Use of baloxavir as adjunctive antiviral therapy to neuraminidase inhibitors in severely immunocompromised individuals infected with influenza.

Immunocompromised patients are at risk of developing severe influenza, with protracted viral shedding and development of resistance-associated mutations under antiviral treatment. We report a case series of severely immunocompromised hematology patients, including allogeneic hematopoietic cell transplantation (HCT) recipients, treated with both baloxavir and oseltamivir and describe clinical and virological outcomes and the safety profile of prolonged combination therapy. Allogeneic HCT recipients with influenza infection treated with baloxavir were retrieved via institutional databases. All hospitalized allogeneic HCT patients treated with a combination therapy of baloxavir and oseltamivir over five influenza seasons between October 2019 and May 2025 were included. Six influenza-infected hematology patients (5/6 allogeneic HCT recipients) were treated with combination therapy of oseltamivir and baloxavir. All patients presented with lower respiratory tract infections. Oseltamivir treatment duration ranged from 5 to 31 days, and the number of administered baloxavir doses ranged between one and five. Baloxavir administration was well tolerated, and no adverse events could be attributed to the administered antiviral treatment. All-cause mortality at 3 months post-infection was 66% (4/6), mainly driven by underlying disease. In two patients with protracted shedding, combination therapy did not prevent the development of resistance mutation(s). Combination treatment with prolonged courses of oseltamivir and repeated doses of baloxavir was well tolerated. No definitive conclusions on the efficacy of this approach could be drawn from this study. More data are required on the best treatment of hematology patients infected with influenza.