In vitro neuraminidase inhibitory concentrations (IC₅₀) of four neuraminidase inhibitors in the Japanese 2023–24 season: Comparison with the 2010–11 to 2022–23 seasons

Introduction: To assess the susceptibility of epidemic influenza viruses to the four most used neuraminidase inhibitors (NAIs) during the 2023–24 influenza season in Japan, we measured the 50 % inhibitory concentration (IC₅₀) of oseltamivir, peramivir, zanamivir, and laninamivir in virus isolates from the sample of 100 patients. Methods: Viral isolation was done using specimens obtained before and after treatment, with the type/subtype determined by RT-PCR using type- and subtype-specific primers. IC₅₀ values were determined by a neuraminidase inhibition assay using a fluorescent substrate. Results: The virus isolates included 16 A(H1N1)pdm09, 19 A(H3N2), and 65 B/Victoria-lineage. The geometric mean (GM) IC₅₀ values of pre-treatment samples for oseltamivir, peramivir, zanamivir, and laninamivir were 0.90 nM, 0.62 nM, 1.09 nM, and 2.77 nM for A(H1N1)pdm09; 0.86 nM, 0.67 nM, 1.64 nM, and 3.61 nM for A(H3N2); and 16.12 nM, 1.84 nM, 3.87 nM, and 11.35 nM for B/Victoria, respectively. These values were comparable to those from the previous eleven seasons, and no virus with significantly reduced susceptibility to any of the NAIs was found either before or after drug administration in the 2023–24 season. Conclusions: These results indicate that susceptibility to these four NAIs has been maintained across the three influenza types/subtypes over the past fourteen seasons in Japan.