This study aims to randomly assign close contacts of hospitalized influenza virus patients to receive baloxavir marboxil prophylaxis, oseltamivir prophylaxis or no antiviral prophylaxis and monitor the incidence of clinical influenza, evaluating the prophylaxis efficiency of antivirals in a hospital setting.

This study will enroll hospitalized patients diagnosed with influenza virus infection (index patients) and their roommates who are not infected with the influenza virus (close contacts). The close contacts will be randomly assigned in a 1:1:1 ratio to receive baloxavir marboxil prophylaxis, oseltamivir prophylaxis or no antiviral prophylaxis. The incidence of clinical influenza (confirmed by reverse-transcriptase polymerase chain reaction (RT-PCR) testing, along with fever and one respiratory symptom) will be monitored to evaluate the efficiency of antiviral prophylaxis in a hospital setting. The index patients will receive standard treatment for influenza infection.

Inclusion Criteria:

• Index patient:
• 1. Patients or accompanying caregivers hospitalized at participating medical institutions.
• 2.The subject and/or their legal guardian both agree to participate in this clinical study, sign the informed consent form, and are able to comply with the study requirements for follow-up and completion of all study procedures and assessments.
• 3.Age ≥ 2 years.
• 4.Influenza nucleic acid positivity confirmed by throat swab, sputum and other respiratory sample through rapid antigen testing (RAT) or PCR.
• Close contact:
• 1.Patients or accompanying caregivers hospitalized at participating medical institutions.
• 2.The subject and/or their legal guardian both agree to participate in this clinical study, sign the informed consent form, and are able to comply with the study requirements for follow-up and complete all study procedures and assessments
• 3.Age ≥ 12 years.
• 4.Expected to remain hospitalized for ≥ 72 hours.
• 5.The room must contain at least one hospitalized index patient diagnosed with influenza virus within the past 72 hours, and the contact is expected to stay with the hospitalized index patient in the ward for more than 24 hours.
• 6.Throat swab on the first day tested negative for influenza virus by PCR.

Exclusion Criteria:

• Close contacts
• 1.With known allergies to the active ingredients or excipients of the investigational drug.
• 2.Close contacts diagnosed with or having experienced an influenza virus infection within the past 12 weeks.
• 3.Close contacts who have used antiviral drugs (including neuraminidase inhibitors, hemagglutinin inhibitors, and M2 ion channel blockers, such as oseltamivir, zanamivir, peramivir, favipiravir, abidol, baloxavir marboxil, amantadine, rimantadine, or any other antiviral drugs approved by NMPA) within the 2 weeks prior to screening.
• 4.Close contacts known to be pregnant or breastfeeding.
• 5. Having experienced fever (axillary temperature ≥37.3°C) in the past 3 days prior to screening.
• 6.Subjects deemed unsuitable to participate in the study by the investigator.