#Oseltamivir #Resistance in #Human #Influenza #H5N1 and #H7N9 Infections: A Mini Review

 

Abstract

Avian influenza viruses (AIVs) have been reported to cause infections in humans following avian-to-human transmission, resulting in a range of clinical outcomes. A(H5N1) and A(H7N9) infections, which constitute the majority of human AIV cases, are responsible for severe infections leading to high mortality. The neuraminidase inhibitor oseltamivir is expected to play a major role for the control of AIV infections in humans. However, the emergence of resistance may compromise the impact of antiviral therapy. The objective of this article is to review human cases of A(H5N1) and A(H7N9) infections for which mutations of oseltamivir resistance were detected. Neuraminidase mutations rapidly occurred in a subtype-specific manner, with H274Y and N294S substitutions predominating in A(H5N1) cases and the R292K substitution in A(H7N9) cases. Serious clinical outcomes and mortality were seen in most A(H5N1) and A(H7N9) cases despite oseltamivir therapy, thus highlighting the need for improving antiviral strategies against these AIVs.

Source: 

Link: https://academic.oup.com/ofid/article/13/7/ofag393/8722865

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Characterization of #oseltamivir-resistant #H5N1 clade 2.3.4.4b, genotype #D1.1 #variants identified in #poultry farms of British Columbia, #Canada

 

ABSTRACT

Highly pathogenic avian influenza A(H5N1) viruses of clade 2.3.4.4b, genotype D1.1, are responsible for widespread outbreaks in poultry and continue to cause sporadic, sometimes severe, human infections. Herein, we characterized a wild-type (WT) influenza A(H5N1) D1.1 isolate (BC-H5N1-WT) and its H275Y neuraminidase (NA) variant (BC-H5N1-H275Y), both of which emerged on farms in British Columbia, Canada, during the fall 2024 outbreak. In vitro analysis assessed replication kinetics in MDCK cells, with supernatants collected at different days post-infection (p.i.) and titrated by TCID50 and qRT-PCR. Neuraminidase inhibitor (NAI) susceptibility was determined by NA inhibition assays, whereas susceptibility to baloxavir acid (BXA) was evaluated by plaque reduction assay. In vivo virulence was evaluated in BALB/c mice infected with serial 10-fold dilutions of each virus to monitor weight loss and mortality. Viral titers in lungs, brain, nose, kidney, spleen, and heart were quantified at day 4 p.i. The BC-H5N1-WT virus was susceptible to the four antivirals tested, whereas BC-H5N1-H275Y displayed resistance to oseltamivir and peramivir but remained susceptible to zanamivir and BXA. The BC-H5N1-WT exhibited significantly higher viral replication titers than BC-H5N1-H275Y at all tested time points and showed larger plaque sizes. In mice, BC-H5N1-WT was more virulent with LD50 values of 1.78 × 103 PFUs compared to 8.71 × 104 PFUs for BC-H5N1-H275Y, and produced higher viral titers in lungs and other organs. Despite the reduced fitness of the resistant H5N1 D1.1 variant, its emergence in the absence of viral selection pressure underscores the need for continued surveillance.

Source: 

Link: https://www.tandfonline.com/doi/full/10.1080/22221751.2026.2686474

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Epidemiological and Virological Characteristics of #H9N2 Avian #Influenza Virus in #Jiangsu Province, #China, 2024

 

Abstract

H9N2 avian influenza viruses inherently carry cross-species transmission potential, making continuous surveillance critical for pandemic prevention. This study focused on monitoring the 2024 H9N2 epidemic in Jiangsu Province’s external environment, analyzing its molecular evolution and receptor binding properties, assessing cross-species transmission and pandemic risks, and investigating serological antibody levels across different human populations. Environmental samples were collected from live poultry markets, farms, slaughterhouses, and bird habitats across Jiangsu, screened via quantitative PCR (qPCR), with positive samples used for virus isolation and whole-genome sequencing. Receptor binding properties were tested by hemagglutination assay, and H9N2 antibody levels were measured in 370 occupationally exposed individuals and 240 non-exposed individuals using hemagglutination inhibition (HI) assays. Among the 5779 collected samples, 6.89% tested H9N2-positive, and 12 strains belonging to the Eurasian lineage Y280-like clade G57 genotype were successfully isolated. All strains carried the HA-Q226L mutation, with 11 showing preferential binding to human α-2,6 receptors and one strain possessing dual receptor binding capability. Internal genes harbored mammalian adaptation mutations, and M2 proteins contained mutations conferring complete resistance to amantadine-class antiviral drugs. Serological tests revealed antibody positive rates of 4.05% in exposed populations and 2.5% in non-exposed populations, with no statistically significant difference between groups. These findings confirm that Jiangsu’s circulating H9N2 viruses have acquired human receptor preference and mammalian adaptation, posing silent infection and pandemic risks. Enhanced surveillance and the development of candidate vaccine stockpiles are strongly recommended.

