Phase I Clinical Trial of Trivalent Inactivated Split Influenza Virus Vaccine (MDCK Cell)

This study is a phase I, randomized, double-blind, placebo and active comparator-controlled clinical trial to evaluate the safety and immunogenicity of a trivalent split influenza virus vaccine (MDCK cell) developed by Sinovac Biotech Co., Ltd., in participants aged 6 months and above.

Inclusion Criteria: 1. Healthy trial participants aged ≥6 months; Among them, for children aged 3-8 years: * Influenza vaccine-naïve: children who have never received any influenza vaccine (licensed or investigational); * Previously vaccinated: children who have received at least 1 dose of influenza vaccine (licensed or investigational); 2. The trial participant and/or the participant's legal representative can understand and voluntarily sign the informed consent form; 3. Willing and able to comply with all visit schedules, sample collection, vaccination and other trial procedures, and maintain contact at all times during the trial; 4. Able to provide legal identification documents. Exclusion Criteria: 1. Receipt of any licensed or investigational influenza vaccine within the previous 18 months, or intention to receive any other influenza vaccine during the study; 2. Clinically or laboratory-confirmed influenza infection within the previous 6 months; 3. Known history of a severe allergic reaction to the study vaccine or any of its components, including urticaria, dyspnea, angioedema, or anaphylaxis; 4. Fever on the scheduled day of study vaccination, defined as an axillary temperature \>37.0°C before vaccination; 5. Skin lesions, inflammation, ulcers, rashes, scars, or other conditions at the intended injection site that may interfere with vaccination or the assessment of local reactions; 6. Clinically significant laboratory abnormalities outside the reference ranges in any of the following parameters: · Hematology: white blood cell count, hemoglobin, or platelet count; Serum biochemistry: alanine aminotransferase (ALT), aspartate aminotransferase (AST), or total bilirubin; Urinalysis: urine protein; 7. Severe congenital malformations or genetic disorders, including but not limited to Down syndrome or phenylketonuria; 8. Poorly controlled chronic disease or a history of serious disease, including but not limited to cardiovascular disease, metabolic disease (e.g., poorly controlled diabetes mellitus), hematologic disease, hepatic or renal disease, gastrointestinal disease, respiratory disease, malignancy, or a history of major organ transplantation. For participants aged ≥18 years, hypertension inadequately controlled with medication is defined as: Systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg for participants aged 18-59 years; Systolic blood pressure ≥150 mmHg and/or diastolic blood pressure ≥100 mmHg for participants aged ≥60 years; 9. Autoimmune disease, immunodeficiency disorder, or a relevant family history, including but not limited to Guillain-Barré syndrome (GBS), systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid disease, anatomic asplenia, functional asplenia, or HIV infection; 10. Coagulation abnormalities, including coagulation factor deficiency or platelet disorders; 11. Current or previous severe neurologic disease, including epilepsy, convulsions, or seizures, except for a history of febrile seizures in infants or young children; severe psychiatric disorder; or a family history of schizophrenia; 12. Receipt of immunosuppressive agents or other immunomodulatory therapy for ≥14 days within the previous 6 months, including prednisone at a dose of ≥20 mg/day or ≥2 mg/kg/day, or another systemic corticosteroid at an equivalent prednisone dose; receipt of cytotoxic therapy; or intention to receive any such therapy during the study. Topical and inhaled corticosteroids are excluded; 13. Receipt of immunoglobulins or other blood products within the previous 3 months, or intention to receive such products during the study; 14. Receipt of any other investigational drug or vaccine within the previous 30 days, or intention to receive any such drug or vaccine during the study; 15. Receipt of a live attenuated vaccine or viral vector vaccine within the previous 14 days, or receipt of a subunit, inactivated, or other non-live vaccine within the previous 7 days; 16. Any acute illness or acute exacerbation of a chronic disease within the previous 7 days, or any known or suspected active infection; 17. Breastfeeding or pregnancy, including a positive urine pregnancy test, or an intention to become pregnant or donate oocytes from the time of signing the informed consent form until 6 months after study vaccine administration; for male participants, an intention to father a child or donate sperm during the same period; 18. Any other condition that, in the investigator's judgment, would make the participant unsuitable for participation in the study.