Immunogenicity and Safety of SYS6057 in the Participants Aged 18 Years and Above

• Aged 18 years and above, male or female ;
• Able to understand the study procedures, comply with the protocol requirements to attend all planned follow-up visits, voluntarily agree to participate in the study, and sign the Informed Consent Form;
• Body mass index (BMI) meeting the requirement: 18.5 ≤ BMI \< 32.5 kg/m²;
• Women of childbearing potential and female spouses of male participants of childbearing potential: no sexual intercourse during the one menstrual cycle prior to enrollment, or use of effective contraception; no pregnancy plan within 6 months after enrollment and agreement to use effective contraception;

Exclusion Criteria:

• Previous vaccination with any RSV vaccine (including investigational RSV vaccines), or planned vaccination with an RSV vaccine during the study period;
• Receipt of RSV monoclonal antibody (including investigational RSV antibodies) within 6 months prior to enrollment, or planned receipt of RSV monoclonal antibody during the study period;
• History of RSV infection, as determined by medical interview, within 6 months prior to enrollment;
• Axillary temperature ≥37.3℃ on the day of enrollment or within 24 hours prior to enrollment;
• Blood pressure measured on the day of enrollment: systolic blood pressure ≥140 mmHg or diastolic blood pressure ≥90 mmHg;
• Abnormal laboratory tests, electrocardiogram (ECG), or physical examination findings with clinical significance prior to enrollment, or findings reaching Grade 1 or above according to the adverse event grading criteria of the National Medical Products Administration's "Guideline for Grading Standards of Adverse Events in Vaccine Clinical Trials (Revised Version)" (2025);
• History of allergy to any component of the investigational vaccine; or a history of severe allergic reactions to vaccines or drugs (including but not limited to anaphylaxis, allergic laryngeal edema, anaphylactoid purpura, thrombocytopenic purpura, or local allergic necrosis [Arthus reaction]);
• History of demyelinating diseases, including but not limited to Guillain-Barré syndrome, multiple sclerosis, neuromyelitis optica, acute disseminated encephalomyelitis, etc.;
• Current epilepsy or seizures, severe neurological diseases, or psychiatric disorders;
• Contraindications to intramuscular injection, such as local infection or skin lesions at the injection site, severe thrombocytopenia, any coagulation disorder, or ongoing treatment with anticoagulants;
• Known malignancy, malignancy not adequately treated, or malignancy with potential risk of recurrence during the study period;
• Active, unstable, severe, or uncontrolled cardiovascular and cerebrovascular diseases, thrombotic diseases, diseases of the blood and lymphatic systems, liver and kidney diseases, respiratory diseases, endocrine and metabolic diseases, musculoskeletal diseases, autoimmune diseases, etc.;
• Confirmed immunodeficiency or immunosuppression prior to enrollment (including HIV infection); congenital or functional asplenia, or splenectomy;
• Long-term (defined as more than 14 days) systemic use of immunosuppressants, immunostimulants, or other immunomodulatory drugs within 6 months prior to enrollment (e.g., corticosteroids, ≥20 mg/day prednisone or equivalent; however, inhaled and topical steroids are permitted); or planned use of the above agents during the study period;
• Receipt of whole blood, plasma, serum, immunoglobulin, or monoclonal antibodies within 3 months prior to enrollment, or planned receipt of the above products during the study period;
• Blood donation or blood loss of ≥450 mL within 1 month prior to enrollment, or planned blood donation during the study period;
• Vaccination with any other vaccine within 30 days prior to enrollment, or planned vaccination with any other vaccine within 30 days after enrollment;
• Current participation, or planned participation during the study period, in other clinical trials;
• Females of childbearing potential: positive pregnancy test, pregnancy, or lactation prior to enrollment;
• In the investigator's judgment, inability to comply with study procedures or appointments; or any other condition unsuitable for participation in this clinical trial.