Source: 

Link: https://www.mdpi.com/1999-4915/18/6/687

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#Pathogens #analysis and modeling of #mortality #risk in #sepsis patients with #COVID19 and without COVID-19

 

Abstract

This study compared isolate-level pathogen profiles, antimicrobial susceptibility, clinical characteristics, and mortality predictors between sepsis patients with and without coronavirus disease 2019 (COVID-19), and developed cohort-specific nomograms for in-hospital mortality. This retrospective intensive care unit (ICU) cohort included 608 adults with sepsis: 158 in group COVID-19 and 450 in group non–COVID-19. All patients were assessed for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) detection status. Patient-level comparisons and mortality modeling used the full cohort, whereas microbiological analyses were restricted to 235 eligible bacterial or fungal isolates from baseline cultures after exclusion of colonizers, contaminants, clinically non-causative organisms, and duplicates. Candidate predictors were selected by least absolute shrinkage and selection operator (LASSO) regression and entered into multivariable logistic regression; coefficients were pooled using Rubin’s rules when multiple imputation was performed. External validation used MIMIC-IV with fixed internal coefficients. Gram-negative organisms predominated, mainly Acinetobacter baumannii and Klebsiella pneumoniae, with substantial antimicrobial resistance. The COVID-19 and non–COVID-19 models showed apparent internal area under the curve values of 0.938 and 0.871, conservative optimism-corrected values of 0.913 and 0.871, and external validation AUC values of 0.841 and 0.859, respectively. Cohort-specific nomograms may provide supplementary risk-stratification information in critically ill patients with sepsis.

Source: 

Link: https://www.nature.com/articles/s41598-026-58450-w

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Identification and characterization of a #SARS-CoV-2 #Mpro G23 deletion #ensitrelvir - #resistant mutant

 

ABSTRACT

Ensitrelvir is an antiviral drug that specifically targets the conserved main protease (Mpro) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, mutations in Mpro could confer resistance to antivirals, including ensitrelvir. Thus, identifying SARS-CoV-2 drug-resistant mutants and elucidating their mechanisms of resistance are critical for guiding the selection of effective antiviral therapies. Here, we utilized a recombinant luminescent attenuated SARS-CoV-2 lacking the open reading frames (ORF) 3a and 7b proteins (Δ3a7b-Nluc WT) to safely identify ensitrelvir drug-resistant mutants (DRM-E) without the need of using virulent forms of SARS-CoV-2. We isolated a DRM-E containing a Mpro G23 deletion (G23del) with high resistance (~1,000-fold) to ensitrelvir, but not to the Mpro inhibitor nirmatrelvir or to the RNA-dependent RNA polymerase (RdRp) inhibitor remdesivir. The contribution of G23del in ensitrelvir resistance was confirmed by generating a Δ3a7b-Nluc containing G23del in Mpro (Δ3a7b-Nluc G23del). Δ3a7b-Nluc G23del exhibited resistance to ensitrelvir in both cultured cells and in K18 hACE2 transgenic mice. Binding affinity revealed that the G23del mutation altered ensitrelvir, but not nirmatrelvir, binding to Mpro. Notably, while Δ3a7b-Nluc G23del was affected in viral fitness, serial passage of Δ3a7b-Nluc G23del in the absence of ensitrelvir resulted in the emergence of substitution L50F in Mpro that restored viral fitness loss caused by G23del without altering resistance to ensitrelvir. Our results demonstrate that G23del in Mpro can confer resistance to ensitrelvir. Positively, G23del in Mpro does not render SARS-CoV-2 resistant to nirmatrelvir or remdesivir, suggesting the feasibility of treating SARS-CoV-2 infections containing G23del Mpro with other approved antivirals.

Source: 

Link: https://journals.asm.org/doi/10.1128/mbio.00584-26

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A mouse #model of #human-derived #H10N3 #influenza enables preclinical evaluation of #antiviral efficacy

 

Highlights

• Revealing one human-derived H10N3 virus was highly lethal to C57BL/6J, ICR, and BALB/c mice;

• Successfully establishing the first human-derived H10N3-infected mice model.

