Historical #Pandemic and Contemporary #Influenza A Viruses Reveal #PB2 M631L as a Convergent #Adaptation to #Human ANP32

 

Abstract

Understanding the genetic changes that allow avian influenza A viruses (IAVs) to switch their natural hosts and establish productive infection in humans is important for pandemic risk assessment. Adaptations in the IAV polymerase are required to overcome species-specific restrictions imposed by host ANP32 proteins. Notably, avian virus polymerase is generally only poorly supported by human ANP32 proteins due to species-specific differences. Consequently, efficient polymerase adaptation to the binding interface of human ANP32 requires distinct amino acid changes, such as PB2 E627K. A separate adaptation, PB2 M631L, has recently been reported in mammalian-adapted IAV; however, its functional role across divergent viral lineages and its relationship to host ANP32-dependent adaptation remain incompletely defined. Here, we examine PB2 M631L in the polymerases of a 1918 pandemic strain, a recombinant contemporary H1N1pdm09, and a recent clade 2.3.4.4b H5N1 virus. Using polymerase activity and protein-interaction assays, we show that PB2 M631L enhances polymerase activity and ANP32 binding in human—but not avian—contexts, and that this effect is conserved across multiple viral backgrounds. In H1N1pdm09, PB2 M631L also increased virus replication in mammalian cells. These findings indicate that PB2 M631L contributes to enhanced polymerase compatibility with human ANP32 proteins and are consistent with a role in adaptation across multiple influenza virus lineages. Our results highlight how analysis of historical pandemic strains can inform risk assessment for future emerging viruses.

Source: 

Link: https://www.mdpi.com/2076-2607/14/4/859

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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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#Immunity to #hemagglutinin and #neuraminidase results in additive reductions in #airborne #transmission of #influenza #H1N1 virus in #ferrets

 

Abstract

Currently, there is limited knowledge on the impact of immunity to hemagglutinin (HA) and/or neuraminidase (NA) on the transmission of influenza viruses. Therefore, using intramuscular vaccination, intranasal vaccination, or infection with reassortant viruses, we induced immunity to each antigen alone or both antigens combined in ferrets. We then assessed transmission of the 2009 pandemic H1N1 virus from these ferrets to naïve respiratory contacts. For all strategies used to induce immunity, combined immunity to HA and NA resulted in the largest reductions in transmission. Moreover, immunity to HA and NA conferred additive rather than synergistic reductions in transmission. No escape variants emerged in our transmission studies, and logistical regression showed that the probability of transmission was less than 50% when viral titers in donors were reduced to 101.5 and 102 median tissue culture infectious dose per ml on days 1 and 3 postinfection, respectively. These studies define the relationship between immunity to HA and NA on transmission and identify a threshold titer indicative of decreased transmission in ferrets.

Source: 

Link: https://www.science.org/doi/10.1126/sciadv.aea8719

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Defining the transmissible dose 50% for two #pandemic #influenza viruses in #ferrets

 

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 H1N1 virus, the ID50 and TD50 were equivalent at <1 TCID50. However, for the 1968 pandemic H3N2 virus, the ID50 and TD50 were 100.5 and 104.08 TCID50 (95% CI: 102.34–105.82), respectively. The increased TD50 for the H3N2 virus was associated with significant reductions in peak viral titers and viral shedding in donors over decreasing virus inoculation doses. Collectively, these studies define a new measure of transmission that permits comparisons of transmissibility between viral strains and subtypes in ferrets. We show that the 1968 pandemic H3N2 virus has a higher TD50 and reduced transmissibility in ferrets relative to the 2009 pandemic H1N1 virus.

Source: 

Link: https://journals.asm.org/doi/full/10.1128/jvi.01635-25?af=R

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A newly emergent N1 #neuraminidase associated with clade 2.3.4.4b highly pathogenic avian #influenza #H5 viruses in North #America

 

Abstract

We investigated the evolutionary history of the newly emergent neuraminidase (am4N1) associated with the D1.1 and D1.2 genotypes of highly pathogenic avian influenza A(H5N1) viruses in North America. Phylogenetic inference places am4N1 in a sister clade to Eurasian avian, swine, and human A(H1N1)pdm09 viruses and distinct from 1918, pre-2009 human seasonal, and classical swine A(H1N1) lineages. Am4N1 descends from diverse avian N1 genes endemic to the Americas. Phylodynamic analysis indicates a monophyletic am4N1 lineage with numerous introductions of viruses carrying the am4N1 gene likely originating from western Canada into the United States during emergence of the D1.1 and D1.2 genotypes. The lineage has diversified and accumulated deletions in the stalk domain. Despite amino acid divergence, structural modeling shows conserved neuraminidase architecture in the globular head. Given its distinct ancestry and amino acid sequence, further studies are needed to assess cross-reactivity of antibodies from prior human A(H1N1)pdm09 infections.

Competing Interest Statement

The authors have declared no competing interest.

