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Baloxavir is an antiviral agent that was licensed in the U.S. in 2018 for influenza treatment. Its mechanism of action is inhibition of activity of the polymerase acidic (PA) protein. In clinical trials leading to licensure of the drug, baloxavir resistance was detected in a minority of recipients and was mediated by changes at amino acid position 38 of the PA protein. Limited data suggested that substitution at this position might diminish viral fitness.
To further examine viral resistance to baloxavir, investigators in Japan sequenced the PA genes from 253 untreated influenza patients and found baloxavir-resistance mutations in only 2 patients, 1 of whom had transmitted drug resistance from a baloxavir-treated patient.
In a subset of 38 pat…