 (TCID50) of the 2009 pandemic H1N1 or 1968 pandemic H3N2 virus and were then paired with respiratory contacts . Using the proportion of donors that became infected across virus doses, we calculated the infectious dose 50 (ID50). Subsequently, by comparing the proportion of contacts that became infected, we calculated the transmissible dose 50% (TD50): the donor inoculation dose that resulted in transmission to 50% of contacts . For the 2009 pandemic H1N...

  {Excerpt} (...) Results NAb geometric mean titers against H1N1 WI/22, H3N2 BA/22, H3N2 CR/23, H3N2-K BA/25, and H3N2-K NY/25 were 200, 231, 119, 50, and 60 at baseline and increased to 582, 661, 356, 85, and 119 at peak immunogenicity, respectively (...), reflecting a significant 2.86- to 2.99-fold increase in NAb titers against the prior H1N1 and H3N2 strains but a lower 1.70- to 1.98-fold increase in NAb titers against the H3N2-K strains . Baseline antibody titers to the H3N2-K strains were 2.0- to 4.6-fold lower than to the prior H1N1 and H3N2 strains (P < .001), and peak antibody titers to the H3N2-K strains following vaccination were 3.0- to 7.8-fold lower than to the prior H1N1 and H3N2 strains (P < .001). (...) Source:  Link:  https://jamanetwork.com/journals/jama/fullarticle/2846268?guestAccessKey=10f1c0a1-4189-438d-9f71-a588fdd0db53&utm_medium=email&utm_source=postup_jn&utm_campaign=article_alert-jama&utm_content=olf-tfl_&utm_term=0309...

  February 2026  WHO convenes technical consultations {1} in February and September each year to recommend viruses for inclusion in influenza vaccines {2} for the northern hemisphere (NH) and southern hemisphere (SH) influenza seasons, respectively.  This recommendation relates to the influenza vaccines for use in the NH 2026-2027 influenza season .  A recommendation will be made in September 2026 relating to vaccines that will be used for the SH 2027 influenza season.  WHO guidance for choosing between the NH and SH formulations for countries in tropical and subtropical regions is available on the WHO Global Influenza Programme website {3}.   National or regional authorities approve the composition and formulation of influenza vaccines used in each country.  National public health authorities are responsible for making recommendations regarding the use of the vaccine.  WHO has published recommendations on the prevention of influenza {4}....

  Abstract As part of its role in the World Health Organization (WHO) Global Influenza Surveillance and Response System (GISRS), the WHO Collaborating Centre for Reference and Research on Influenza in Melbourne received 12,180 human influenza-positive samples during 2024 . Viruses were analysed for their antigenic, genetic, and antiviral susceptibility properties . Selected viruses were propagated in qualified cells or embryonated hens’ eggs for potential use in seasonal influenza virus vaccines. During 2024 , influenza A( H1N1 )pdm09 and A( H3N2 ) viruses predominated , accounting for 33% and 42%, respectively, of all viruses received, compared to 5% for influenza B/Victoria . Of note, one influenza A(H5N1) virus was also received in 2024 . The majority of A(H1N1)pdm09 (98%), A(H3N2) (88%) and influenza B (100%) viruses analysed at the Centre were found to be antigenically and genetically similar to the respective WHO recommended vaccine strains for the Southern Hemisphere in 2024...

  Abstract Background :  Genetic drift and host-associated adaptation in influenza A viruses threaten the long-term reliability of RT-qPCR-based diagnostics , particularly when nucleotide mismatches arise within primer and probe binding regions . Conventional assay evaluations often emphasize sequence conservation but rarely assess functional mismatch tolerance across divergent subtypes and hosts.  Methods :  We performed an in silico evaluation of a matrix (M) gene–targeted RT-qPCR assay by aligning primer and probe binding regions against 22 H1N1 isolates and representative H3N2 and H5N1 reference strains, including recent zoonotic isolates from avian and bovine hosts . Nucleotide mismatches were identified, quantified, and mapped relative to assay components and oligonucleotide termini. Mismatch burden was summarized by subtype and assay region.  Results :  H1N1 isolates exhibited complete conservation across primer and probe regions. In contrast, H3N2 a...