Funding Statement

This study did not receive any external funding.

Source: 

Link: https://www.medrxiv.org/content/10.64898/2026.03.09.26347929v1

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Recommended #composition of #influenza virus #vaccines for use in the 2026 – 2027 northern hemisphere influenza season (#WHO, Feb. 27 '26)

 

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}.  

Seasonal influenza activity 

From September 2025 through January 2026, influenza activity was reported in all transmission zones. 

Overall influenza virus detections were higher compared to the same reporting period in 2024-2025 but peaked in December 2025 for this recent period compared to February 2025 for the previous period. 

During this reporting period, influenza A viruses predominated, although the proportion of virus detections varied among transmission zones. 

In Africa, influenza activity increased during the start of the reporting period, with a predominance of influenza A viruses in all transmission zones. 

In Eastern, Northern, and Western Africa, among subtyped influenza A viruses, A(H1N1)pdm09 viruses accounted for the majority of detections early in the reporting period while A(H3N2) viruses predominated later in the reporting period. 

Influenza detections peaked in November in Western Africa and December in Eastern and Northern Africa. 

In Middle Africa, influenza detections remained low throughout the reporting period with a slight predominance of A(H1N1)pdm09 viruses early in the reporting period. 

In Southern Africa, influenza detections remained low throughout the reporting period, with a predominance of influenza A viruses. 

In Northern and Middle Africa, there was low and sustained influenza B activity throughout the reporting period. 

In Asia, influenza activity increased during the start of the reporting period in South East and Western Asia, from October in Central and Eastern Asia, and from November in Southern Asia, with a predominance of influenza A viruses in all transmission zones. 

Most influenza detections were reported from Eastern Asia, where activity peaked in early December. 

In Southern Asia, influenza activity also peaked in December; in Central Asia influenza activity peaked in November, and in Western and South East Asia, influenza activity peaked in October. 

Among subtyped influenza A viruses, A(H3N2) viruses accounted for the majority of detections in all transmission zones; detections of A(H1N1)pdm09 and influenza B viruses remained low in most transmission zones throughout the reporting period, except in Eastern Asia where there was a substantial rise in influenza B viruses in recent weeks. 

In Europe, influenza activity increased from mid-September in Northern Europe, from October in South West Europe and from mid-November in Eastern Europe, with a predominance of influenza A viruses in all transmission zones. 

Influenza detections peaked in December in Northern and South West Europe but remained elevated through January. 

Influenza detections continued to increase through January in Eastern Europe. 

Among subtyped influenza A viruses, A(H3N2) viruses predominated. 

In South West Europe, detections of A(H1N1)pdm09 viruses slightly increased in mid-November. 

In Eastern and Northern Europe, detections of A(H1N1)pdm09 and influenza B viruses remained low throughout the reporting period.  

In the Americas, influenza activity increased from the start of the reporting period in Temperate and Tropical South America and from November in North America and Central America Caribbean. 

Influenza A viruses accounted for most detections, and influenza B virus detections remained low throughout the reporting period in all transmission zones, except in North America where there was a substantial rise in influenza B viruses in recent weeks. 

In North America, activity peaked in late December. 

Among subtyped influenza A viruses, there was a predominance of A(H3N2) viruses. 

In Central America Caribbean, influenza activity remained elevated through mid-January with A(H3N2) virus detections predominant from December. 

In Tropical South America, influenza activity peaked in early November and slowly declined until the end of the reporting period. 

Among subtyped influenza A viruses, A(H3N2) predominated through November then co-circulated at similar proportions to A(H1N1)pdm09 until the end of the reporting period. 

In Temperate South America, influenza activity peaked in mid-November and among subtyped influenza A viruses, A(H3N2) viruses predominated throughout the reporting period.  

In Oceania, influenza activity decreased until mid-October, increased in December and decreased since mid-December. A(H1N1)pdm09 and B viruses were detected at similar levels until mid-September and A(H3N2) virus detections predominated since then. 

Influenza A 

Globally, influenza A virus detections greatly outnumbered those of influenza B. 

Among subtyped influenza A viruses, A(H3N2) viruses predominated throughout the reporting period in most transmission zones. 

In Eastern, Northern, Western Africa, Central America Caribbean and Oceania, influenza A(H1N1)pdm09 virus detections predominated during the first part of the reporting period, and A(H3N2) virus detections predominated in the latter part of the reporting period. 

Influenza A(H1N1)pdm09 virus detections increased slightly towards the latter part of the reporting period in Eastern and South West Europe, Central America Caribbean and Tropical South America. 

The overall number of influenza detections was low in Middle and Southern Africa. 

Influenza B 

Globally, influenza B virus detections remained low throughout the reporting period. 

Increases in influenza B virus detections in January were reported in North America and Eastern Asia. 

All influenza B viruses where lineage was confirmed belonged to the B/Victoria lineage. 

(...)

