Study to Assess the Efficacy and Safety of GP681 Versus Placebo for Postexposure Prophylaxis Against Influenza

Inclusion Criteria: Index patients(IPs) 1. Male or female patients aged≥2 years at the time of signing the informed consent form. 2. The first patient in a household with a diagnosis of influenza virus infection confirmed by all of the following in the influenza season: 3. Positive Influenza rapid antigen test or Polymerase chain reaction (PCR), and; 4. Fever (axillary temperature ≥37.3℃) in the predose examinations or \> 4 hours after dosing of antipyretics if they were taken. 5. Patients with onset of fever within 48 hours or less at informed consent. 6. Patients live in a household where all household contacts are expected to meet the key household contacts inclusion criteria (criteria 2, 3, \& 4). 7. Patients and/or their guardian who are willing to provide written informed consent and consent to participate in the study, able to understand the study and comply with all study procedures. Household contacts of index patients: 1. Male or female patients aged≥12 years at the time of signing the informed consent form. 2. Household contacts who are able to provide informed consent within 24 hours or less from informed consent in index patients. 3. Subjects who had lived with the index patient for 3 months or more prior to informed consent. 4. Subjects who meet all of the following criteria and are judged not to have influenza virus infection by the investigator. 5. Negative Influenza rapid antigen test or Polymerase chain reaction (PCR), and; 6. Subjects who have a body temperature (axillary) \< 37.3°C at Screening, and; 7. Subjects who have no influenza like symptoms (cough, sore throat, headache, nasal discharge/nasal congestion, feverishness or chills, muscle or joint pain, and fatigue) at Screening. 8. Household contacts intended for full study have not received influenza vaccine within 6 months prior to screening; 9. Subjects and/or their guardian who are willing to provide written informed consent and consent to participate in the study, able to understand the study and comply with all study procedures, including patient health diary records. Exclusion Criteria: Household contacts of index patients: 1. History of allergic reactions attributed to GP681 or any of the ingredients of its formulation. 2. Subjects with household members other than the index patient that was diagnosed with or strongly suspected to have influenza in the past 12 weeks. 3. subjects with concurrent bacterial or other virus infections requiring systemic antimicrobial and/or antiviral therapy at the pre-dose examinations. 4. Subjects who are unable to live with the index patient from Screening until Day 10. 5. Known history of dysphagia or any gastrointestinal disease that affects drug absorption (including but not limited to reflux esophagitis, chronic diarrhea, inflammatory bowel disease, intestinal tuberculosis, gastrinoma, short bowel syndrome, stomach after subtotal resection, etc.). 6. Subjects who have any underlying diseases requiring systemic , treatment of antipyretics/analgesics, corticosteroids, or immunosuppressive agents. 7. Subjects with human immunodeficiency virus \[HIV\] infection. 8. Subjects with severe (Grade 3 or higher of Common Terminology Criteria for Adverse Events \[CTCAE\] ver. 5) underlying diseases. 9. Treatment with anti-influenza virus drugs (oseltamivir, zanamivir, peramivir, favipiravir, abidol, baloxavir marboxil, amantadine, or rimantadine) within 2 weeks before screening. 10. Known history of alcohol abuse (Average weekly intake of alcohol is more than 14 units alcohol (1 units ≈ 360 mL beer, or 45 mL spirits with 40% content, or 150 mL wine) or drug abuse at screening; 11. Women who are pregnant, breastfeeding, or have a positive pregnancy test at the predose examinations. The following female patients who have documentation of either a or below do not need to undergo a pregnancy test at the predose examinations: 1. Postmenopausal women (defined as cessation of regular menstrual periods for 2 years or more and aged more than 50 years old) 2. Women who are surgically sterile by hysterectomy, bilateral oophorectomy, or tubal ligation 12. Has received any investigational agents or devices for any indication within 30 days prior to Screening. 13. Subjects who, in the opinion of the Investigator, may not be qualified or suitable for the study.