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: 1. Axillary temperature \<37°C at screening; 2. 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.