Zoonotic influenza  

From 23 September 2025, sporadic zoonotic influenza infections were reported, in most cases, following exposure to infected birds, swine or contaminated environments. 

Single cases of A(H5N1) from Bangladesh, A(H5N2) from Mexico, and A(H5N5) from the United States of America were reported. 

Three A(H5N1) cases were reported from Cambodia. 

Fourteen cases of A(H9N2) and one case of A(H10N3) were reported from China. 

Single cases of A(H1N1)v and A(H1N2)v were reported from China, a case of A(H1N2)v from the United States of America, and a case of A(H3N2)v from Brazil. 

Genetic and antigenic characteristics of recent seasonal influenza viruses, human serology and antiviral susceptibility 

Influenza A(H1N1)pdm09 viruses  

Since September 2025, A(H1N1)pdm09 viruses circulated globally, but did not predominate in any region. 

The haemagglutinin (HA) genes of viruses that were genetically characterized belonged to the 5a.2a and 5a.2a.1 clades. 

Viruses from clade 5a.2a subclades C.1, C.1.9 and C.1.9.3 circulated in low numbers, with the largest proportion of detections in Africa {5}. 

Since September 2025, clade 5a.2a.1 subclades D.3.1 and D.3.1.1 viruses circulated globally. 

The D.3.1 subclade with substitutions T120A, I372V, I460T and V520A predominated in Western Pacific, Africa, South East Asia and several countries in the Americas. 

D.3.1.1 viruses characterized by R113K and more recently acquired substitutions A139D, E283K and K302E predominated in some countries in Europe, the Middle East and North America. 

The antigenic properties of A(H1N1)pdm09 viruses were assessed in haemagglutination inhibition (HI) assays with post-infection ferret antisera. 

HI results for viruses with collection dates since September 2025 showed that ferret antisera raised against cell culture-propagated A/Wisconsin/67/2022-like and eggpropagated A/Victoria/4897/2022-like viruses from the 5a.2a.1 clade recognized viruses in both 5a.2a and 5a.2a.1 clades well. 

However, post-infection ferret antisera raised against viruses from clade 5a.2a showed some reduction in recognition of the now predominating D.3.1 and D.3.1.1 subclade viruses. 

Post-infection ferret antisera raised against viruses from subclade D.3.1 (e.g., A/Missouri/11/2025) recognized recently circulating viruses from both 5a.2a and 5a.2a.1 clades well.  

Human serology studies used 15 serum panels from children, adults (18 to 64 years) and older adults (≥65 years) who had received egg-propagated inactivated (standard, high dose or adjuvanted), cell culture-propagated inactivated or recombinant trivalent or quadrivalent vaccines with NH 2025-2026 influenza vaccine formulations. 

-- NH 2025-2026 egg-based vaccines contained A/Victoria/4897/2022 (H1N1)pdm09like, 

-- A/Croatia/10136RV/2023 (H3N2)-like, 

-- B/Austria/1359417/2021-like (B/Victoria lineage) and, in quadrivalent vaccines, 

-- B/Phuket/3073/2013-like (B/Yamagata lineage) virus antigens. 

Cell culture-propagated and recombinant vaccines contained A/Wisconsin/67/2022 (H1N1)pdm09-like, A/District of Columbia/27/2023 (H3N2)-like and B/Austria/1359417/2021-like (B/Victoria lineage) virus antigens. 

Recent A(H1N1)pdm09 viruses with HA genes from clades 5a.2a (subclade C.1.9.3) and 5a.2a.1 (subclades D.3.1 and D.3.1.1) were analysed in HI assays using these human serum panels. 

When compared to the responses to cell culture-propagated A/Wisconsin/67/2022 (H1N1)pdm09-like vaccine reference viruses, post-vaccination geometric mean titres (GMTs) were significantly reduced for some recently circulating viruses from D.3.1 and D.3.1.1 subclades. 

Of 1 161 A(H1N1)pdm09 virus clinical samples and isolates examined by genetic and/or phenotypic analyses, 15 viruses showed evidence of reduced susceptibility to neuraminidase inhibitors (NAIs): seven had an H275Y neuraminidase (NA) substitution and eight had I223V and S247N substitutions. 

Of 1 331 A(H1N1)pdm09 viruses examined by genetic and/or phenotypic analyses, no viruses showed evidence of reduced susceptibility to the endonuclease inhibitor baloxavir marboxil. 

Influenza A(H3N2) viruses  

Phylogenetic analysis of the HA gene sequences of A(H3N2) viruses collected since September 2025 showed that the vast majority of viruses belonged to one of the J.2 subclades {6}, expressing HA N122D and K276E substitutions. 

HA genes have diversified with many subclades; J.2.2 (characterized by S124N), J.2.3 (characterized by N158K, K189R and S378N), J.2.4 (characterized by T135K [a potential loss of an N-glycosylation site] and K189R), and K (formerly designated as J.2.4.1; characterized by K2N, S144N [a potential addition of an N-glycosylation site], N158D, I160K, Q173R, T328A and S378N). 

