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.