• In vitro antiviral assay revealed that this human-origin H10N3 virus exhibits natural resistance to oseltamivir, but remains sensitive to peramivir and baloxavir.

• Oseltamivir or BM immediate treatment after infection effectively prevented mortality caused by H10N3,but their efficacy with a 24h delay were significantly weaker than that of peramivir.

• BM one-dose treatment with a 24h delay has no protective effect, but BM combination with NAIs exhibits significant additive effect on mortality caused by H10N3.

• BM and NAIs combination may be a promising therapy for combating novel H10N3 virus infection.

Abstract

    Human infections with avian influenza A (H10N3) have recently been reported, representing a notable global public health concern. To seek effective strategies for emerging H10N3 virus infection and provide tools for vaccine and antiviral drugs development, we established a mouse model with a novel human-derived H10N3 virus. Our findings revealed that this human-derived H10N3 virus was highly lethal to C57BL/6J, ICR, and BALB/c mice. Neuraminidase inhibitors (oseltamivir or peramivir) effectively conferred protection for H10N3 low-lethal infection, but the efficacy of peramivir is superior to that of oseltamivir. One single dose of baloxavir marboxil (BM) treatment at 2 h post-infection provides complete protection against mortality, but BM treatment with a 24h delay has no protective effect against mortality caused by H10N3 virus infection. Furthermore, BM multiple doses treatment with a 24h delay for H10N3 infection remains effective in preventing weight loss and enhancing viral clearance, but its protective efficacy against mortality was significantly attenuated. However, both in vitro and in vivo combination of BM with NAIs exhibit significant additive effect against H10N3 virus infection than BM or NAIs monotherapy. Our findings suggest that combination of BM with NAIs represents a promising therapeutic strategy for emerging H10N3 infections in clinical practice.

Source: 

Link: https://www.sciencedirect.com/science/article/abs/pii/S0166354226000951?via%3Dihub

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#Niclosamide Inhibits the #Replication of Highly Pathogenic Avian #Influenza #H5Nx Viruses and Antiviral-Resistant #Mutants

 

Highlights

• Niclosamide blocks the replication of highly pathogenic avian influenza H5 viruses

• Niclosamide is effective against H5 viruses with antiviral-resistant substitutions

• Niclosamide has potential as host-targeting anti-influenza drug

Abstract

The recurrent spillover of highly pathogenic avian influenza (HPAI) H5 viruses into humans represents a major public health concern that is exacerbated by the emergence of drug-resistant viral variants. Host-targeting antiviral approaches, including drug repurposing, offer a promising alternative to conventional virus-directed therapeutics. Here, we evaluated the antiviral activity of niclosamide, an FDA-approved anthelmintic drug, against four HPAI A(H5Nx) viruses, two A(H5N1), one A(H5N6), and one A(H5N8), recently isolated from human cases. Niclosamide inhibited all four viruses in plaque reduction assays with MDCK cells, with low inhibitory concentration 50% (IC50) values (0.68–1.40 μM) and minimal cytotoxicity at effective concentrations. These values were more potent than the IC50 values observed for the RdRp inhibitor favipiravir. Niclosamide treatment plus either baloxavir marboxil or favipiravir resulted in additive or near-additive interactions, as indicated by synergy scores of ±10. Importantly, niclosamide retained antiviral activity against HPAI A(H5Nx) viruses bearing resistance-associated amino acid substitutions (i.e., PA-I38T, baloxavir resistance and PB1-K229R, favipiravir resistance), consistent with its host-directed mechanism of action. Although there are barriers to be overcome such as a narrow therapeutic window, largely attributable to its poor bioavailability and some cytotoxicity, our findings suggest niclosamide has potential as a host-targeting therapeutic option against emerging zoonotic influenza viruses, particularly in settings involving antiviral-resistant escape mutants.

