Subunit Influenza Vaccine Immune Responses and Safety in Healthy Adults

Study start date: October 11, 2026

Inclusion Criteria: * Adults aged between 18 and 59 years at the time of informed consent signature. * Judged by the investigator to be in good health, based on medical history and physical examination findings obtained at screening. * Able to comprehend and voluntarily provide written informed consent, and willing to comply with all study procedures and requirements. Exclusion Criteria: * Known allergy to any component of the vaccine, including excipients, formaldehyde, or Triton N-101. * Confirmed influenza infection or influenza-like illness within the past 6 months, or close contact with a confirmed influenza case within 14 days prior to the screening visit. * Receipt of any influenza vaccine (licensed or investigational) within the past 6 months. * Receipt of any licensed vaccine within 28 days prior to study vaccination, or planned receipt of any licensed vaccine within 28 days after study vaccination. * Receipt of blood products or immunoglobulin within the past 3 months, or planned receipt within 1 month after study vaccination. * Female participants who are known to be pregnant or breastfeeding; female participants of childbearing potential with a positive pregnancy test at screening; or those planning pregnancy within the next 3 months. * Presence of acute illness, acute exacerbation of a chronic disease, or fever (axillary temperature ≥37.3 °C) within 3 days prior to vaccination. Use of anti-inflammatory, antibiotic, anti-allergic, or antiviral medications within 3 days prior to vaccination due to illness, including but not limited to acetaminophen, ibuprofen, loratadine, penicillins, cephalosporins, ribavirin, oseltamivir, and acyclovir. * Systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg on the day of vaccination. * Severe chronic cardiovascular or pulmonary disease: * Heart failure with a history of acute exacerbation within the past 2 years; * Chronic obstructive pulmonary disease (COPD) with a history of acute exacerbation within the past 2 years; * Chronic interstitial lung disease with acute exacerbation or requirement for treatment adjustment within the past 2 years; * \- History of asthma exacerbation within the past 2 years, or worsening asthma symptoms requiring adjustment of controller medication within the past 2 months; * Uncontrolled serious arrhythmia, or hospitalization for acute arrhythmia or regular antiarrhythmic medication within the past 6 months; * Unstable coronary artery disease within the past 3 months, as evidenced by percutaneous coronary intervention (e.g., stent implantation), coronary artery bypass grafting (CABG), newly initiated cardiac medication for symptom control, or unstable angina. * Uncontrolled epilepsy or other progressive neurological diseases (e.g., multiple sclerosis, dementia, Parkinson's disease, neurodegenerative disorders, neuropathy); History of Guillain-Barré syndrome; Stroke with residual neurological deficits within the past 12 months; Any other neurological disease with residual symptoms within the past 12 months. * Currently receiving anti-tumor therapy, including chemotherapy/radiotherapy, targeted therapy, or immunotherapy; Recurrence or metastasis of malignancy within the past 12 months; History of solid organ transplantation or hematopoietic stem cell transplantation. * Chronic conditions potentially affecting study outcomes: * Diagnosed hypertension or diabetes mellitus; * Symptoms of myocarditis or pericarditis; * Anemia, thrombocytopenia, or other coagulation disorders; * Gout with joint redness and swelling, or use of systemic corticosteroids, high-dose colchicine, or non-steroidal anti-inflammatory drugs (NSAIDs) within the past month; * Autoimmune diseases (e.g., systemic lupus erythematosus, rheumatoid arthritis, systemic vasculitis); * Severe immunodeficiency, or requirement for immunosuppressive or corticosteroid therapy during the study period, or any medical history associated with impaired immunity (e.g., HIV infection, malignancy, particularly leukemia or lymphoma, history of radiation therapy, or use of immunosuppressive or anti-neoplastic agents); * Chronic kidney disease/renal insufficiency, or chronic gastrointestinal and hepatic diseases (e.g., chronic gastritis, irritable bowel syndrome \[IBS\], inflammatory bowel disease \[IBD\], Crohn's disease, colitis, hepatitis B, hepatitis C, non-alcoholic fatty liver disease, cirrhosis, hepatocellular carcinoma) requiring regular medication; * Uncontrolled allergic diseases (e.g., allergic asthma, allergic rhinitis). Presence of active, unhealed systemic or extensive cutaneous allergic conditions at screening, including but not limited to allergic dermatitis, severe eczema, urticaria, acute flare of psoriasis, or exfoliative dermatitis. Frequent episodes of unexplained pruritus, rash, or wheals within the past month, or unstable skin allergic conditions requiring continuous antihistamine therapy, as judged by the investigator to make the participant unsuitable for enrollment. * Planned relocation outside the study site area before completion of all study procedures, or anticipated prolonged absence during scheduled follow-up visits that would prevent completion of the study schedule. * Currently enrolled in any other clinical trial, or planning to participate in another clinical trial before completion of the present study. * Any other condition or circumstance that, in the investigator's opinion, renders the participant unsuitable for enrollment.