  ABSTRACT Dogs are considered mixing vessels for influenza viruses , posing a pandemic potential via viral reassortment . Our previous studies indicated that the avian-origin H3N2 canine influenza virus (A/canine/Zhejiang/1/2010, abbreviated C1) is virulent in canine and mice . Furthermore, we found that the HA and NA genes of C1 share a close genetic relationship with an H3N2 avian influenza virus (A/duck/Shanghai/06/2009, abbreviated D6), but they exhibit distinct pathogenicity . However, the understanding mechanisms remain unclear. In the present study, we explored the genetic determinants that contribute to the different pathogenicity between the C1 and D6. By using the reverse genetics approaches, we rescued several single-gene and position-substituted reassortant viruses based on the C1. The replication in Madin–Darby canine kidney cells and pathogenic trial in mice showed that the neuraminidase (NA) gene played a critical role in C1 virulence. Further analysis demonstrated ...

  Highlights •  A swine influenza virus H3N2 subtype was isolated during epidemiological survey. •  It is a complex and novel reassortant , and acquired accumulation of adaptive mutations. •  Both rescue and parent strains demonstrated efficient replication in mammalian cells. •  Key residues of the H3N2 HA collectively enhance the binding preference for human-type receptor. •  The rescued H3N2 cause significant pulmonary pathological damage in mice. Abstract Pigs serve as key "mixing vessels" for influenza A viruses, playing a critical role in cross-species transmission , while the H3N2 subtype represents an important lineage within the swine influenza virus (SIV) family. In this study, a novel reassortant H3N2 SIV strain , designated A/Swine/Jiangsu/YZ07/2024 , was isolated from pigs exhibiting clinical symptoms in Northern Jiangsu , China during epidemiological survey . Genetic analysis revealed that the virus is a complex reassortant, with the internal ...

  Abstract Background   Seasonal human influenza viruses can escape from antibody-mediated neutralization when amino acid changes occur in the hemagglutinin protein . Routine surveillance identified circulation of an A(H3N2) virus variant in the Netherlands with amino acid substitutions at hemagglutinin positions 158 and 189 . These amino acid positions were previously responsible for antigenic change of influenza A(H3N2) viruses and potentially lead to escape of this variant from vaccine-mediated immunity .  Aim   To characterize the emergence and antigenic properties of N158K and K189R double substitution virus variants .  Methods   We analyzed the geographical and temporal dynamics of the double-substitution variant using a phylogeographic approach and used hemagglutination inhibition assays and antigenic cartography methods to map its antigenic properties.  Results   A(H3N2) viruses carrying K189R were first detected in Guatemala in June 2024,...

  Abstract The polymerase acidic (PA) protein is a subunit of the trimeric influenza A virus (IAV) RNA-dependent RNA polymerase and the target of the anti-influenza drug baloxavir marboxil (BXM). As with other direct-acting antivirals , treatment with BXM can lead to selection of viruses carrying resistance mutations . If these mutations have negligible fitness costs, resistant viruses can spread widely and render existing treatments obsolete . Multiple BXM resistance mutations in the nuclease domain of PA have been identified, with I38T and I38M amino acid substitutions occurring frequently. These mutations have minimal to no effects on viral polymerase activity , virus replication , or transmission . However, for reasons that are not well understood, viruses with BXM resistance substitutions have not been able to compete with parental wild-type strains . The IAV genome segment encoding PA also encodes the host shutoff nuclease PA-X, which shares the endonuclease domain with PA bu...

  Abstract Introduction Seasonal influenza causes significant global morbidity, mortality, and economic burden . Ongoing viral evolution can lead to vaccine mismatch and the emergence of antiviral resistance , highlighting the importance of genomic surveillance. The 2024–2025 influenza season was characterized by high incidence and increased hospitalizations. Methods We analyzed influenza A virus (IAV) genomes and clinical characteristics from the 2024–2025 season . Whole-genome sequencing was performed on 648 influenza A–positive clinical specimens collected between October 2024 and April 2025. Results Hemagglutinin (HA) sequences were recovered from 74.23% (481/648) of samples and used for subtyping and phylogenetic analysis. A(H1N1)pdm09 and A(H3N2) viruses co-circulated , representing 55.5% and 44.5% of cases, respectively. Among A(H1N1)pdm09 viruses, the HA1 substitution T120A , located near the Sa antigenic site , increased more than twofold compared with the prior season. Ci...