Source: 

Link: https://www.sciencedirect.com/science/article/pii/S016635422600080X?via%3Dihub

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Decade-long #warming accelerates #antibiotic #resistance in #grassland soils

 

Abstract

Soils are critical reservoirs of antibiotic-resistance genes (ARGs), which are strongly shaped by microbial interactions and environmental conditions and are therefore highly sensitive to disturbance. Although climate warming is recognized as one of the most significant disturbances to microbial communities and their functions, its impacts on soil resistomes remain poorly understood. Here we investigated the effects of decade-long experimental warming on ARGs in grassland soils using integrated experimental and computational approaches. Our results revealed that ARG abundance substantially increased (23.9%) under warming—particularly glycopeptide- and rifamycin-resistance genes. Warming specifically enriched Actinomycetota hosts, including various potential plant pathogens, and enhanced ARG mobility. Large-scale unprecedented isolates-based phenotypic analyses also validated that warming increased bacterial resistance to multiple antibiotics. Further mechanistic analyses revealed that warming increased ARG abundance primarily through co-selection of resistance genes physically linked to adaptive traits (for example, thermal tolerance and nitrogen assimilation) and positive selection for thermal tolerance genes, which could be further amplified via horizontal gene transfer. Together, these findings convincingly demonstrate that climate warming substantially accelerates soil antibiotic resistance at genomic, ecological and evolutionary levels, with broad implications for public health and environmental sustainability in a warming world.

Source: 

Link: https://www.nature.com/articles/s41586-026-10413-x

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#Genetic and #biological characterization of a #duck-origin clade 2.3.4.4b #H5N6 avian #influenza virus reveals partial #mammalian #adaptation

 

Highlights

• Duck-origin H5N6 virus A/Duck/Jiangsu/628/2022 shares high homology with the human strain A/Yangzhou/125/2022.

• The 628 strain shows mammalian adaptation markers: HA mutations enhance human receptors affinity and NA mutations reduce sensitivity to neuraminidase inhibitors.

• Limited airborne transmission but detectable droplet-mediated spread suggests increased mammalian transmission risk.

Abstract

Clade 2.3.4.4b H5Nx highly pathogenic avian influenza viruses (HPAIVs) have caused extensive outbreaks in poultry worldwide. H5 HPAIVs have caused sporadic but severe human infections in China, representing a persistent zoonotic threat. Here, we identified a duck-origin H5N6 HPAIV (A/Duck/Jiangsu/628/2022) through routine surveillance and assessed its biological characteristics and mammalian pathogenesis. Phylogenetic analysis revealed > 98% nucleotide identity between strain 628 and the concurrent human H5N6 strain A/Yangzhou/125/2022. Molecular characterization identified multiple mammalian adaptation markers: hemagglutinin substitutions (S137A, T160A, T192I) associated with enhanced human receptor binding; neuraminidase mutations (I117T, D198N) linked to reduced neuraminidase inhibitor susceptibility; and polymerase complex changes (PB1-D622G, PA-K142Q) conferring increased mammalian cell replication. In vitro studies demonstrated that 628 virus replicated more efficiently in mammalian than in avian cells and exhibited dual receptor-binding specificity. Mouse pathogenicity assays revealed moderate virulence with progressive lung pathology. Critically, transmission experiments confirmed both direct contact and airborne transmission capabilities of 628 in guinea pigs. These findings demonstrate that circulating H5N6 viruses have acquired partial mammalian adaptation while retaining avian fitness, significantly elevating pandemic potential. Enhanced surveillance of wild bird populations, poultry farms, and live poultry markets is urgently needed to develop effective prevention and control strategies.

Source: 

Link: https://www.sciencedirect.com/science/article/abs/pii/S037811352600146X?via%3Dihub

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Use of #baloxavir as adjunctive #antiviral #therapy to neuraminidase inhibitors in severely immunocompromised individuals infected with #influenza

 

ABSTRACT

Immunocompromised patients are at risk of developing severe influenza, with protracted viral shedding and development of resistance-associated mutations under antiviral treatment. We report a case series of severely immunocompromised hematology patients, including allogeneic hematopoietic cell transplantation (HCT) recipients, treated with both baloxavir and oseltamivir and describe clinical and virological outcomes and the safety profile of prolonged combination therapy. Allogeneic HCT recipients with influenza infection treated with baloxavir were retrieved via institutional databases. All hospitalized allogeneic HCT patients treated with a combination therapy of baloxavir and oseltamivir over five influenza seasons between October 2019 and May 2025 were included. Six influenza-infected hematology patients (5/6 allogeneic HCT recipients) were treated with combination therapy of oseltamivir and baloxavir. All patients presented with lower respiratory tract infections. Oseltamivir treatment duration ranged from 5 to 31 days, and the number of administered baloxavir doses ranged between one and five. Baloxavir administration was well tolerated, and no adverse events could be attributed to the administered antiviral treatment. All-cause mortality at 3 months post-infection was 66% (4/6), mainly driven by underlying disease. In two patients with protracted shedding, combination therapy did not prevent the development of resistance mutation(s). Combination treatment with prolonged courses of oseltamivir and repeated doses of baloxavir was well tolerated. No definitive conclusions on the efficacy of this approach could be drawn from this study. More data are required on the best treatment of hematology patients infected with influenza.

