This is a randomised, double-blind, placebo-controlled Phase 1/2 clinical trial to evaluate the safety, tolerability and immunogenicity of recombinant respiratory syncytial virus in participants aged 18 years and older.

Three dose-levels of SCTV02 will be evaluated in participants aged 18 years and older. Solicited adverse events within 7 days post study vaccination and unsolicited adverse events within 30 days post study vaccination will be collected. Immunogenicity including Neutralizing antibody against RSV-A and RSV-B will be tested 14, 30, 90, 180 and 365 days post study vaccination will be assessed. T cell response will be tested 30 days post study vaccination.

Inclusion Criteria:

• Phase I: male and female participants ≥18 years old at the time of signing the ICF; Phase II: male and female participants ≥50 years of age at the time of signing the ICF.
• Be able to sign a written Informed consent form (ICF) and participate in the trial voluntarily, and can fully understand the trial procedures and risks of participating in the trial.
• Be able to complete the diary card, contact card and diary/contact card by own or with the assistance of others.
• Phase I: healthy subjects or healthy subjects judged by the investigator according to the clinical data of the subjects; Stage II: Subjects who are healthy or have a stable underlying disease.
• Fertile men and women of childbearing age voluntarily agree to use effective contraception from the time they sign the ICF until 6 months after immunization; Pregnancy test results for women of childbearing age during the screening period were negative.

Exclusion Criteria:

• Acute illness and/or fever (axillary temperature ≥37.3 ° C) occurred within 3 days prior to vaccination with the study vaccine, or antipyretic, analgesic, or antiallergic drugs.
• Use any investigational or unregistered product within 30 days prior to immunization, or plan to use any such product during the study period.
• Concurrent participation in another clinical study where the subject has been or will be exposed to an investigational or non-investigational vaccine/product at any time during the study.
• Long-term or high-dose glucocorticoid therapy (duration ≥15 days, or dose ≥1 mg/kg/ day of prednisone or equivalent doses of other glucocorticoids), or other immunosuppressive and cytotoxic therapy within 90 days prior to vaccination.
• The study received a non-attenuated vaccine within 14 days prior to vaccination, or a live attenuated vaccine within 28 days; Have previously received the experimental respiratory syncytial virus (RSV) vaccine or been infected with RSV within 1 year prior to receiving the investigational vaccine.
• History or family history of epilepsy and mental illness.
• A family history of congenital or hereditary immunodeficiency, or a history and physical examination confirmed or suspected immunosuppression or immunodeficiency, or human immunodeficiency virus (HIV) infection, asplenia or functional asplenia.
• Have the following organ-specific or systemic autoimmune diseases: Guillain-Barre syndrome, myasthenia gravis, autoimmune hepatitis, ulcerative colitis, systemic lupus erythematosus, rheumatoid arthritis, Sjogren's syndrome, and systemic sclerosis.
• Allergic to any component of the study vaccine, or any previous history of severe allergy to vaccine vaccination, such as anaphylactic shock, allergic laryngeal edema, anaphylactoid purpura, thrombocytopenic purpura, local allergic necrosis reaction, angioneurotic edema, etc.
• Severe chronic disease or active chronic disease, as assessed by the investigator, including but not limited to myocardial infarction, severe arrhythmia, unstable angina, uncontrolled blood pressure after medication (systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg in subjects < 60 years of age; Hypertensive disease with systolic blood pressure > 160 mmHg and/or diastolic blood pressure > 100 mmHg in subjects ≥60 years of age, diabetes mellitus with serious complications, cancer or precancerous lesions, and other serious cerebrovascular diseases, heart diseases, respiratory diseases, liver and kidney diseases, and thyroid diseases.
• Pregnant women (positive pregnancy test) or breastfeeding women, or those with pregnancy plans during the study period, or less than 6 weeks after the end of pregnancy (including ectopic pregnancy).

Plan to donate eggs or sperm during the study.

• Unable to follow the trial procedures, or plan to relocate or stay away for a long period of time during the study and cannot complete the trial follow-up.
• Any medical condition that makes intramuscular injection unsafe due to other abnormalities, conditions that may obfuscate the results of the study, or conditions that are not in the best interest of the subject, is determined by the investigator to be unsuitable for clinical study.