Duration of fever and PA/I38X-substituted virus emergence in patients treated with baloxavir in the 2018-2019 influenza season.

Duration of fever and virus persistence after baloxavir administration were investigated in 81 outpatients, 16 with A(H1N1)pdm09 and 65 with A(H3N2) in the Japanese 2018-2019 influenza season. Only eight cases of A(H3N2) viruses were detected post-dose. PA/I38T-substituted viruses were detected in four (6.2%) of 65 A(H3N2) patients, at days 3 and 4, constituting 50% (4/8) of A(H3N2) detected post-dose. The median duration of fever was 26.0 h for A(H1N1)pdm09 and 20.3 h for A(H3N2). The median duration of fever for patients with PA/I38T-substituted viruses was 22.0 h, without significant difference to that of the patients in whom the mutated virus was not detected. Emergence of PA/I38T-substituted viruses after treatment with baloxavir was confirmed, but no significant prolongation of fever was observed in the four patients with PA/I38T-substituted virus emergence.