Previously, three influenza A(H3N2) isolates with a reduced susceptibility to the neuraminidase inhibitors (NAIs) zanamivir and oseltamivir were identified during screening by the Neuraminidase Inhibitor Susceptibility Network (NISN). The isolates were from untreated patients from the first three years post licensure of the NAIs. We plaque-purified progeny from each of these isolates and determined the NAI sensitivity of each plaqued population. Sequencing and serology for each population revealed that the isolates contained a mix of wild type influenza A(H3N2) and influenza B. The NAI susceptibility reductions that had originally been reported were a consequence of influenza B neuraminidases that have lower relative NAI sensitivities, rather than being due to resistant influenza A(H3N2) viruses. Our study highlights the need to check for mixed influenza infections when isolates with potentially lower sensitivities to NAIs are identified.
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