Source: 

Link: https://journals.asm.org/doi/10.1128/aac.01659-25

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#Survival #trends in patients with difficult-to-treat, #antibiotic-resistant, Gram-negative #infections in the era of next-generation antibiotics in the #USA: a retrospective cohort study

 

Summary

Background

Difficult-to-treat resistant (DTR) Gram-negative infections show resistance to all first-line antibiotics (ie, β-lactams and fluoroquinolones) and have a 40% greater mortality rate than susceptible infections. New antibiotics are now available with improved safety and efficacy and with in-vitro activity against DTR infections; however, their influence on the outcomes of patients with DTR infections remains unclear. We aimed to evaluate whether and why mortality in patients with DTR infections has changed since the introduction of these newer antibiotics in the USA.

Methods

In this retrospective cohort study in the USA, adult patients (aged ≥18 years) with a DTR Gram-negative infection, defined as microbiological evidence of DTR Enterobacterales, Pseudomonas aeruginosa, or Acinetobacter baumannii and receipt of at least 3 consecutive days of any antibiotic therapy, were identified from hospitals reporting microbiology data in the PINC-AI Healthcare Database. We characterised the proportion of inpatient encounters receiving newer DTR-active antibiotics, traditional DTR-active antibiotics, and non-DTR-active antibiotics. We used a generalised linear mixed model with marginal predictions to examine changes in in-hospital mortality, defined as death or discharge to hospice, over the study period when adjusting for patient-related and treatment-related factors (including receipt of a new antibiotic and receipt of in-vitro discordant initial therapy), hospital-related factors (including the availability of newer antibiotics and corresponding susceptibility testing), and COVID-19 pandemic-related factors. A three-way interaction term for time (year), pathogen, and infection site (ie, bloodstream and non-bloodstream) was included given the expected differences in mortality.

Findings

Between Jan 1, 2016, and Aug 31, 2023, 8 319 398 adult inpatient encounters with available microbiology data were recorded from 471 hospitals, of which 9384 (0.11%) encounters had microbiological evidence of an eligible DTR organism. 5065 (54·0%) of these 9384 encounters, from 262 hospitals, met the inclusion criteria for DTR Gram-negative infections and were included in the study. Among this cohort, the prescription of newer antibiotics, as well as the availability of newer antibiotics and their corresponding susceptibility tests, increased substantially from 2016 to 2023. Although the proportion of encounters in which the patient received a newer antibiotic as initial therapy increased from 4% (21 of 589) in 2016 to 15% (34 of 234) in 2023, in most cases (196 [84%] of 234) patients continued to receive in-vitro discordant initial antibiotic therapy, even in 2023. We observed no change in the average marginal effect (the average percentage change per year) for adjusted mortality between 2016 and 2023 for Enterobacterales (0.1% [95% CI −1.1 to 1.4]), P aeruginosa (−0.7% [−1.7 to 0.3]), or A baumannii (−0.4% [−1.8 to 0.9]) infections. When dichotomised into bloodstream and non-bloodstream infections, the marginal effect for adjusted mortality remained unchanged over time for most pathogen and site combinations, with the exception of P aeruginosa bloodstream infections, for which a decrease was observed (−4.5% [−8.2 to −0.60]).

Interpretation

Despite the availability of newer antibiotic agents, the estimated mortality and ongoing use of in-vitro discordant initial antibiotics remains unacceptably high among patients with DTR infections in US hospitals. Prompt recognition of both the pathogen and resistance phenotype could be a crucial component in reducing mortality. Although notable, the decrease over time in adjusted mortality for P aeruginosa bloodstream infections should be considered hypothesis-generating because the cohort of patients with such infections was small.

Funding

US National Institutes of Health (NIH) Clinical Center; US National Cancer Institute; the Intramural Research Program of the US National Heart, Lung, and Blood Institute; the US National Institute of Allergy and Infectious Diseases; and the US Food and Drug Administration.