During this reporting period, subclade K viruses were detected in all regions and predominated in many countries. 

There was still circulation of other J.2 subclades, notably J.2 or J.2.3 in South America, J.2.2 or J.2.4 in Africa and Asia.  

Post-infection ferret antisera raised against cell culture-propagated A/District of Columbia/27/2023-like and egg-propagated A/Croatia/10136RV/2023-like (clade 2a.3a.1, subclade J.2) viruses, representing the A(H3N2) component for the NH 2025-2026 influenza vaccines, showed poor recognition with recently circulating subclade J.2.3 (e.g., A/Netherlands/10685/2024), J.2.4 (e.g., A/Sydney/1359/2024) and K (e.g., A/Darwin/1415/2025) viruses. 

Ferret antisera raised against reference viruses from J.2.3 subclades showed good recognition of viruses expressing HA from J.2.3, but poor recognition of other subclades.  

Post-infection ferret antisera raised against cell culture-propagated A/Sydney/1359/2024-like and eggpropagated A/Singapore/GP20238/2024-like J.2.4 viruses, representing SH 2026 influenza vaccines, recognized most J.2.4 viruses and many subclade K viruses well. 

However, subclade K viruses and J.2.4 viruses with HA substitutions F79V, S144N (addition of a potential N-glycosylation site), N158D, I160K, T328A were better recognized by post-infection ferret antisera raised against cell culture-propagated A/Darwin/1415/2025-like and egg-propagated A/Darwin/1454/2025-like (subclade K) viruses. 

Human serology studies were conducted using the serum panels as described above by HI and virus neutralization (VN) assays with recent circulating A(H3N2) viruses with HA genes from subclades J.2, J.2.2, J.2.3, J.2.4, J.2.5 and K. 

When compared to titres against cell-propagated A/District of Columbia/27/2023-like vaccine reference viruses, post-vaccination HI GMTs or VN GMTs against many of the recent viruses in all subclades tested were significantly reduced in many serum panels.  

(...)

Of 4 458 influenza A(H3N2) viruses examined by genetic and/or phenotypic analyses, two viruses showed evidence of reduced susceptibility to NAIs; both had an NA E119V substitution. 

Of 4 828 A(H3N2) viruses examined by genetic and/or phenotypic analyses, nine viruses showed evidence of reduced susceptibility to the endonuclease inhibitor baloxavir marboxil: three had a PA I38T substitution, three had a PA I38I/T substitution, two had a PA I38I/M substitution and one had a PA E199E/G substitution.  

Influenza B viruses  

Since September 2025, influenza B viruses were detected in all WHO regions, and all those characterized belonged to the B/Victoria lineage. 

There have been no confirmed detections of circulating B/Yamagata lineage viruses after March 2020.  

All HA genes of B/Victoria lineage viruses characterized during this reporting period belonged to clade 3a.2 with HA substitutions A127T, P144L, and K203R. 

Viruses with clade 3a.2 HA genes have diversified further, forming several subclades (C.1-C.5)7. 

Viruses designated as C.5, C.5.1, C.5.6, C.5.6.1 and C.5.7, all of which had the HA substitution D197E, circulated at varying proportions in different regions. 

Viruses designated as C.3 have HA substitutions E128K, A154E and S208P. 

Subclade C.3.1 viruses shared additional mutations D197N (addition of a potential N-glycosylation site) and P208S. 

Recent C.3 viruses which had additional changes D197N (addition of a potential N-glycosylation site), S255P and I267V and C.3.1 viruses have been detected in increasing proportions in Eastern Asia and North America in recent weeks. 

Antigenic analysis showed that post-infection ferret antisera raised against B/Austria/1359417/2021-like viruses (3a.2), representing the vaccine viruses for the SH 2026 and NH 2025-2026 influenza seasons, recognized viruses within the C.5.1, C.5.6, C.5.6.1 and C.5.7 subclades well. 

C.3 and C.3.1 subclade viruses with the HA substitution D197N were recognized poorly. 

Post-infection ferret antisera raised against cell culture-propagated viruses from subclade C.3.1 (e.g., B/Pennsylvania/14/2025) recognized recently circulating viruses from C.3, C.3.1 and other 3a.2 subclades well. 

All available egg isolates for subclade C.3 and C.3.1 viruses acquired substitutions that remove the potential N-glycosylation site at HA 197 to 199. 

Post-infection ferret antisera raised against egg-propagated viruses from subclade C.3.1 (e.g., B/Tokyo/EIS13-175/2025, B/Tokyo/EIS13-011/2025, B/Perth/115/2025) showed reduced recognition of recently circulating viruses from C.3 and C.3.1 subclades compared to that of the cell equivalent.  

(...)

In human serology studies, recently circulating B/Victoria lineage viruses with HA genes from clade 3a.2 subclades C.3, C.3.1, C.5.1, C.5.6, C.5.6.1 and C.5.7 were tested using the serum panels described above. 

