Study to Assess the Prophylactic Efficacy and Safety of Sebaloxavir Marboxil Tablets/Oral Suspension for Influenza Prevention in Household Contacts Aged ≥2 Years.

Index Participants:

Inclusion criteria

：

• 1. The index participant (aged ≥18 years), or the index participant (aged <18 years) together with their legal guardian, understands and voluntarily provides written informed consent form (ICF). The index participant must be ≥1 year of age at the time of ICF signing, with no gender restriction.
• 2. The index participant must be the first case of influenza virus infection in the household during the current influenza season.
• 3. The index participant must have a positive result from a rapid influenza diagnostic test (RIDT) or polymerase chain reaction (PCR) assay.
• 4. At the time of HHC randomization, the interval from the onset of the index participant's initial influenza symptoms to informed consent must be ≤48 hours. Symptom onset is defined as: first axillary temperature ≥37.5°C, or the presence of at least one other influenza symptom.
• 5. The index participant must not have used any anti-influenza antiviral medications (including traditional Chinese medicines or Western medicines) since symptom onset, and must be willing to receive oseltamivir phosphate treatment after signing informed consent form.

Exclusion criteria

：

• 1. Participants who are diagnosed with bacterial or other non-influenza virus infections at screening requiring systemic antibacterial or antiviral treatment;
• 2. Participants with suspected hypersensitivity to the active ingredients or excipients of oseltamivir phosphate oral suspension/capsules.

Household contact:

Inclusion criteria

：

• 1. Aged ≥2 years at the time of randomization, with no gender restriction;
• 2. Has been living in the same household with the index participant for at least 48 hours at the time of randomization;
• 3. Is able to reside with the index participant from screening/randomization through Day 10 (must live in the same household with the index participant for at least 7 days);
• 4. Tests negative by rapid influenza diagnostic test (RIDT) or polymerase chain reaction (PCR);
• 5. Meets all of the following criteria and is judged by the investigator to have no influenza virus infection:

• Axillary temperature <37°C at screening;
• No influenza-related symptoms at screening (such as nasal congestion, sore throat, cough, myalgia or arthralgia, fatigue, headache, chills/sweating, or other influenza symptoms assessed by the investigator);
• 6. The HHC is ≥18 years of age, or the HHC is <18 years of age and together with their parent(s)/legal guardian(s) understands and voluntarily signs the informed consent form (ICF), and agrees to comply with all study procedures, including completion of diary cards (the guardian may assist in assessment/completion).
• 7. The HHC must sign the informed consent form (ICF) within 24 hours after the index participant signs the ICF;
• 8. The HHC and their partner must agree to practice complete abstinence or use effective contraceptive measures from screening through 3 months after the end of study drug administration.

Exclusion criteria

：

• 1. Participants with suspected hypersensitivity to the active pharmaceutical ingredients or excipients of the investigational product;
• 2. Participants whose household contacts or other family members (excluding the index participant) have been diagnosed with influenza within 3 months before randomization;
• 3. Participants who have been cohabiting with other family members exhibiting influenza-like symptoms (axillary temperature ≥37.5°C, nasal congestion, sore throat, cough, myalgia/arthralgia, fatigue, headache, chills/sweating, or other influenza symptoms as assessed by the investigator) other than on the screening day;
• 4. Participants requiring systemic (oral or injectable) or intranasal antipyretic/analgesic agents, corticosteroids, or immunosuppressants (those using topical preparations are allowed);
• 5. Participants who have received an influenza vaccine within 6 months prior to screening;
• 6. Participants who have taken anti-influenza Chinese or Western medications within 30 days prior to screening, including but not limited to neuraminidase inhibitors, hemagglutinin inhibitors, M2 ion channel blockers, cap-dependent endonuclease (CEN) inhibitors, and heat-clearing/detoxifying Chinese herbal medicines, such as oseltamivir, zanamivir, peramivir, laninamivir, umifenovir, favipiravir, rimantadine, amantadine, arbidol, nitazoxanide, baloxavir, Lianhua Qingwen capsules/granules, Jinhua Qinggan granules*, etc.
• 7. Participants with active infection or requiring systemic anti-infective treatment at screening (topical therapy for mild skin infections is allowed).
• 8. Participants with other respiratory viral infections.
• 9. Participants with difficulty swallowing study medication or a history of gastrointestinal disorders that may significantly impair drug absorption, including but not limited to reflux esophagitis, chronic diarrhea, inflammatory bowel disease, intestinal tuberculosis, gastrinoma, short bowel syndrome, or post-gastrectomy.
• 10. Participants with severe underlying comorbidities, defined as Grade 3 or higher per the Common Terminology Criteria for Adverse Events (CTCAE) Version 6.0.
• 11. Immunocompromised participants, including those with malignancies, history of organ or bone marrow transplantation, HIV infection, or use of immunosuppressants within the previous 3 months.
• 12. HHCs requiring long-term use of aspirin- or salicylate-containing products, defined as daily regular use for more than 14 consecutive days.
• 13. Participants with suspected or confirmed alcohol abuse (defined as weekly consumption of >14 units of alcohol; 1 unit = 360 mL beer, 45 mL of 40% spirits, or 150 mL of 12% wine) or history of drug abuse.
• 14. Participants who have participated in another interventional clinical trial of a drug or medical device within 30 days before screening.
• 15. Women who have a positive pregnancy test result or are currently lactating.
• 16. Participants with any other conditions that, in the judgment of the investigator, would preclude enrollment in the study.