Phase I/II Clinical Trial of Influenza Vaccine (Split Virion), Inactivated (High Dose)

This is a randomized, double-blind, active-controlled Phase I clinical trial to evaluate the safety and immunogenicity of Influenza Vaccine (Split Virion), Inactivated (High Dose) developed by Sinovac.

Inclusion Criteria: 1. Participants aged 60 years and older who are in stable health condition. 2. Participants who are fully informed and voluntarily sign the informed consent form. 3. Participants who are willing and able to comply with all study procedures and remain contactable throughout the study. 4. Participants who are able to provide valid legal identification. Exclusion Criteria: 1. Participants who have received a seasonal influenza vaccine within 6 months before enrollment, have received a seasonal influenza vaccine for the current influenza season, or plan to receive a seasonal influenza vaccine during the study. 2. Participants who have had seasonal influenza within 6 months before enrollment. 3. Participants with known severe allergy to any vaccine or vaccine component, such as urticaria, dyspnea, angioedema, or anaphylactic shock. 4. Participants with poorly controlled chronic diseases or a history of serious diseases, including but not limited to cardiovascular diseases, such as hypertension not controlled by medication (systolic blood pressure ≥160 mmHg and/or diastolic blood pressure ≥100 mmHg measured at the study site) or coronary heart disease; metabolic diseases, such as poorly controlled diabetes; hematologic diseases; hepatic or renal diseases; digestive system diseases; respiratory system diseases; malignant tumors; a history of major organ transplantation; or any other disease or physiological condition that, in the investigator's opinion, may interfere with the study results. 5. Participants with autoimmune diseases or immunodeficiency diseases, including but not limited to systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid disease, asplenia, functional asplenia, or HIV infection. 6. Participants with coagulation disorders, such as coagulation factor deficiency or platelet abnormalities. 7. Participants with current or previous severe neurological disease or psychiatric disease, or a family history of psychiatric disease. 8. Participants with a history of long-term alcohol abuse or drug abuse. 9. Participants with a history of Guillain-Barré syndrome (GBS). 10. Participants who have received immunosuppressive agents or other immunomodulatory therapy for ≥14 days within the past 6 months (prednisone ≥20 mg/day or equivalent), cytotoxic therapy, or plan to receive such therapy during the study. Corticosteroid spray therapy for allergic rhinitis and topical corticosteroid therapy for acute uncomplicated dermatitis are excluded from this criterion. 11. Participants who have received immunoglobulin or other blood products within 3 months before receiving the study vaccine, or plan to receive such treatment during the study. 12. Participants who have received other investigational drugs or vaccines within 30 days before receiving the study vaccine, or plan to receive such drugs or vaccines during the study. 13. Participants who have received live attenuated vaccines or RNA vaccines within 14 days before receiving the study vaccine. 14. Participants who have received subunit or inactivated vaccines within 7 days before receiving the study vaccine. 15. Participants with any acute disease, acute exacerbation of a chronic disease, or known or suspected active infection within the past 7 days. 16. Participants with fever on the planned day of study vaccination, defined as axillary temperature \>37.0°C before vaccination. 17. Participants who, in the investigator's opinion, have any other factors that make them unsuitable for participation in the clinical trial.