The purpose of this clinical trial is to find out if the medication called baloxavir marboxil (sold under the brand name Xofluza®) can help to reduce the time needed to recover from flu when patients need an hospitalization. Patients infected by influenza and requiring a hospitalization will be approched to be included in the study. The main questions are: 1. When someone is hospitalized with a severe influenza infection, does baloxavir help to reduce the time needed to recover? 2. Can baloxavir marboxil help to shorten the amount of time people need to stay in the hospital with severe flu? 3. Can baloxavir marboxil help to reduce the risk of life-threatening complications as well as of death due to severe flu? 4. Can baloxavir reduce duration of contagiousness? To be able to measure the above, the investigators will compare two groups of patients: One group receiving baloxavir marboxil, the other group receiving a mock treatment called placebo. Participants will: * Take one single dose of baloxavir marboxil or placebo soon after hospitalization. * Vital signs will be followed three times per day during hospital stay. * Have a nose swab to detect the presence of influenza virus on the first and third day of trial participation. * Answer to a short quality of life questionnaire on the phone 3 months after receiving the study treatment.
Inclusion Criteria: * Age ≥ 18 years * Participant or participant representative capable of giving signed informed consent. * Positive reverse transcriptase-polymerase chain reaction (RT-PCR) for influenza A and/or B confirmed on arespiratory tract sample. * Patient requiring hospitalization. * National Early Warning Score 2 (NEWS2) of ≥4 at planned randomization Exclusion Criteria: * Ongoing pregnancy or breastfeeding (self-reported by the participant or diagnosed by the treating phyisician) * Known contraindication to baloxavir or to the placebo * Participant weighing \< 40 kg * Patients already on NAI therapy for the current influenza episode for \> 24 hours at the time of randomization. * Prior treatment with baloxavir for the current influenza epidose * Immunosuppression defined as 1) cancer treatment with significant negative effect on the immune system; 2) immunosuppressive therapy (treatments comprising a dose of ≥20 mg/day prednisone or equivalent when administered for ≥ 2 weeks, biological therapies, steroid sparing drugs); 3) HIV infection if CD4+ T cell count \< 500/µL; 4) organ or stem cell transplantation; 5) patients on the waiting list for a transplant * Severe underlying respiratory comorbidity requiring long-term oxygenotherapy at home. * Severe disease requiring ICU care directly at hospitalization. * Severe hepatic insufficiency or any other severe medical condition when participation in the study puts the patient at risk according to the investigator's judgment. * History of inclusion in this study during a previous influenza season * Inclusion in another interventional study with an investigational drug 30 days before inclusion in the study. * Unability to consent or patient representative unable to consent.