Neuraminidase inhibitors (NAIs), such as oseltamivir and zanamivir, serve as the primary treatment for influenza virus infection. NAI-resistant influenza A(H1N1) strains were widespread during the 2008–09 influenza season, especially in Japan, where 100% of the strains were resistant to oseltamivir. However, after the NAI-susceptible 2009 pandemic subtype (A[H1N1]pdm09) replaced the previous seasonal A(H1N1) subtype in 2009, the incidence of A(H1N1)pdm09 with reduced inhibition (10-fold to 100-fold) or highly reduced inhibition (>100-fold) was only approximately 1% in the 2019–20 influenza season.