This phase 1 study in China will evaluate the safety and immunogenicity of the the Respiratory Syncytial Virus (RSV) Vaccine, LYB005 in adults aged 18 years and older.

A randomized, observer-blinded, placebo-controlled, dose escalation trial will be conducted to observe the safety and immunogenicity of LYB005 in adults aged 18 years and older. A total of 90 healthy subjects will be enrolled and stratified by age (18-59 years and ≥60 years in a 1:1 ratio). Six formulations of LYB005 will be provided, three dose levels of antigen with or without A01B adjuvant.

A sentinel and escalating dosing approach will be used for close monitoring of safety to minimize risk to participants. Participants will be enrolled in one of six cohorts, including Cohort 1 (18-59 years, low dose, n=15), Cohort 2 (18-59 years, middle dose, n=15), Cohort 3 (≥60 years, low dose, n=15), Cohort 4 (18-59 years, high dose, n=15), Cohort 5 (≥60 years, middle dose, n=15), and Cohort 6 (≥60 years, high dose, n=15). In each cohort, five sentinels were set up, and they were randomly vaccinated with the investigational vaccine without A01B adjuvant, the investigational vaccine with A01B adjuvant, or placebo in a 2:2:1 ratio. The remaining participants were randomly vaccinated in a 2:2:1 ratio with the investigational vaccine without A01B adjuvant, the investigational vaccine with A01B adjuvant, or placebo. A single-dose immunization schedule will be adopted.

Inclusion Criteria:

1. Residents aged 18 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. Females of childbearing potential should use effective contraceptive measures one month before enrollment; females of childbearing potential (excluding those who have undergone tubal ligation, bilateral oophorectomy, or hysterectomy) and male participants should practice effective contraception and avoid pregnancy plans, as well as sperm or egg donation plans from the time of enrollment until 6 months after the full course of vaccination. Effective contraceptive methods include oral contraceptives (excluding emergency contraceptives), injectable or implantable contraceptives, sustained-release local contraceptives, contraceptive patches, intrauterine devices, sterilization, abstinence, condoms, diaphragms, cervical caps, etc.

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:

   • Has acute diseases or are in the acute exacerbation period of chronic diseases within 3 days before vaccination;
   • Diagnosed with congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc.;
   • History 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;
   • Neurological diseases or family history (seizures, epilepsy, encephalopathy, etc.); history of psychiatric disorders or family history;
   • Asplenia or functional asplenia;
   • Severe 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;
   • Contraindications for intramuscular injection and blood drawing, such as coagulation disorders, thrombosis or hemorrhagic diseases, or situations requiring continuous use of anticoagulants;
   • Severe 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;

12. Exclusion criteria for specific populations: lactating or pregnant women during the clinical research period, or women of childbearing age with a positive pregnancy test before vaccination.