The objective of this study was to evaluate the safety of quadrivalent subunit influenza vaccine (adjuvant) in people 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.