Between July and October 2025, mutation increased oseltamivir resistance among influenza A(H1N1)pdm09 strains in Catalonia, highlighting growing concerns.
Between July and October 2025, among the total 117 influenza A(H1N1)pdm09 strains characterised in Catalonia, 20% to 100% per week were carrying the NA:S247N substitution. The mutation, conferring reduced susceptibility to oseltamivir, was phenotypically confirmed (IC 50 between 0.82 and 1.63 nM, compared to median IC 50 of 0.3 nM for susceptible strains). An increased proportion of S247N variants was also observed in sequence data (10,944 sequences) from other parts of Spain and five of 35 submitting countries across Europe.
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Nadal‐Barón et al. (2025) studied this question.
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