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.