Research in context

Evidence before this study

Difficult-to-treat resistance is a resistance phenotype describing Gram-negative pathogens that show resistance to all first-line, safe and effective traditional antibiotic options (ie, β-lactams [including carbapenems] and fluoroquinolones). As a result, difficult-to-treat resistant (DTR) infections are associated with high mortality, of which they are a better predictor than susceptible infections. Several new antibiotics have been introduced into the US market since 2014 and could provide more treatment options to patients with antimicrobial-resistant (AMR) infections such as DTR infections, for whom no safe and effective treatment exists. Evidence supporting a reduction in mortality after treatment with these new antibiotics, specifically in patients with AMR infections, is scarce. Understanding the population-level change in mortality following the implementation of these new antibiotic therapies could inform current priorities in therapeutic and diagnostic development to improve outcomes in this high-risk patient population. We searched PubMed from database inception to Dec 16, 2025, for studies with the Medical Subject Headings Major Topics “Bacterial Infections/mortality” AND “Drug Resistance, bacterial”, with no language restrictions. This search returned 426 publications. 34 articles evaluated trends in bacterial resistance over time and associated mortality among drug-resistant bacterial infections, of which 13 examined only Gram-positive bacterial infections and eight examined a period before the introduction of any of the newer antibiotics (ie, before 2014). Mortality estimates over time in the remaining 13 publications considered only discrete phenotypes for which alternative effective treatment options might exist, did not characterise the use of newer antibiotics within the study population, and did not adjust for relevant COVID-19 pandemic-related factors that are likely to affect the interpretation of mortality trends since 2020.

Added value of this study

This study leverages an administrative and clinical database to identify, to our knowledge, the largest cohort to date of hospitalised patients with DTR Gram-negative infections in the USA. We evaluate adjusted mortality trends in patients with DTR infections after the real-world introduction of six new antibiotics (ceftolozane–tazobactam, ceftazidime–avibactam, meropenem–vaborbactam, imipenem–relebactam, eravacycline, and cefiderocol) with broad spectra of activity and variable side-effect profiles. Trends in mortality were examined using a generalised linear mixed model adjusted for relevant patient-related, hospital-related, and COVID-19 pandemic-related covariates. The estimated probability of mortality across DTR Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii infections did not change from 2016 to 2023. When dichotomised into bloodstream and non-bloodstream infections, we identified a 4.5% annual reduction in the adjusted mortality for P aeruginosa bloodstream infections, albeit the sample size for this population was small (n=87). The trends in adjusted mortality for all other pathogen and infection-site combinations remained unchanged. These findings occurred in the setting of persistent and frequent use of in-vitro discordant initial empirical therapy despite a substantial increase in access to and use of newer antibiotic options.

Implications of all the available evidence

Our results underscore the importance of better understanding why survival has not improved across all DTR Gram-negative infections despite the availability of newer, safe, and effective broad-spectrum antibiotic options. The development and real-world implementation of improved rapid diagnostic platforms for early detection of resistant phenotypes could improve patient outcomes. Comparative trials of new antibiotics with increased representation of patients with highly resistant bacterial infections, namely DTR infections, could elucidate the true clinical benefit of newer antibiotics in their respective target populations. Exploration of additional non-antibiotic, host-directed therapies could address the immune dysregulation that might contribute to poor patient outcomes, even among those who die with susceptible infections.

Source: 

Link: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(26)00020-4/fulltext?rss=yes

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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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#Report on #influenza viruses received and tested by the #Melbourne #WHO CC for #Reference and Research on #Influenza during 2024

 

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. Of 4,007 samples tested for susceptibility to the neuraminidase inhibitors oseltamivir and zanamivir, twelve A(H1N1)pdm09 viruses and one B/Victoria virus showed highly reduced inhibition against oseltamivir or zanamivir. Of 3,294 total samples sequenced for baloxavir susceptibility, 18 of the 1,825 A(H3N2) samples were identified with genetic evidence of reduced susceptibility to baloxavir marboxil in the PA gene.

Source: 

Link: https://ojs.cdi.cdc.gov.au/index.php/cdi/article/view/3449

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Impaired #host shutoff is a fitness cost associated with #baloxavir marboxil #resistance #mutations in #influenza A virus PA/PA-X nuclease domain

 

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 but has a unique C-terminal domain generated by ribosomal frameshifting during translation. Unlike their effects on PA activity, the effects of BXM or the I38T/M substitutions on PA-X function remain uncharacterized. In our work, for the first time, we directly examine the effects of baloxavir and the I38T/M substitutions on PA-X activity and show that baloxavir inhibits PA-X activity in a dose dependent manner. Most importantly, we also demonstrate that the I38T/M mutations significantly impair the host shutoff activity of PA-X proteins from different IAV strains of H1N1, H3N2, and H5N1 subtypes. Our work reveals that the deleterious effects of I38T/M on PA-X function may represent an important barrier to the spread of BXM-resistant viruses.

