Study to Assess the Tolerability, Safety and Efficacy of GP681 Powder for Oral Suspension

Inclusion Criteria: 1. Male or female participants aged 2 to 18 years (≥2 years and \<18 years). 2. For participants aged 5 to 18 years (excluding 18 years): body weight ≥20 kg; for participants aged 2 to 5 years (excluding 5 years): 10 kg≤body weight \<20 kg. 3. Diagnosed with influenza virus infection based on the following criteria: • Positive rapid antigen test (RAT) result from a throat or nasal swab for influenza virus (rapid nucleic acid tests or other rapid molecular diagnostic tests are also acceptable); and * Fever at screening (axillary temperature ≥37.5°C). If antipyretics have been administered, axillary temperature should be ≥37.5°C at least 1 hour after administration, or fever (axillary temperature \<37.5°C) that returns to ≥37.5°C more than 4 hours after antipyretic use; and * At least one moderate or severe respiratory symptom at screening, including cough and/or nasal congestion or runny nose. 4. Time interval between the onset of illness symptoms and enrollment is ≤48 hours. Onset of illness: defined as the time of the first temperature increase (axillary temperature ≥37.5°C) or the time when at least one respiratory symptom related to influenza virus infection is first noticed by caregiver. 5. The participant's legal guardian agrees to the child's participation in the clinical study and signs the informed consent form (ICF). Participants aged ≥8 years must also voluntarily sign the ICF. 6. The investigator assesses that the participant and/or the caregiver can comply with the protocol requirements, follow-up visits, and complete all study procedures and evaluations, including diary card entries. Exclusion Criteria: 1. Known allergy to the active ingredients or excipients of the investigational drug GP681. 2. Patients diagnosed with severe or critical influenza at screening: (1) Severe influenza cases were defined by the presence of one or more of the following conditions: a. Breathing difficulty and/or increased respiratory rate: \>30 breaths/min for children \>5 years old, \>40 breaths/min for children aged 2-5 years; b. Consciousness alteration: slow response, somnolence, agitation, seizures, etc; c. Severe vomiting or diarrhea with moderate or severe dehydration; d. Pneumonia; e. Significant exacerbation of underlying diseases. (2) Critical influenza cases were defined by the presence of one or more of the following conditions: a. Respiratory failure; b. Acute necrotizing encephalopathy; c. Septic shock; d. Multiple organ dysfunction; e. Other severe clinical conditions requiring intensive care treatment. Note: Refer to the "Expert Consensus on Diagnosis and Treatment of Influenza in Children (2020 Edition)" for the criteria of severe/critical influenza. 3. History of any gastrointestinal disease known to affect drug absorption (including but not limited to gastroesophageal reflux disease, chronic diarrhea, inflammatory bowel disease, intestinal tuberculosis, gastrinoma, short bowel syndrome, or post-gastrectomy conditions). 4. Suspected or confirmed bronchitis, pleural effusion, or interstitial pneumonia via clinical or radiological examination at screening. 5. Use of anti-influenza antiviral drugs within two weeks before screening (including neuraminidase inhibitors, hemagglutinin inhibitors, and M2 ion channel blockers such as oseltamivir phosphate, zanamivir, peramivir, favipiravir, arbidol, baloxavir, amantadine, or rimantadine, or other anti-influenza antiviral drugs approved by the NMPA). 6. Clinically relevant abnormal results in physical examination, 12-lead ECG, vital signs, hematology, clinical biochemistry, or urinalysis at screening, which in the investigator's judgment may pose a risk to the participant's safety, affect the study results, or impact the participant's ability to complete the study. 7. Acute respiratory tract infection, otitis media, or sinusitis within two weeks before screening. 8. Co-infection requiring systemic antibiotics or other systemic therapy at screening. 9. Known or suspected active primary or secondary immunodeficiency, including a history of HIV infection or other severe immunodeficiencies. 10. Known or suspected congenital abnormalities of the heart or lungs, or severe primary diseases affecting the cardiovascular, hepatic, renal, or hematopoietic systems, or evidence of active liver disease, including but not limited to jaundice or AST/ALT levels exceeding two times the upper limit of normal (ULN) at screening. 11. Positive serology for hepatitis B surface antigen or a history of hepatitis B infection at screening. Note: Children and adolescents with a history of hepatitis B vaccination without a history of hepatitis B infection do not need to be screened for this criterion (assessed via medical history). 12. Obesity as determined by the investigator: For participants \<6 years old: BMI \>97th percentile for the corresponding age and gender group; For participants ≥6 years old: BMI ≥obesity threshold for the corresponding age and gender group. Note: Refer to "WS/T 423-2022: Growth Standards for Children Under 7" or "WS/T586-2018: Screening for Overweight and Obesity in School-Aged Children and Adolescents." 13. History of mental illness, intellectual disability, substance abuse, or other adverse conditions (e.g., inability to read, understand, or write) that, in the investigator's judgment, may limit participation in the study. 14. Vaccination with an influenza vaccine within six months before screening or during the study period. Note: Participants' influenza vaccination history should be carefully reviewed before study enrollment (assessed via medical history). 15. Participation in a clinical trial of any investigational drug or device for any indication within 30 days prior to screening. 16. Intolerance to skin puncture, fainting at the sight of blood or needles, or poor venous access for blood collection at screening. 17. Other conditions judged by the investigator to be unsuitable for participation in the study.