Inclusion Criteria:
* Males or females aged 18 years and older at the time of enrollment (aged 18 years and older for the phase I part; aged 60 years and older for the phase II part), who are able to provide legal proof of identity.
* Voluntarily agree to participate in the trial, able to fully understand and sign the informed consent form.
* Able to attend all scheduled follow-up visits and comply with the requirements of the clinical trial protocol to complete the study.
* Female participants must meet the following criteria: In the phase I part, women of childbearing potential\* must have a negative pregnancy test prior to enrollment and be willing to use effective contraceptive measures for 12 months after receiving the investigational vaccine; in the phase II part, only women of non-childbearing potential will be enrolled.
* Women of childbearing potential: Defined as females who have experienced menarche and have not yet entered menopause, unless permanently sterile, such as documented bilateral salpingectomy, bilateral oophorectomy, or hysterectomy; menopause is defined as amenorrhea for 12 consecutive months without other medical cause.
\[Effective contraceptive measures include: oral contraceptives, injectable contraceptives, subdermal implants or hormonal patches, intrauterine device (IUD), sterilization surgery, abstinence (no sexual intercourse), male condoms, etc.; rhythm method, withdrawal, and emergency contraception are not considered effective contraceptive measures.\]
* Axillary body temperature ≤ 37.0°C measured on site prior to vaccination on the day of vaccination.
Exclusion Criteria:
* Clinically significant laboratory abnormalities that, in the investigator's comprehensive judgment, preclude enrollment (applicable only to the phase I part).
* Pregnant or breastfeeding women.
* A clear diagnosis of RSV infection or a history of RSV infection-related respiratory disease within 6 months prior to vaccination.
* Use of immunoglobulins or/and any blood products or plasma derivatives within 3 months prior to vaccination, or planned use during the study.
* Treatment with immunomodulators (including immunosuppressants and immunostimulants) within 6 months prior to vaccination (e.g., long-term use of systemic glucocorticoids for ≥14 days at a dose of ≥2 mg/kg/day or ≥20 mg/day prednisone or equivalent) (excluding inhaled, intra-articular, and topical steroids).
* Administration or planned use of long-acting immunomodulatory drugs (e.g., infliximab) at any time during the study.
* A history of severe allergic reactions (e.g., anaphylactic shock, allergic laryngeal edema, Henoch-Schönlein purpura, thrombocytopenic purpura, Arthus reaction) following any previous vaccination or drug use, or a family history of severe allergies.
* Impaired immune function or a diagnosis of congenital or acquired immunodeficiency, or human immunodeficiency virus (HIV) infection.
* A personal or family history of convulsions, epilepsy, encephalopathy, psychiatric disorders, or neurological diseases (such as Guillain-Barré syndrome, Miller Fisher syndrome).
* A diagnosis of lymphoproliferative disease or malignant tumor within 5 years.
* Clinically significant electrocardiogram (ECG) abnormalities as determined by the investigator (applicable only to the phase I part).
* Previous vaccination with any licensed or investigational RSV vaccine prior to enrollment, or planned vaccination during the study.
* Receipt of any vaccine within 14 days prior to vaccination, or any live vaccine within 30 days prior to vaccination.
* An acute disease or an acute exacerbation of a chronic disease within 3 days prior to vaccination, or use of antipyretic, analgesic, or anti-allergy medications.
* Suspected or known alcohol abuse or alcohol dependence (referring to a drinking pattern that causes serious mental or physical health problems) or drug abuse.
* A history of thrombocytopenia or other coagulation disorders that may contraindicate intramuscular injection.
* Severe or unstable chronic diseases, including but not limited to cardiovascular diseases \[e.g., hypertension uncontrolled by medication (on-site blood pressure measurement prior to vaccination: systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg), coronary heart disease, myocarditis, pericarditis, atrial fibrillation\], metabolic diseases (e.g., poorly controlled diabetes), hematological disorders (e.g., severe anemia, hemophilia), hepatic or renal diseases, digestive system diseases, respiratory system diseases (e.g., chronic obstructive pulmonary disease, active pulmonary tuberculosis, other severe respiratory diseases).
Note: For participants aged 60 years and older, those with pre-existing stable disease may be enrolled. This is defined as participants who may have underlying conditions such as hypertension or diabetes, provided that symptoms and signs are stable and medically controllable as assessed by the investigator prior to vaccination, and who have not required changes to the treatment/medication regimen or hospitalization due to the underlying condition within 3 months.
* A history of confirmed immune-mediated/autoimmune diseases (e.g., cold agglutinin hemolytic anemia, lymphadenopathy, eosinophilia, systemic lupus erythematosus, gout, hyperuricemia, acute disseminated encephalomyelitis, immune thrombocytopenia, autoimmune aplastic anemia, autoimmune neutropenia, autoimmune lymphoproliferative syndrome, thrombocytopenic purpura).
* Asplenia or functional asplenia, as well as any condition resulting in asplenia or splenectomy.
* Currently participating in or planning to participate in another clinical study involving investigational or unregistered products (drugs, vaccines, medical devices, etc.) during this study.
* Any other condition that, in the investigator's judgment, makes the individual unsuitable for participation in this clinical trial.
