This randomized, double-blind, placebo-controlled study is designed to evaluate the efficacy and safety of Onradivir Tablets in participants aged 12 to 17 years with influenza A virus infection.

The trial plans to enroll approximately 150 adolescent participants (12-17 years) with uncomplicated influenza A, confirmed by rapid antigen/nucleic acid testing and within 48 hours of symptom onset. Eligible participants who meet the inclusion criteria and do not meet any exclusion criteria will be randomized in a 2:1 ratio to receive either Onradivir Tablets or placebo once daily for 5 consecutive days.

The study consists of a screening period, a treatment period, and a safety follow-up period. Each participant will be followed up until Day 14 after the first dose. Nasopharyngeal swabs will be collected at screening, Day 2, Day 4, Day 5, and Day 10 for virological testing, including:

Viral load measurement by reverse transcription quantitative polymerase chain reaction (RT-qPCR)

Viral titer determination by culture assay (50% tissue culture infectious dose, TCID₅₀)

Throughout the treatment and safety follow-up periods, participants will record body temperature, influenza symptom scores, and daily activity scores using a diary card. Clinical laboratory tests will be performed at screening, Day 5, and Day 15. Physical examinations and vital signs measurements will be conducted at each follow-up visit. Adverse events and concomitant medications will be documented throughout the study.

Inclusion Criteria:

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All participants must meet all of the following criteria:

Aged 12 to 17 years (inclusive), regardless of gender.

Diagnosed with influenza A based on the following criteria:

• Positive for influenza A virus infection by Rapid Antigen Test (RAT) or nucleic acid testing of nasopharyngeal/oropharyngeal swab at screening.
• Axillary temperature ≥38°C at screening. If this criterion is not met due to antipyretic use, reassessment may be performed >4 hours after the most recent antipyretic administration.
• Presence of at least one moderate or severe influenza-related systemic symptom (headache, fever or chills, muscle or joint pain, fatigue) at screening.
• Presence of at least one moderate or severe influenza-related respiratory symptom (cough, sore throat, nasal congestion) at screening.

Onset of the first influenza symptom/sign occurring within ≤48 hours before randomization. Influenza symptoms/signs include fever (based on participant-reported data, body temperature ≥37.5°C), headache, muscle or joint pain, fatigue, cough, sore throat, and nasal congestion.

Participant's parent/legal guardian and the participant themselves provide signed informed consent before the trial, with full understanding of the trial content, procedures, and potential adverse reactions.

In the investigator's judgment, the participant and their parent/legal guardian are willing and able to comply with the study protocol requirements.

Exclusion Criteria:

• Participants will be excluded from the study if they meet any of the following criteria:

Judged by the investigator as having severe or critical influenza.

Having allergic constitution, known allergy to Anladiwei or paracetamol, and/or clinically suspected intolerance, or having contraindications.

Difficulty swallowing the investigational product, or history of gastrointestinal diseases that severely affect drug absorption (including but not limited to reflux esophagitis, chronic diarrhea, inflammatory bowel disease, intestinal tuberculosis, gastrinoma, short bowel syndrome, partial gastrectomy, etc.).

History of febrile seizures.

High-risk population for severe cases, defined as having any of the following conditions:

• Respiratory diseases judged clinically significant by the investigator (e.g., interstitial lung disease, pulmonary hypertension, cystic fibrosis, asthma), metabolic diseases, hematological diseases, cardiovascular and cerebrovascular diseases, liver diseases, kidney diseases, neurological and neuromuscular diseases, psychiatric disorders, or other serious underlying conditions.
• Immunocompromised status, such as malignant tumors, organ or bone marrow transplantation, HIV infection, or use of systemic immunosuppressants within the past 3 months.

Concurrent conditions requiring treatment with medications containing aspirin or salicylates.

History of allergic diseases requiring regular use of antihistamines or other prohibited medications; or occurrence of acute respiratory infection, bronchitis, otitis media, or sinusitis within 2 weeks before onset (appearance of the first influenza symptom/sign); or presence of respiratory symptoms (cough, nasal congestion, runny nose, sore throat) within 1 week before onset.

Presence of suspected pharyngoconjunctival fever, herpangina, measles, purulent tonsillitis, or other respiratory infectious diseases or communicable diseases.

Suspected co-infection requiring systemic treatment, in addition to influenza A virus infection.