Shionogi & Co., Ltd. has announced that the appearance rate and clinical course of mutated viruses at position 38 in PA protein for influenza A/H1N1pdm and A/H3N2 in special drug use survey for Xofluza® (baloxavir malboxil, hereafter baloxavir) at Options X for the Control of Influenza (OPTIONSX), held in Singapore on August 28 ­ September 1, 2019. The outline of the results are as: An observational study was conducted in 96 influenza A-infected patients (A/H1N1pdm: 32, A/H3N2: 64) under 20 year of age; One patient infected with A/H3N2 influenza had PA/I38T mutated virus prior to treatment with baloxavir . The source of infection and information about prior contact with baloxavir-treated patients is unknown. The patient's fever resolved approximately 1 day after trearment with baloxavir; the rate of appearance of PA/I38T-mutated virus after treatment with baloxavir and of PA-mutated virus after treatment with baloxavir was as follows. The incidence were respectively 6.3% (2/32) and 12.5% (4/32) in A/H1N1pdm-infected patients and 10.9% (7/64) and 14.1% (9/64) in A/H3N2- infected patients if calculated based on all treated patients. If calculated based on post-treatment sequence-positive patients at second visit, 28.6% (2/7) and 57.1% (4/7) in A/H1N1pdm infected patients and 25.9% (7/27) and 33.3% (9/27) in A/H3N2-infected patients; The averate duration of fever in patients with PA/I38Tmutated virus (9), that for no PA-mutated virus (21), and that for PCR-negatives patients (62) werenot different. (0.99±1.21 days, 1.02± 1.06 days and 0.76±0.86 days, respectively); and PA/I38X viruses isolated from patients showed reduced susceptibility to baloxavir by 50-250 fold but the duration of fever in patients was not prolonged and was approximately 1 day.