A Study to Evaluate the Safety, Efficacy and Pharmacokinetics of Suraxavir Marboxil for Suspension

Inclusion Criteria: 1. Male or female participants aged 2 to 12 years (≥2 years and \<12 years). 2. body weight≥10 kg at screening. 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 (tympanic temperature ≥38.0°C or axillary temperature ≥37.5°C). If antipyretics were taken, fever must be present \>1 hour after administration, or If fever subsided (tympanic \<38.0°C or axillary \<37.5°C) after antipyretic use, fever must have recurred \>4 hours after administration; and * Presence of at least one moderate or more severe respiratory symptom at screening (including cough, and/or nasal congestion or rhinorrhea). 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 (tympanic temperature ≥38.0°C or axillary temperature ≥37.5°C) or time of first appearance (or caregiver observation) of at least one respiratory symptom associated with influenza virus infection. 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 (e.g., completing the diary card). Exclusion Criteria: 1. Known hypersensitivity to the active ingredient or excipients of the investigational product. 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. Dyspnea and/or tachypnea without fever: \>30 breaths/min (age \>5 years), \>40 breaths/min (age 2-5 years).b. Altered mental status (e.g., lethargy, agitation, seizures).c. Severe vomiting/diarrhea with moderate-to-severe dehydration.d. Concurrent pneumonia.e. Significant exacerbation of pre-existing comorbidities.(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. Multiorgan dysfunction.e. Other life-threatening conditions requiring intensive care. Note: Based on "Expert Consensus on Diagnosis and Treatment of Influenza in Children (2020 Edition)". 3. History of gastrointestinal diseases affecting drug absorption (e.g., reflux esophagitis, chronic diarrhea, inflammatory bowel disease, intestinal tuberculosis, gastrinoma, short bowel syndrome, post-gastrectomy). 4. Bronchitis, pleural effusion, or interstitial pneumonia suspected by investigator or confirmed by chest imaging. 5. Use of anti-influenza antiviral drugs within two weeks before screening (e.g., neuraminidase inhibitors \[oseltamivir, zanamivir, peramivir\], cap-dependent endonuclease inhibitors \[baloxavir marboxil\], hemagglutinin inhibitors \[arbidol\], M2 inhibitors \[amantadine, rimantadine\], or other NMPA-approved anti-influenza agents). 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 infection (excluding current influenza), otitis media, or sinusitis within 2 weeks pre-screening. 8. Co-infection requiring systemic antibiotics or other systemic therapy at screening. 9. Known/suspected active primary/secondary immunodeficiency (including HIV infection, hematologic malignancies causing severe immunodeficiency). 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: participants with HBV vaccination history and no prior HBV infection are exempt from this exclusion after medical history review). 12. 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. 13. Influenza vaccination within 6 months pre-screening or planned during the study. (Note: Vaccination history must be verified by medical history review). 14. Participation in any interventional clinical trial (drug/device) within 30 days pre-screening. 15. Intolerance to skin puncture, fainting at the sight of blood or needles, or poor venous access for blood collection at screening. 16. Any other condition deemed unsuitable by the investigator.