Clinical Trial of Quadrivalent Subunit Influenza Vaccine (Adjuvant) in Chinese Population Aged 65 Years and Older

Inclusion Criteria: * Age at enrollment is ≥65 years (including the 65th birthday). * The participant voluntarily agrees to take part in the trial and has signed the informed consent form. * The participant is able to comply with the trial follow-up schedule as required by the protocol (i.e., no plans for long-term absence or relocation from the study site). * Medically stable: The participant may have underlying chronic conditions such as hypertension, diabetes, ischemic heart disease, or hypothyroidism, but symptoms/signs must be controlled. If the participant is on any medication, the dosage must have remained stable for at least three weeks prior to vaccination. Exclusion Criteria: * Axillary temperature \>37.0°C. * Confirmed influenza virus infection by laboratory test or self-test kit positive within the past 12 months. * Received any influenza vaccine (licensed or investigational) within the past 12 months or plans to receive any influenza vaccine during the study. * Known or suspected allergic to any component of the investigational vaccine. * A history of severe allergic reactions to any vaccines or medications (for example, but not limited to: anaphylactic shock, allergic laryngeal edema, allergic purpura, thrombocytopenic purpura, local allergic necrotic reaction (Arthus reaction)). * Congenital malformations or developmental disorders, and genetic defects or severe malnutrition. * History of convulsions or seizures, epilepsy, psychiatric disorders, or relevant family history. * Severe liver and kidney diseases, malignant tumors, various acute diseases, or being in the acute exacerbation stage of a chronic disease. * Diagnosed with congenital or acquired immunodeficiency, HIV infection, lymphoma, leukemia, or other immune-mediated/autoimmune diseases. * A history of asthma, unstable within the past two years requiring emergency treatment, hospitalization, intubation, or oral/intravenous corticosteroids. * Individuals who have received immunostimulant or immunosuppressant therapy within the past 6 months (continuously administered orally or via drip infusion for more than 14 days). * Suffering from severe cardiovascular diseases (e.g., heart disease, cor pulmonale, pulmonary edema). * Blood pressure ≥150/100 mmHg despite medication control. * History of live attenuated vaccine within 28 days prior to vaccination, or any other vaccine within 14 days prior to vaccination. * Diagnosed with progressive neurological diseases, or a history of Guillain-Barré syndrome. * Asplenia or functional asplenia, including splenectomy due to any reason, or resection/partial removal of other vital organs. * Received blood or blood-derived products within the past 6 months. * A diagnosed history of coagulation disorders (e.g., coagulation factor deficiencies, coagulopathies, platelet disorders). * Currently undergoing anti-tuberculosis treatment. * Clinically significant abnormalities in laboratory tests (blood biochemistry, blood routine and urine routine) as determined by the investigator. * Any condition that, in the opinion of the investigator, makes the participant unsuitable for this clinical trial.