When compared to titres against egg- or cell culture-propagated B/Austria/1359417/2021-like vaccine reference virus, titres against most viruses with HA genes from C.5.1, C.5.6, C.5.6.1 and C.5.7 subclades were not significantly reduced; however, titres against viruses with HA genes from C.3 and C.3.1 were significantly reduced in most serum panels. Serology studies were not performed for B/Yamagata lineage viruses.  

Of 549 influenza B/Victoria lineage viruses examined by genetic and/or phenotypic analyses, two showed evidence of reduced or highly reduced susceptibility to NAIs, both with an NA M464T substitution. 

Of 760 B/Victoria lineage viruses examined by genetic and/or phenotypic analyses, no viruses showed evidence of reduced susceptibility to the endonuclease inhibitor baloxavir marboxil.  

Recommended composition of influenza virus vaccines for use in the 2026-2027 northern hemisphere influenza season  

Since September 2025, A(H1N1)pdm09 viruses circulated globally. The majority of viruses had HA genes belonging to the 5a.2a.1 clade which has further diversified into the D.3.1 and D.3.1.1 subclades. 

Postinfection ferret antisera raised against the northern hemisphere (NH) 2025-2026 A(H1N1)pdm09 vaccine viruses (cell culture-propagated A/Wisconsin/67/2022 and egg-propagated A/Victoria/4897/2022) and the southern hemisphere (SH) 2026 A(H1N1)pdm09 vaccine viruses A/Missouri/11/2025 recognized D.3.1 and D.3.1.1 viruses well. 

In human serology studies, post-vaccination geometric mean titres (GMTs) were significantly reduced for some recently circulating A(H1N1)pdm09 viruses when compared to the responses to cell culture-propagated A/Wisconsin/67/2022 A(H1N1)pdm09-like vaccine reference viruses. 

Since September 2025, A(H3N2) viruses circulated and predominated globally. 

The vast majority of A(H3N2) viruses collected had HA genes from subclades of J.2 and have continued to diversify with subclade K (previously designated as J.2.4.1) viruses predominating in most regions. 

Post-infection ferret antisera raised against NH 2025-2026 influenza season vaccine viruses (cell culture-propagated A/District of Columbia/27/2023 and egg-propagated A/Croatia/10136RV/2023) recognized some J.2 viruses well but showed poor recognition of viruses from subclades J.2.3, J.2.4 and K. 

Post-infection ferret antisera raised against subclade K viruses (cell culture-propagated A/Darwin/1415/2025 and egg-propagated A/Darwin/1454/2025) showed improved recognition of K viruses compared to post-infection antisera raised against NH 2025-2026 and SH 2026 A(H3N2) vaccine viruses. 

When compared to titres against cell culture-propagated A/District of Columbia/27/2023-like vaccine reference viruses, human post-vaccination haemagglutinin inhibition (HI) GMTs or virus neutralisation (VN) GMTs against many of the recent viruses in J.2.3, J.2.4 and K subclades were significantly reduced. 

Since September 2025, influenza B virus detections remained low globally, although some countries had increased detections in recent weeks. All circulating influenza B viruses characterized belonged to the B/Victoria lineage, and had HA genes belonging to clade 3a.2 with substitutions A127T, P144L and K203R. 

Post-infection ferret antisera raised against B/Austria/1359417/2021-like viruses (3a.2), representing the vaccine viruses for the SH 2026 and NH 2025-2026 influenza seasons, recognized viruses within the C.5.1, C.5.6, C.5.6.1 and C.5.7 subclades well. C.3 and C.3.1 subclade viruses with HA substitution D197N were recognized poorly. 

Post-infection ferret antisera raised against cell culture-propagated viruses from subclade C.3.1 (e.g., B/Pennsylvania/14/2025) recognized recently circulating viruses from C.3, C.3.1 and other 3a.2 subclades well. All available egg isolates for subclade C.3 and C.3.1 viruses (e.g., B/Tokyo/EIS13-175/2025) acquired egg-adaptive mutations that remove the potential N-glycosylation site at HA 197 to 199, leading to post-infection ferret antisera raised against egg-propagated viruses from subclade C.3.1 (e.g., B/Tokyo/EIS13-175/2025) showing reduced recognition of recently circulating viruses from C.3 and C.3.1 subclades compared to that of the cell equivalent. 

Human serology assays showed that post-vaccination titres against most recent B/Victoria lineage viruses with HA genes from subclades C.5.1, C.5.6, C.5.6.1 and C.5.7 were not significantly reduced when compared to titres against egg- or cell culturepropagated B/Austria/1359417/2021-like vaccine reference viruses. Titres against viruses with HA genes from subclade C.3 and C.3.1 were significantly reduced in most serum panels.  

For vaccines for use in the 2026-2027 northern hemisphere influenza season, WHO recommends the following:  

Egg-based vaccines  

• an A/Missouri/11/2025 (H1N1)pdm09-like virus;  

• an A/Darwin/1454/2025 (H3N2)-like virus; and  

• a B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus.  

