A Cluster of Influenza A(H3N2) Viruses in 2024 From Tasmania, Australia, With Reduced Susceptibility to Baloxavir.

The emergence of viruses with reduced susceptibility to antivirals poses an ongoing risk which could limit their effectiveness. Here we report a cluster of A(H3N2) viruses detected in Tasmania, Australia, with a PA gene mutation A37T that showed mild reductions in susceptibility in vitro to baloxavir (3.47- to 16.33-fold increased EC50) when compared to wild-type influenza A(H3N2) viruses. When the PA-A37T and PA-I38T substitutions were combined and tested in IRINA assay, eightfold higher inhibition was seen compared to PA-I38T alone. On-going monitoring of influenza viruses for reduced susceptibility to antivirals remains an important part of seasonal and zoonotic influenza surveillance.