Source: 

Link: https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1013550

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Genomic #Evolution of #Influenza A Virus During the 2024-2025 Season, the Johns Hopkins Health System: Antigenic Drift Reduces Serum Neutralization

 

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. Circulating A(H3N2) viruses belonged to multiple HA subclades and exhibited distinct amino acid substitutions at key antigenic sites. Neutralization assays using sera from individuals vaccinated with the 2024–2025 seasonal influenza vaccine demonstrated reduced neutralization of three dominant A(H1N1)pdm09 isolates and two A(H3N2) isolates compared with vaccine strains, consistent with antigenic drift. In addition, the neuraminidase substitution S247N, previously associated with reduced oseltamivir susceptibility, was detected in 13.9% of A(H1N1)pdm09 samples.

Discussion

These findings demonstrate ongoing antigenic drift and the presence of antiviral resistance–associated mutations during the 2024–2025 influenza season, underscoring the need for continued genomic surveillance to guide vaccine and antiviral strategies.

Source: 

Link: https://academic.oup.com/jid/advance-article/doi/10.1093/infdis/jiag069/8461561#google_vignette

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#Macrolide #Resistance and P1 Cytadhesin Genotyping of #Mycoplasma pneumoniae during #Outbreak, #Canada, 2024–2025

 

Abstract

We investigated macrolide resistance and P1 genotypes of Mycoplasma pneumoniae during the 2024–2025 outbreak in Hamilton, Ontario, Canada. Macrolide resistance remained stable at ≈10%–20%, but significant shifts in P1 genotype distribution and resistance rates in P1 types occurred, indicating notable changes in M. pneumoniae molecular epidemiology in Ontario since 2011–2012.

Source: 

Link: https://wwwnc.cdc.gov/eid/article/31/12/25-0872_article

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#Influenza PA #Substitutions and Genetic Diversity of #H1N1pdm09, #H3N2, and B/Victoria Viruses in #Japan During the 2023–2024 Season

 

Abstract

We characterized influenza A(H1N1)pdm09, A(H3N2), and B/Victoria viruses circulating in Japan during 2023–2024, focusing on lineage placement relative to WHO-recommended vaccine strains and on baloxavir resistance (PA/I38T substitutions). We enrolled 210 outpatients with influenza-like illness across eight clinics in six prefectures (October 2023–September 2024). Of these, 209 had an analyzable pre-treatment respiratory specimen for RT-PCR; hemagglutinin (HA) and neuraminidase (NA) genes were sequenced by next-generation sequencing (NGS). PA/I38T substitutions that confer baloxavir resistance were assessed by cycling-probe RT-PCR, Sanger sequencing, and NGS. HA phylogenies were constructed with global datasets and WHO vaccine reference strains. Of 209 pre-treatment specimens, 181 were influenza-positive (A(H1N1)pdm09 44.2%, A(H3N2) 37.6%, B/Victoria 18.2%); 51 follow-up specimens were collected ≈4–5 days after baloxavir or neuraminidase inhibitor therapy. HA phylogeny placed A(H1N1)pdm09 in clades 5a.2a/5a.2a.1 with predominance of subclade D.2. A(H3N2) clustered exclusively in clade 2a.3a.1 (J lineage, mostly J.1), indicating a mismatch with the season’s A/Darwin/9/2021 vaccine component and supporting the subsequent J-lineage update. All B/Victoria genomes fell within V1A.3a.2 on a C.5 backbone (C.5.1 and C.5.7). No PA/I38T variant was detected in any pre-treatment specimen. Post-baloxavir, PA/I38T emerged in one A(H3N2) case (confirmed by all three methods) and in one B/Victoria case detected by NGS only (minority variant in a low-load sample). NA genes showed no substitutions associated with reduced susceptibility to laninamivir (e.g., E119A, G147E). During 2023–2024, A(H1N1)pdm09 and B/Victoria remained genetically aligned with their vaccine components, whereas A(H3N2) shifted to the J lineage, consistent with the 2024–2025 vaccine update. Although pre-treatment PA/I38T was absent, low-frequency on-therapy selection was observed, including a rare PA/I38T in influenza B/Victoria detected by NGS, suggesting the value of deep sequencing when viral loads are low. These integrated genomic–clinical data support vaccine strain realignment for H3N2 and continued monitoring of baloxavir resistance in outpatient care.