Cell culture-, recombinant protein- or nucleic acid-based vaccines  

• an A/Missouri/11/2025 (H1N1)pdm09-like virus;  

• an A/Darwin/1415/2025 (H3N2)-like virus; and  

• a B/Pennsylvania/14/2025 (B/Victoria lineage)-like virus.  

Lists of prototype viruses for egg-, cell culture-, recombinant protein- and nucleic acid-based vaccines together with candidate vaccine viruses (CVVs) suitable for the development and production of human influenza vaccines are available on the WHO website {8}. 

A list of reagents for vaccine standardization, including those for this recommendation, can also be found on the WHO website.  

CVVs and reagents for use in the laboratory standardization of inactivated vaccines may be obtained from:  

• Therapeutic Goods Administration, P.O. Box 100, Woden, ACT, 2606, Australia (email: influenza.reagents@health.gov.au; website: http://www.tga.gov.au).  

• Medicines and Healthcare products Regulatory Agency (MHRA), Blanche Lane, South Mimms, Potters Bar, Hertfordshire, EN6 3QG, the United Kingdom of Great Britain and Northern Ireland  • (email: enquiries@mhra.gov.uk; website: http://www.nibsc.org/science_and_research/virology/influenza_resource_.aspx). 

• Division of Biological Standards and Quality Control, Center for Biologics Evaluation and Research, Food and Drug Administration, 10903 New Hampshire Avenue, Silver Spring, Maryland, 20993, the United States of America (email: cbershippingrequests@fda.hhs.gov).  

• Research Centre for Influenza and Respiratory Viruses, National Institute of Infectious Diseases, 4-7-1 Gakuen, Musashi-Murayama, Tokyo 208-0011, Japan (email: flu-vaccine@nih.go.jp).  

Requests for reference viruses should be addressed to:  

• WHO Collaborating Centre for Reference and Research on Influenza, VIDRL, Peter Doherty Institute, 792 Elizabeth Street, Melbourne, Victoria 3000, Australia (email: whoflu@influenzacentre.org; website: http://www.influenzacentre.org).  

• WHO Collaborating Centre for Reference and Research on Influenza, National Institute of Infectious Diseases, Japan Institute for Health Security 4-7-1 Gakuen, Musashi-Murayama, Tokyo 208-0011, Japan (email: whocc-flu@nih.go.jp).  

• Influenza Division, Centers for Disease Control and Prevention, 1600 Clifton Road, Mail Stop H17-5, Atlanta, GA 30329, the United States of America (email: InfluenzaVirusSurvei@cdc.gov; website: http://www.cdc.gov/flu/).  

- WHO Collaborating Centre for Reference and Research on Influenza, The Francis Crick Institute, 1 Midland Road, London NW1 1AT, the United Kingdom of Great Britain and Northern Ireland (Tel: +44 203 796 1520 or +44 203 796 2444, email: whocc@crick.ac.uk;  • website: http://www.crick.ac.uk/research/worldwideinfluenza-centre).  

• WHO Collaborating Centre for Reference and Research on Influenza, National Institute for Viral Disease Control and Prevention, China CDC, 155 Changbai Road, Changping District, 102206, Beijing, China. (tel: +86 10 5890 0851; email: fluchina@ivdc.chinacdc.cn; website: https://ivdc.chinacdc.cn/cnic/en/).  

WHO provides weekly updates {9} of global influenza activity. Other information about influenza surveillance, risk assessment, preparedness and response can be found on the WHO Global Influenza Programme website {10}.  

Acknowledgements  

The WHO recommendation on vaccine composition is based on the year-round work of the WHO Global Influenza Surveillance and Response System (GISRS). We thank the National Influenza Centres (NICs) of GISRS, and non-GISRS laboratories including the World Organization for Animal Health (WOAH) and the Food and Agriculture Organization of the United Nations (FAO) Network of Expertise on Animal Influenza (OFFLU), who contributed information, clinical specimens, viruses and associated data; WHO Collaborating Centres of GISRS for their in-depth characterization and comprehensive analysis of viruses; University of Cambridge for performing antigenic cartography and phylogenetic analysis; WHO Essential Regulatory Laboratories of GISRS for their complementary virus analyses and contributions from a regulatory perspective; and laboratories involved in the production of high growth/yield reassortants as candidate vaccine viruses. We also acknowledge the GISAID Global Data Science Initiative for the EpiFluTM database and other sequence databases which were used to share gene sequences and associated information; modelling groups for virus fitness forecasting; and the Global Influenza Vaccine Effectiveness (GIVE) Collaboration for sharing estimates of influenza vaccine effectiveness on a confidential basis.  

