#19,331
Regular readers know that we've been following an uptick in reports of `reduced susceptibility' of the seasonal H1N1 flu virus to the antiviral drug oseltamivir (aka `Tamiflu') around the globe for the past 3 years.
In March 2024 The Lancet published - Global Emergence of Neuraminidase Inhibitor-Resistant Influenza A(H1N1)pdm09 Viruses with I223V and S247N Mutations - which reported a much higher incidence of oseltamivir resistance among samples tested in Hong Kong in 2023.
Concerning, but not enough of a hit to invalidate the clinical use of the drug.
In July of 2025, Virus Research: A 15-year Study of Neuraminidase Mutations and the Increasing of S247N Mutation in Spain, we looked at a study that found a sharp increase in detections of the NA:S247N mutation beginning in 2024, but not including I223V.
Last December, in Eurosurveillance: Expansion of influenza A(H1N1)pdm09 NA:S247N Viruses with Reduced Susceptibility to Oseltamivir, Catalonia, Spain, and in Europe, July to October 2025. a Rapid Communications reported on another dramatic surge in H1N1 viruses carrying the NA:S247N mutation in Catalonia Spain, and other parts of Europe.
Some weeks (see above graphic), as many as 100% of viruses tested showed this NA:S247N mutation.
The good news, however, was that once again, a second permissive mutation - NA:I223V - was not detected in these recent Catalonia isolates.
As for detections in the United States (see Feb 2026 blog), over the entire 2024-2025 flu season - out of 1697 H1N1 viruses tested - only one carried the NA-I223V and NA-S247N amino acid substitutions.
But shortly after the New Year we began to seen an uptick in reduced inhibition detections in the United States. By late February (FluView week 7) - based on 517 H1N1 isolates tested since October - the CDC reported 10 isolates with reduced inhibition and 4 with highly reduced inhibition (due to NA:H275Y).
In early June we looked at the last full FluView report of the 2025-2026 flu season, which reported a total 20 reduced inhibition results, and 11 highly reduced results:
Nine A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution conferring highly reduced inhibition by oseltamivir and peramivir. Nineteen A(H1N1)pdm09 viruses had amino acid substitutions NA-I223V and NA-S247N and showed reduced inhibition by oseltamivir. One A(H1N1)pdm09 virus had amino acid substitutions NA-I223T and NA-S247N and showed reduced inhibition by oseltamivir. Two A(H3N2) viruses had amino acid substitution NA-E119V conferring highly reduced inhibition by oseltamivir. Three B viruses had amino acid substitution NA-M464T and showed reduced inhibition by peramivir.
While the number of S247N+I223V mutations remains low (2.1%), this was more than a 35-fold increase over the previous year. And equally concerning - after an extended absence - the tag team of S247N+I223V was being reported again in a newer NA-clade H1N1 virus.
All of which brings us to this week's Eurosurveillance dispatch, which reports on a similar rise of S247N+I223V in the Netherlands; first in 2024 and then - after a brief absence - again in 2026.
Zandra Felix Garza1 , Dirk Eggink1 , Mariam Bagheri1 , Sharon van den Brink1 , Gabriel Goderski1 , Mark Pronk2 , Pascal Lexmond2 , Mariëtte Hooiveld3 , Rianne van Gageldonk-Lafeber1 , Björn Koel2 , Ron Fouchier2 , Adam Meijer1
In 2023/24, a neuraminidase (NA)-clade of A(H1N1)pdm09 influenza viruses carrying the NA-I223V amino-acid substitution emerged, followed by acquisition of NA-S247N [1-3]. Both substitutions individually increase the 50 per cent inhibitory concentration (IC50) by oseltamivir but the resulting IC50-fold-increase remains below the threshold for reduced inhibition (RI) (IC50-fold-change > 10 compared with median IC50 of wildtype (WT) viruses [4]). In double-mutant viruses, the substitutions act synergistically, causing phenotypically RI by oseltamivir, but not by zanamivir [1,2].
In late 2025, Saubi et al. [5] reported re-emergence of A(H1N1)pdm09 viruses carrying NA-S247N in Spain and elsewhere in Europe. Here, we show the re-emergence of A(H1N1)pdm09 NA-I223V and NA-S247N double-mutant viruses in 2025/26 in the Netherlands and assess their phenotypic susceptibility, global spread and evolution from 2023 to 2026.
