This study will evaluate the efficacy, safety, and population pharmacokinetics of a single, oral dose of GP681 compared with placebo in patients aged 12 to 65 years with acute uncomplicated influenza virus infection

Inclusion Criteria:

1. Male or female patients aged≥5 to≤65 years at the time of signing the informed consent form.

2. Patients with a diagnosis of influenza confirmed by all of the following:

   • Positive Influenza rapid antigen test or Polymerase chain reaction (PCR), and;

   • Fever (axillary temperature ≥37.3℃) in the predose examinations or > 4 hours after dosing of antipyretics if they were taken,and;

   • At least one of the following systemic symptoms and respiratory symptoms respectively associated with influenza are present with a severity of moderate or greater:

     1. Systemic symptoms: headache, fever or chills, muscle or joint aches, fatigue;
     2. Respiratory system symptoms: cough, sore throat, nasal congestion.

3. The time interval between the onset of symptoms and random enrollment is 48 hours or less. The onset of symptoms is defined as either:

   1. Time of the first increase in body temperature (axillary temperature ≥37.3℃)
   2. Time when the patient experiences at least one systemic or respiratory symptom related to influenza

4. Subjects of child-bearing potential who agree to use a highly effective method of contraception for 1 month after drug withdrawal.

5. 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:

1. History of allergic reactions attributed to GP681 or any of the ingredients of its formulation.

2. Patients with influenza virus infection requiring inpatient treatment;

3. 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.).

4. Treatment with anti-influenza virus drugs (oseltamivir, zanamivir, peramivir, favipiravir, arbidol, baloxavir marboxil, amantadine, or rimantadine) within 2 weeks before screening, or immunization with influenza vaccine within 6 months prior to enrollment;

5. Investigator suspects or confirms that patients with bronchitis, pneumonia, pleural effusion or interstitial disease through chest imaging examination.

6. Acute respiratory infection, otitis media or sinusitis within 2 weeks prior to Screening.

7. Patients with concurrent bacterial or (non-influenza) infections requiring systemic antimicrobial and/or antiviral therapy at the pre-dose examinations.

8. Positive nucleic acid test for COVID-19 in screening period.

9. Patients with severe or uncontrollable underlying diseases, including blood disorders, severe chronic obstructive pulmonary disease(COPD), liver disorders, kidney disorders, chronic congestive heart failure(NYHA III-IV), mental disorders;

10. Immunodeficiency,including malignant tumor, organ or marrow transplant, human immunodeficiency virus [HIV] infection, or patients receiving immunosuppressant therapy 3 months prior to enrollment.

11. Concomitant therapy with aspirin or salicylic acid.

12. Morbid obesity (Body mass index [BMI]≥30kg/m2)

13. 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 b 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

14. 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;

15. Patients weighing <20 kg.

16. Has received any investigational agents or devices for any indication within 30 days prior to Screening.

17. Patients who, in the opinion of the Investigator, may not be qualified or suitable for the study.