In vitro neuraminidase inhibitory activity of four neuraminidase inhibitors against clinical isolates of the influenza virus circulating in the Japanese 2013–2014 season

To assess the extent of viral resistance, we measured the 50% inhibitory concentration (IC50) of neuraminidase inhibitors (NAIs) for the influenza virus isolates in the 2013–2014 influenza season and compared the results to those of the 2010–2011 to 2012–2013 influenza seasons.Viral isolation was done with specimens obtained prior to treatment, and the type and subtype of influenza was determined by RT-PCR using type- and subtype-specific primers. The IC50 was determined by a neuraminidase inhibition assay using a fluorescent substrate.A total of 327 influenza viruses were isolated: 172 influenza A(H1N1)pdm09 (52.6%), 49 A(H3N2) (15.0%), and 106 B (32.4%). Numbers of Victoria and Yamagata lineage isolates were 36 and 70, respectively.Two A(H1N1)pdm09 isolates showed a high IC50 for oseltamivir (130 and 150 nM) exceeding by 100 times the geometric mean of the IC50 of oseltamivir for A(H1N1)pdm09 isolates (0.76 nM). No isolate showed a very high IC50 for A(H3N2) or B.The IC50 of the NAIs except for oseltamivir for A(H1N1) pdm09 were significantly higher than those of the 2010–2011 season (P < 0.05). The IC50 of all four NAIs for A(H3N2) were significantly lower than those of the 2012–2013 season (P < 0.001). The IC50 of the NAIs for B except for oseltamivir were significantly lower than those of the 2012–2013 season (P < 0.001).Although there are some isolates that show highly reduced sensitivity to oseltamivir among A(H1N1)pdm09 isolates, the currently epidemic influenza A(H1N1)pdm09, A(H3N2) and B viruses are susceptible to all four NAIs with no trend toward decreased sensitivity.