Source: 

Link: https://www.mdpi.com/1999-4915/18/1/13

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Post #COVID19 #resurgence of #Mycoplasma pneumoniae infections in French #children (ORIGAMI): a retrospective and prospective multicentre cohort study

 

Summary

Background

Following a decline during the COVID-19 pandemic, Mycoplasma pneumoniae infections resurged in several countries. We aimed to characterise the clinical presentation of paediatric patients admitted to hospital for M pneumoniae during 2023 and 2024 in France.

Methods

We conducted a nationwide, multicentre, retrospective, and prospective observational study across 37 French paediatric hospitals (September, 2023–September, 2024). Children younger than 18 years who were hospitalised with laboratory-confirmed M pneumoniae infection (PCR or serology) were included. Demographics (excluding race), clinical features, laboratory and radiological findings, management, and outcomes data were described and analysed. Logistic regression was used to identify factors associated with paediatric intensive care unit (PICU) admission. The trial was registered at ClinicalTrials.gov (NCT06260371) and is complete.

Findings

We included 969 children and adolescents with M pneumoniae infection (7·3 years [SD 4·5], 426 [44%] of 966 patients were female and 540 [56%] of 966 were male). 936 (97%) of all patients were positive by PCR for M pneumoniae. Pneumonia was diagnosed in 628 (87%) of the 726 patients with respiratory involvement, and cutaneous manifestations were reported in 132 (14%) of 969 patients, including 56 (42%) of 132 who had erythema multiforme. Macrolides were prescribed in 884 (95%) of the 931 patients who were prescribed antibiotics, primarily azithromycin (563 [64%] of 884). Macrolide resistance was detected in one (5%) of the 21 tested samples. In total, 57 (6%) of 969 patients required PICU admission and four (<1%) died. Factors significantly associated with PICU admission included being older than 11 years (adjusted odds ratio 2·0 [95% CI 1·1–3·6]; p=0·023), asthma (2·2 [1·2–4·0]; p=0·0072), other underlying conditions (2·1 [1·2–3·7]; p=0·013), and erythema multiforme (3·7 [1·6–8·8]; 0·0025).

Interpretation

The 2023–2024 M pneumoniae epidemic in France resulted in a substantial paediatric hospitalisation burden. Although severe cases were uncommon, children older than 11 years, those with asthma, other comorbidities, and erythema multiforme were at increased risk of PICU admission. Ongoing surveillance and targeted management strategies are warranted for future epidemics.

Funding

Association Clinique et Thérapeutique Infantile du Val de Marne (ACTIV).

Source: 

Link: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(25)00598-5/abstract?rss=yes

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Molecular basis of #SARS-CoV-2 proofreading enzyme–mediated #resistance to #remdesivir

 

Abstract

SARS-CoV-2’s remarkable resistance to nucleotide analog antivirals such as remdesivir, which thwarts RNA synthesis by inhibiting viral polymerase (RdRp), challenges available therapies. We reveal that remdesivir incorporation destabilizes RdRp–RNA complex while enhancing RNA binding to the proofreading exoribonuclease (ExoN), facilitating remdesivir excision. Conserved ExoN determinants for remdesivir recognition and excision underpin ExoN-mediated resistance across all coronaviruses. These findings inform the design of next-generation antivirals and combination therapies capable of overcoming ExoN-mediated resistance.

Source: Proceedings of the National Academy of Sciences of the United States of America, https://www.pnas.org/doi/full/10.1073/pnas.2519755122

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Impaired host shutoff is a #fitness cost associated with #baloxavir marboxil #resistance mutations in #influenza A virus PA/PA-X nuclease domain.

 

Abstract

The polymerase acidic (PA) protein is a subunit of the trimeric influenza A virus (IAV) RNAdependent 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 but has a unique C-terminal domain generated by ribosomal frameshifting during translation. Unlike their effects on PA activity, the effects of BXM or the I38T/M substitutions on PA-X function remain uncharacterized. In our work, for the first time, we directly examine the effects of baloxavir and the I38T/M substitutions on PA-X activity and show that baloxavir inhibits PA-X activity in a dose dependent manner. Most importantly, we also demonstrate that the I38T/M mutations significantly impair the host shutoff activity of PA-X proteins from different IAV strains of H1N1, H3N2, and H5N1 subtypes. Our work reveals that the deleterious effects of I38T/M on PA-X function may represent an important barrier to the spread of BXM-resistant viruses.

Competing Interest Statement

The authors have declared no competing interest.

Source: BioRxIV, https://www.biorxiv.org/content/10.1101/2025.09.22.677688v1

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