Objectives: To investigate the susceptibility of epidemic influenza viruses to neuraminidase inhibitors (NAIs) and the emergence of resistant viruses after treatment, a prospective, observational study was done in the 2019-20 Japanese influenza season.

Methods: Influenza viruses were isolated before and twice after treatment, the first at day 5 and the second at day 10. The 50% inhibitory concentrations (IC50s) to oseltamivir, zanamivir, peramivir, and laninamivir were measured and compared with those of 2010-11 to 2018-19 seasons. NA amino acid sequences were analyzed by next generation sequencing (NGS).

Results: The IC50 geometric means of the NAIs for 128 A(H1N1)pdm09, 2 A(H3N2), and 33 B were comparable to those of the previous seasons. Only 2 (1.6%) A(H1N1)pdm09 with significantly high IC50 to oseltamivir were found pretreatment. No A(H3N2) or B had resistance. Treatment-emergent oseltamivir resistance was found in 2 among 33 oseltamivir-treated A(H1N1)pdm09, only at the second follow-up. The NGS indicated a rapid increase in the proportion of H275Y to wild type, from 0% to almost 100? between days 5 and 10.

Conclusions: These results suggest the continued effectiveness of these NAIs for epidemic influenza in Japan. Treatment-emergent resistant H275Y A(H1N1)pdm09 viruses were detected after oseltamivir treatment, rapidly replacing the wild type.