Discussion
The recent expansion of A(H1N1)pdm09 influenza virus clusters carrying either NA-I223V (2023/24) or NA-S247N (2025/26) was followed in both seasons by the emergence and spread of double-mutant viruses combining these substitutions, causing RI by oseltamivir. This pattern suggests a fitness advantage of single and double mutants [2], although they have not become permanently dominant. The appearance of double mutants is likely independent of antiviral selection, as NA inhibitors are only sporadically used in several countries reporting high proportions of these viruses and such viruses have not been reported from Japan, where NA inhibitor use by capita is highest [14,15], in publicly available GISAID data.
NA-I223V and NA-S247N, alone or in combination, have been proposed as changes that compensate for the fitness cost of NA-H275Y, which confers highly RI by oseltamivir [16]. Both substitutions individually, and especially together, further drastically reduce inhibition by oseltamivir in NA-H275Y mutants [16,17].
During the study period, NA-S247N together with NA-H275Y was detected 33 times and once as a triple mutant including NA-I223V, confirming that such variants showing very high RI can arise and are of concern. Phenotypic data for Dutch viruses show that the 2025/26 NA-I223V combined with NA-S247N double mutant display oseltamivir RI while retaining normal inhibition by zanamivir, similar to the 2023/24 double-mutant and reports elsewhere [1,2,17]. The shown impact of different reference IC50 values on fold-change calculations highlights the need for caution in using WT data and interpreting IC50 values near the 10-fold RI threshold, given the arbitrary nature of the defined threshold values [11]. Synergistic IC50 increases in double mutants should nonetheless be carefully monitored and reported. The impact of these double mutants on clinical management of influenza patients is unknown and should be part of future studies.
Dominance of double mutants within defined HA/NA-subclusters, with gradual accumulation of additional changes, suggests global dissemination from one or several initial emergence(s) or introduction(s), although occasional detection of double mutants in other NA-clades indicates that parallel evolution of similar A(H1N1)pdm09 variants showing oseltamivir RI is also possible. Awareness is therefore warranted whenever NA-I223V or NA-S247N become fixed in spreading subclusters. The recent emergence and spread of NA-S247N in NA-clade D.1 on an HA-clade D.3.1.1 background should be closely monitored during the progressing 2026 southern hemisphere and coming 2026/27 northern hemisphere seasons. Similar to the abundancy of NA-clade D.3 with NA-S247N in HA-clade D.3.1.1 background, HA-clade D.3.1.1 viruses could provide a favourable context for the expansion of such viruses carrying NA of clade D.1 with NA-S247N.
Our global assessment is limited by reliance on non‑embargoed sequences and metadata in GISAID. Country and regional representativeness depends on local laboratory capacity to sequence at least the HA and NA gene segments, and on whether data are submitted to GISAID or other databases (e.g. GenBank, Pathoplexus). While WHO Collaborating Centres partly compensate for gaps at NICs by sequencing of representative viruses shared by NICs and submitting these sequence data to GISAID, our analysis may still over‑estimate mutant presence in some regions and under‑estimate it in others. Our study outcomes should therefore be interpreted in light of these sampling and database‑related limitations.
Conclusion
Our findings highlight the need for continued surveillance on the evolution of A(H1N1)pdm09 influenza viruses and the possible emergence of mutants, especially double and triple ones, with antiviral (highly) RI. The capacity to phenotypically evaluate the impact of individual and combinations of amino-acid substitutions associated with antiviral (highly) RI remains a critical component of this surveillance.
In 2024 we saw sporadic reports of I223V+S247N in H1N1 NA-Clade C.5.3.3 viruses which produced a 12-13 fold reduction in oseltamivir susceptibility. By 2025 they had largely disappeared, replaced by reports of less impactful single S247N mutations spreading in Europe.
Fast forward to 2026 and we find that the I223V+S247N combo has reappeared in a genetically different NA-clade (D.3) H1N1 virus, suggesting the earlier appearance was more than just a fluke.
Where this story goes from here is anyone's guess, but should the I223V+S247N combo team up with additional permissive mutations (including H275Y) - and also become more prevalent in seasonal H1N1 - it could compromise the clinical effectiveness of our primary influenza antiviral drug; oseltamivir.
A lot of `ifs', but given the stakes, this is a story very much worth following.
The CDC should resume reporting on antiviral mutations in October, and hopefully we'll be getting similar reports out of Europe later this fall.