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{1} Recommendations for influenza vaccine composition: https://www.who.int/teams/global-influenza-programme/vaccines/who-recommendations 

{2} Description of the process of influenza vaccine virus selection and development: http://www.who.int/gb/pip/pdf_files/Fluvaccvirusselection.pdf 

{3} Vaccines in tropics and subtropics: https://www.who.int/teams/global-influenza-programme/vaccines/vaccine-in-tropics-and-subtropics 

{4} Vaccines against influenza WHO position paper – May 2022. Wkly Epidemiol Rec 2022; 97 (19): 185 - 208. Available at: https://iris.who.int/handle/10665/354264 

{5} Real-time tracking of influenza A(H1N1)pdm09 evolution: https://nextstrain.org/seasonal-flu/h1n1pdm/ha/2y?c=subclade 

{6} Real-time tracking of influenza A(H3N2) evolution: https://nextstrain.org/seasonal-flu/h3n2/ha/2y?c=subclade 

{7} Real-time tracking of influenza B/Victoria lineage evolution: https://nextstrain.org/seasonal-flu/vic/ha/2y?c=subclade 

{8} Candidate vaccine viruses: https://www.who.int/teams/global-influenza-programme/vaccines/who-recommendations/candidate-vaccine-viruses 

{9} Current respiratory virus update: https://www.who.int/teams/global-influenza-programme/surveillance-and-monitoring/influenza-updates 

{10} Global Influenza Programme: https://www.who.int/teams/global-influenza-programme 

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Source: 

Link: https://www.who.int/publications/m/item/recommended-composition-of-influenza-virus-vaccines-for-use-in-the-2026-2027-northern-hemisphere-influenza-season

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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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Longitudinal #assessment of functional #antibodies to a novel #influenza virus strain across age groups

 

Abstract

Newly emerging influenza virus strains pose a constant threat as they encounter a population lacking neutralizing antibodies against the new strain. However, cross-reactive non-neutralizing antibodies (nnABs) may be present and assist in mitigating disease symptoms via various effector mechanisms, including antibody-dependent cellular cytotoxicity (ADCC). Although nnABs to influenza virus have received more attention lately, little information is available on their age-related prevalence, steady-state levels, functional properties, and changes in these parameters over time. Using longitudinal samples from adolescents, adults, and older adults, collected before and after the 2009 swine flu pandemic, we comprehensively characterized the specificity and functionality of nnAB responses against H1N1 pandemic 2009 (H1N1pdm09) virus. Remarkably, all participants exhibited cross-reactive antibodies to this virus before having encountered it through infection or vaccination, with the highest baseline levels observed in older adults. The levels of these IgG antibodies showed a strong correlation with engagement of fragment crystallizable γ receptor IIIa (FcγRIIIa) and ADCC activity, both of which were notably lower in adolescents compared to adults and older adults. Without infection or vaccination, average amounts of H1N1pdm09-reactive antibodies remained relatively stable on population level over the 5-year study period. However, on an individual level, substantial increases and decreases occurred. H1N1pdm09 infection or vaccination significantly enhanced specific antibody levels and the FcγRIIIa-engaging capacity of these antibodies in all age groups. ADCC-mediating antibodies increased however only in adolescents, reaching the same level as observed in the adult groups. Taken together, our results demonstrate the presence of cross-reactive, non-neutralizing, functional, and boostable antibodies against a never-encountered influenza virus strain across all age groups. These antibodies can potentially contribute to protection from severe disease. Accordingly, in case of a newly emerging virus, their further enhancement by vaccination could be beneficial as an immediate protective measure before a strain-specific vaccine becomes available.

Competing Interest Statement

The authors have declared no competing interest.

Funding Statement

This study did not receive any funding

Source: 

Link: https://www.medrxiv.org/content/10.64898/2026.02.21.26346781v1

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Evaluating #primer and #probe #mismatch tolerance in an #Influenza A #matrix gene RT #qPCR using contemporary human and zoonotic strains

 

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 and H5N1 strains demonstrated subtype-specific sequence variability, with a total of eleven mismatches identified across seven non-H1N1 isolates (mean mismatch per isolate = 2.43). Probe mismatches predominated (63.6%), occurring primarily at internal positions, while primer mismatches were infrequent and largely avoided 3′ terminal nucleotides. Recent H5N1 isolates (2023–2024) shared conserved internal mismatches in the probe and forward primer, whereas a historical H5N1 isolate (2016) exhibited a distinct profile including a terminal probe mismatch. Despite this variability, mismatch patterns were consistent with preserved amplification potential. 

Conclusion: 

This study demonstrates that the evaluated influenza A M gene RT-qPCR assay exhibits inherent mismatch tolerance across human and zoonotic subtypes. By shifting diagnostic evaluation from strict sequence identity to functional resilience, our findings provide a framework for designing and maintaining robust molecular assays suitable for surveillance and pandemic preparedness amid ongoing viral evolution.

