Suraxavir Marboxil And Oseltamivir for Influenza Prophylaxis Under a Hospital-based Setting

• Suraxavir marboxil (Drug): Suraxavir marboxil (40mg on the first day)
• Oseltamivir Placebo (Drug): Oseltamivir placebo(75mg each day during the first five days)
• Oseltamivir (Drug): Oseltamivir (75mg each day during the first five days)
• Suraxavir marboxil placebo (Drug): Suraxavir marboxil placebo (40mg on the first day).

Primary outcomes

• The proportion of room contacts who develop clinical influenza within 5 days, with a virus subtype consistent with that of the index case. [within 5 days]: The proportion of room contacts who develop clinical influenza* within 5 days, with a virus subtype consistent with that of the index case. *Clinical influenza is defined as a positive influenza virus detection by Rapid Antigen Test (RAT) or Nucleic Acid Amplification Test (NAAT), accompanied by at least one respiratory symptom (fever or chills, muscle or joint pain, headache, fatigue) and at least onesystemic symptom (cough, nasal congestion, sore throat).

Secondary outcomes

• The proportion of room contacts who develop clinical influenza (as defined above) within 10 days [within 10 days]: The proportion of room contacts who develop clinical influenza (as defined above) within 10 days, with a virus subtype consistent with that of the index case.
• The proportion of room contacts with laboratory-confirmed influenza within 5 days [Within 5 days]: The proportion of room contacts with laboratory-confirmed influenza (defined as a positive influenza virus RAT or NAAT result, regardless of symptoms) within 5 days, with a virus subtype consistent with that of the index case.
• The incidence of post-treatment changes in the influenza virus sequence [Within 10 days]: Among room contacts who become infected with influenza virus within 10 days, the incidence of post-treatment changes in the influenza virus sequence (based on viral sequence analysis). This includes the incidence of events such as the I38T amino acid substitution, the H275Y amino acid substitution, and other emerging amino acid substitutions in the PA and NA sequences.

Sex: All, ages: 12 Years to any

1.Inclusion Criteria:

(1)Inclusion Criteria for Index Cases:

① Patients hospitalized at a participating medical institution.

* The subject and/or their legal guardian agrees to participate in this clinical study, has signed the informed consent form, and is able to comply with the protocol requirements for follow-up and complete all study procedures and assessments.

  * Age ≥ 2 years.

    * Positive for influenza virus as determined by a Rapid Antigen Test (RAT) or Nucleic Acid Amplification Test (NAAT) on a respiratory specimen (e.g., oropharyngeal swab, nasopharyngeal swab, sputum, or bronchoalveolar lavage fluid).

      (2) Inclusion Criteria for Room Contacts:

      ① Patients hospitalized at a participating medical institution.
* The subject and/or their legal guardian agrees to participate in this clinical study, has signed the informed consent form, and is able to comply with the protocol requirements for follow-up and complete all study procedures and assessments.

  ③ Age ≥ 12 years.
  * Expected to remain hospitalized for ≥ 72 hours.

    * Shares a room with at least one hospitalized index case who has been confirmed with influenza virus infection by RAT or NAAT on a respiratory specimen within the last 120 hours and developed influenza-related symptoms within the last 120 hours, and is expected to share the same room for ≥ 24 hours.

      ⑥ Negative for influenza virus as determined by a Nucleic Acid Amplification Test (NAAT) on a throat swab collected on Day 1.

      3.Exclusion Criteria

      (1) Index Cases There are no exclusion criteria for index cases. (2) Exclusion Criteria for Room Contacts:
      * Room contacts with a known allergy to the active ingredients or excipients of the investigational drugs.
* Room contacts who have been diagnosed with or have had an influenza virus infection in the past 12 weeks.

  * Use of any anti-influenza antiviral drugs within 2 weeks prior to screening, including: neuraminidase inhibitors, polymerase inhibitors, hemagglutinin inhibitors, and M2 ion channel blockers (e.g., Oseltamivir, Zanamivir, Peramivir, Favipiravir, Arbidol, Baloxavir Marboxil, Amantadine, or Rimantadine, or other anti-influenza drugs approved by the NMPA).

    * Known to be pregnant or breastfeeding. (Subjects who suspect they may be pregnant must report this to the investigator, who will confirm with a pregnancy test).

      * History of fever (axillary temperature ≥ 37.3°C) within 72 hours prior to screening.

        * Child-Pugh class B or C, or an eGFR ≤ 60 mL·min-¹·(1.73 m²)-¹. ⑦ Any subject deemed unsuitable for participation in the study by the investigator.