This is a randomized, observer-blinded, placebo-controlled Phase Ⅱ clinical trial to evaluate the immunogenicity and safety of the Respiratory Syncytial Virus (RSV) Vaccine, LYB005 in participants aged 60 years and older.

A randomized, observer-blinded, placebo-controlled trial will be conducted to observe the immunogenicity and safety of LYB005 in adults aged 60 years and older. A total of 700 participants aged 60 years and older will be enrolled. Six formulations of LYB005 will be provided, three dose levels of antigen with or without A01B adjuvant. All participants will randomly receive six investigational vaccines and the placebo in a 1:1:1:1:1:1:1 ratio.

Inclusion Criteria:

1. Residents aged 60 years and older (at the time of screening), regardless of gender;
2. Participants can provide valid identification, voluntarily agree to participate in the study, and sign the Informed Consent Form, and are able to attend all planned follow-up visits and comply with the protocol requirements;
3. Axillary temperature < 37.3°C on the day of enrollment;
4. Female participants must be postmenopausal (postmenopausal status defined as amenorrhea for 12 months without other medical causes) and must not intend to become pregnant by any means. Male participants must practice strict contraception and avoid plans for procreation or sperm donation from the screening period until 1 month after vaccination. Acceptable methods of contraception include oral contraceptives (excluding emergency contraception), injectable or implantable contraception, sustained-release local contraceptives, contraceptive patches, intrauterine devices, sterilization, abstinence, condoms, diaphragms, and cervical caps.

Exclusion Criteria:

1. Allergy to the investigational vaccine or its excipients, or a history of anaphylactic shock or other serious adverse reactions to other vaccines;
2. Previous vaccination against Respiratory Syncytial Virus;
3. A confirmed diagnosis or etiological evidence of respiratory syncytial virus infection and related diseases caused by the infection within 12 months before enrollment;
4. Has taken antipyretics, analgesics or anti-allergy drugs within 24 hours before enrollment;
5. Has received any vaccine within 14 days before vaccination, or have received a live vaccine within 28 days;
6. Has received blood or blood-related products, including immunoglobulin, within 3 months prior to enrollment; or plan to use them during the study period;
7. Individual with the following diseases: ①Has acute diseases or are in the acute exacerbation period of chronic diseases within 3 days before vaccination; ②Diagnosed with congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc.; ③History of congenital or acquired immunodeficiency or autoimmune diseases; Chronic administration (≥14 consecutive days) of corticosteroids (dose ≥ 20 mg/day prednisone or equivalent dose) or other immunosuppressants within the past 3 months, with the exception of inhaled or topical steroids, or short-term use (<14 consecutive days) of oral corticosteroids; ④Neurological diseases or family history (seizures, epilepsy, encephalopathy, etc.); history of psychiatric disorders or family history; ⑤Asplenia or functional asplenia; ⑥Severe or uncontrolled or hospitalization-required cardiovascular diseases, diabetes, blood and lymphatic system diseases, immune system diseases, liver and kidney diseases, respiratory system diseases, metabolic and skeletal system diseases, or malignant tumors; ⑦Contraindications for intramuscular injection and blood drawing, such as coagulation disorders, thrombosis or hemorrhagic diseases, or situations requiring continuous use of anticoagulants; ⑧Severe hypertension that cannot be controlled by medication (measured on-site: systolic blood pressure ≥ 160 mmHg and/or diastolic blood pressure ≥ 100 mmHg);
8. History of major surgery within 12 weeks prior to enrollment (as determined by the investigator), or not fully recovered from the surgery, or having plans for major surgery during the anticipated period of the subject's participation in the study;
9. History of long-term alcohol abuse and/or drug abuse;
10. Individual who is currently participating in other research or unregistered product (drugs, vaccines, or devices, etc.) clinical studies, or plan to participate in other clinical studies before the end of this clinical study;
11. Other conditions that may impact the subject's safety or influence the assessment of vaccine response, as determined by the investigator.