Source: 

Link: https://www.biorxiv.org/content/10.64898/2026.02.23.707407v1

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Impact of an #aminoacid #deletion detected in the #hemagglutinin (HA) #antigenic site of swine #influenza A virus field strains on HA antigenicity

 

ABSTRACT

Swine influenza A virus (swIAV) is an important pathogen with regard to both the swine industry and public health. The pandemic A(H1N1) 2009 outbreak was caused by the swine-origin pandemic A(H1N1) 2009 [A(H1N1)pdm09] virus. Several reports have shown that several amino acid substitutions in the hemagglutinin (HA) antigenic sites can alter HA antigenicity. However, the impact of the amino acid deletion at position 155 on HA antigenicity remains unknown. In this study, we have isolated 11 samples of swIAVs from seven pig farms in Japan and found an amino acid deletion at position 155 of the HA region in one of the isolates of the H1N2 subtype. To examine the impact of this amino acid deletion on viral replication and HA antigenicity, we generated recombinant influenza A viruses possessing the H1 HA gene encoding either an artificial insertion or deletion of glycine at position 155. The growth kinetics of these recombinant viruses in two different cell lines demonstrated that the effect of amino acid deletion at position 155 of H1 HA on viral replication is limited. In contrast, microneutralization assay-based neutralization titers revealed that amino acid deletion significantly altered HA antigenicity. These results demonstrate that a naturally occurring amino acid deletion at position 155 in an H1 HA antigenic site can markedly alter HA antigenicity with only a limited impact on replication in vitro, highlighting the need to monitor such variants in swine populations and to assess their zoonotic potential.

Source: 

Link: https://journals.asm.org/doi/full/10.1128/jvi.01820-25?af=R

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Characterization of a reassortant #H3N2 swine #influenza virus with 2009 pandemic internal #genes and enhanced potential for zoonotic #risk

 

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 genes (M, NP, PB1, PB2, PA) originated from the 2009 pandemic H1N1 lineage, the NS gene exhibiting a North American triple reassortant origin (human-avian-swine origin), and the HA and NA genes belonging to the human-like lineage. Although neither the rescued virus nor its parental strain could replicate effectively in chicken embryos and chicken cells, both demonstrated efficient replication in mammalian cells, reflected by the much higher polymerase activity in mammalian versus chicken cells. The key residues of HA protein (190D, 225D and 228S) collectively enhanced the binding preference for human-type α-2,6-linked sialic acid receptors, which was confirmed by receptor binding assays. Furthermore, mouse infection experiments using the rescued H3N2 demonstrated efficient viral replication in nasal turbinates and lung tissues, accompanied by significant pulmonary pathological damage. These findings indicate that the YZ07 strain, through the vast reassortment and accumulation of adaptive mutations, has acquired potential zoonotic risk, underscoring the importance of surveillance of swine influenza viruses.

Source: 

Link: https://www.sciencedirect.com/science/article/abs/pii/S0378113526000684?via%3Dihub

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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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T cell #immunity to seasonal #Influenza A and #H5N1 viruses in #laboratory #workers receiving annual seasonal Influenza #vaccines

 

{Abstract}

Background: 

Emerging threats such as highly pathogenic influenza strains like H5N1 emphasize the need for vaccines that induce cross-reactive immunity against conserved epitopes. Existing influenza vaccines primarily elicit strain-specific responses, leaving gaps in protection against pandemic subtypes. This study aimed to evaluate T cell responses to seasonal influenza A and H5N1 and compare them to SARS-CoV-2 specific T cell responses to understand differences shaped by distinct exposure histories and vaccination strategies.

Methods: 

T cell responses were assessed in 41 laboratory workers who received annual seasonal influenza vaccines using ELISpot to quantify responses to peptide pools derived from influenza (H1N1 hemagglutinin [HA], H3N2 HA, H5N1 HA, matrix protein 1 [MP1], nucleoprotein [NP]) and SARS-CoV-2 (spike [S2S], nucleocapsid [S2N]). Ten-day expansion assays were used to evaluate functional cross-reactivity between H1, H3, and H5 HA. Intracellular cytokine staining was performed to assess antigen-specific T cell functionality. We used the IFN-γ ELISpot assay and intracellular cytokine staining to evaluate T cell responses to H5N1 HA peptides and assessed cross-reactivity and functional similarity in H1N1 HA-expanded cells.

Results: 

The percentage of individuals with effector T cell responses to influenza peptide pools, was markedly lower than the percentage of individuals with S2S-specific T cells. However, HA-specific memory cells that cross-recognized H1, H3, and H5 HA were present in many individuals. T cells expanded with H1 or H5 HA proteins cross-recognized homologous epitopes in the 2 proteins and cytokine production profiles were comparable between H1- and H5-expanded T cells.

Conclusion: 

These results highlight the potential for influenza vaccines to elicit cross-reactive immunity against H5N1 viruses. These findings also demonstrate differences between T cell responses to influenza and SARS-CoV-2, highlighting distinct immune profiles that could inform future vaccine strategies.

Source: 

Link: https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2025.1718805/full

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