A phase 1, randomized, observer-blinded, parallel-controlled, dose escalation study in Australia will evaluate the safety and immunogenicity of the RSV vaccine candidate LYB005 with or without adjuvant in healthy adults aged 18 years and older.

The study design includes an age- and dose-escalation (low/middle/high dose) in two adult age groups (young adults [18-59 years] and older adults [≥60 years]). Study will be conducted in two parts, part 1 will enrolled young adults and part 2 will enroll older adults. A sentinel dosing approach will be used for close monitoring of safety to minimize risk to participants. Participants will be divided into the sentinel cohort and the remainder of cohort. Participants in part 1 will receive of one of two RSV vaccine formulations at one of 3 antigen dose levels or placebo. Participants in part 2 will receive of one of two RSV vaccine formulations at one of 3 antigen dose levels or positive control AREXVY. Detailed characterization of safety (including safety laboratory evaluation) and immune responses will be observed.

Inclusion Criteria:

1. Part 1-A male or female aged 18-59 years at screening; Part 2-A male or female aged 60 years and older at screening.

2. Written informed consent obtained from the subject before any assessment is performed.

3. Subjects who the investigator believes that they can and will comply with the requirements of the protocol. (e.g., complete the diary cards, and complete follow-up visits).

4. Subjects must have a Body Mass Index (BMI) between ≥18.0 and ≤35.0 kg/m^2 at screening.

5. Female subjects who are not pregnant or lactating. Female subjects with childbearing potential and their partners should use highly effective, medically accepted double-barrier contraception and will not have pregnancy and fertility plan and refrain from donating ovum from at least 28 days prior to study vaccination until study completion.

   • A woman is considered of childbearing potential, i.e. fertile, following menarche and until becoming post-menopausal unless permanently sterile. Permanent sterilisation methods include hysterectomy, bilateral salpingectomy and bilateral oophorectomy. A postmenopausal state is defined as no menses for 12 months without an alternative medical cause.
   • Highly effective double-barrier contraception is defined as use of a condom AND one of the following: birth control pills (The Pill), depot or injectable birth control, intrauterine device (IUD), NuvaRing®, implantable contraception (e.g., Implanon).
   • Note: There is no contraception requirement for female subjects with non-childbearing potential (WNCBP). There is no contraception requirement for female subjects with non-childbearing potential (WNCBP) and WNCBP subjects' male partners must use a condom from study vaccination/Day 1 until study completion.

6. Males participating in this study who are involved in heterosexual sexual activity with a female partner of childbearing potential must agree to use highly effective, medically accepted double-barrier contraception (as described above) and refrain from donating sperm from at least 28 days prior to study vaccination until study completion; male participants with WNCBP partners must use a condom only from study vaccination/Day 1 until study completion.

Exclusion Criteria:

1. Tympanic temperature > 37.5°C at screening or prior to vaccination.
2. History or presence of any respiratory infection symptoms within 7 days prior to vaccination.
3. Previous vaccination against Respiratory Syncytial Virus (RSV). Planned administration of RSV vaccination during the study (including an investigational or non-registered vaccine), except for the investigational vaccine in this trial.
4. Received a live attenuated vaccine within 28 days before vaccination or received other vaccines within 14 days before vaccination.
5. Received any immunoglobulins or blood/plasma products within 3 months prior to vaccination.
6. Individuals with the following diseases: 1）Any acute disease or acute attack of chronic diseases or using antipyretic, analgesic or anti-allergic drugs (e.g., acetaminophen, ibuprofen, aspirin, loratadine, cetirizine, etc.) within 24 h prior to enrolment; 2）Allergies to any component of the investigational vaccine; 3）Subject has any clinically significant history of allergic conditions to other vaccines; 4）History of neurological disorders (convulsions, epilepsy, encephalopathy, etc.) or psychiatric disorders (bipolar disorder, schizophrenia, etc.) that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the participant inappropriate for entry into this study; 5）Asplenia, or functional asplenia; 6）Congenital or acquired immunodeficiency or autoimmune disease; 7）Chronic administration (≥14 consecutive days) of glucocorticoid (reference value for dose: ≥20 mg/day prednisone or equivalent) or other immunosuppressive agents within the past 3 months, with the exception of inhaled or topical steroids, or short-term use (<14 consecutive days) of oral corticosteroids; 8）Have severe cardiovascular diseases (cardiopulmonary disease, pulmonary edema), severe hepatic or renal diseases, and diabetes complications that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the participant inappropriate for entry into this study; 9）History of severe thrombocytopenia or other coagulation disorders which may be contraindications for an IM; 10）Severe hypertension uncontrolled by medication with systolic blood pressure ≥160 mmHg and/or diastolic blood pressure ≥100 mmHg; 11）Positive test for hepatitis C virus (HCV), hepatitis B surface antigen (HbsAg), human immunodeficiency virus (HIV) at screening.
7. Clinically significant laboratory abnormalities determined by the investigator at screening.
8. A positive urine drug test or alcohol breath test at screening or Day 1.
9. Recent participated in another clinical trial, with receipt of the investigational drug/vaccine within 30 days prior to screening. Currently participating in or those planning to participate in another clinical trial during the study.
10. Have donated blood or plasma within 2 weeks prior to screening.
11. Other conditions that may impact the subject's safety or influence the assessment of vaccine response, as determined by the investigator.