The purpose of this study is to assess the safety and immunogenicity of two dose levels of the single dose Recombinant RSV vaccine(CHO cells), when administered intramuscularly (IM) in healthy adults aged 18 years and older.

A total of 522 subjects in the phase 1/2 study will be enrolled. The study will be conducted in 2 parts (phase 1 for dose escalation and phase 2 for expansion), with first evaluation of safety of 2 two dose levels of Recombinant RSV vaccine(CHO cells) in healthy participants aged 18-59 and ≥60 in phase 1 before preceding with vaccination of the participants aged 50-59 and ≥60 in phase 2. To ensure the safety of the study participants, phase 1 will follow a staggered enrolment with 3 steps. All subjects in each age group in phase 1 will be randomly receive the investigational vaccine(half or full dose) and the placebo in a 2:1 ratio, while all subjects in each age group in phase 2 will be randomly receive the half dose vaccine, the full dose vaccine and the placebo in a 1:1:1 ratio.

Inclusion Criteria:

1. A male or female, in the opinion of the investigator, aged 18 and older for phase 1 and aged 50 and older for phase 2 at the time of the enrollment;
2. Be able to understand the trial procedures, risks and benefits and voluntarily agree to participate in the study and signed an informed consent;
3. Be able to participate in all scheduled visits and comply with the protocol requirements;
4. Women of childbearing potential are willing to use effective contraception (e.g. oral contraceptives, injectable progestogen, implants of levonorgestrel, percutaneous contraceptive patches, intrauterine device (IUD), female and male sterilization, abstinence, condoms, or diaphragms), and the rhythm method, withdrawal and emergency contraception pills are not acceptable;
5. Subjects with stable conditions considered by the investigator.

Exclusion Criteria:

1. Axillary temperature>37.0℃;
2. History of RSV infection within 6 months before enrollment;
3. New onset of respiratory tract infection symptoms like cough, sputum, shortness of breath, wheezing, fever, runny nose or nasal congestion within 7 days before enrollment;
4. Acute diseases or acute exacerbation of chronic disease within 3 days before vaccination;
5. A known allergy to any components of the study vaccine, or history of severe allergy (e.g. anaphylactic shock, allergic laryngeal edema, anaphylactoid purpura, thrombocytopenic purpura, Arthus reaction, severe urticaria) or serious adverse reactions to any previous vaccination or drug use;
6. Pregnant (urine pregnancy test was positive) or lactating female, or planned pregnancy within 12 months after vaccination;
7. Any confirmed or suspected immunosuppressive or immunodeficient condition due to diseases or immunosuppressive therapy, based on medical history and physical examination;
8. Serious or unstable chronic illness, including but not limit to cardiovascular diseases (such as uncontrolled hypertension, coronary heart disease, myocarditis, pericarditis), metabolic diseases (such as poorly controlled diabetes), hematological diseases (such as severe anemia, hemophilia), liver and kidney diseases, digestive diseases, respiratory diseases (such as chronic obstructive pulmonary disease, active tuberculosis, other severe respiratory diseases ), malignant tumor, major functional organ transplantation history;
9. Significant underlying illness that in the opinion of the investigator would be expected to prevent completion of the study;
10. History of thrombocytopenia or other coagulation disorders;
11. History of convulsions, epilepsy, congenital brain dysplasia, mental illness or family history, or history of brain nerve tissue damage due to other severe neurological disorders(e.g. brain tumor, cerebral hemorrhage, cerebral infarction, brain infection disease, chemical drug poisoning);
12. History of cognitive dysfunction, or any moderate or severe cognitive impairment;
13. Asplenia or functional asplenia, or autoimmune thyroid diseases, such as Hashimoto thyroiditis, toxic diffuse goiter;
14. Receipt of live vaccine within 28 days, or any other vaccine within 14 days prior to vaccination;
15. Previous vaccination with an RSV vaccine;
16. Administration of long-acting immune-modifying drugs(e.g. Infliximab) or planned administration at any time during the study period;
17. Administration of immunoglobulins and/or any blood products during the period starting 3 months before vaccination or planned administration during the study period;
18. Chronic administration of immunosuppressants or other immune-modifying drugs (such as long-term use of systemic glucocorticoid ≥14 days, ≥20mg/day prednisone or equivalent dose) during the period starting 3 months before vaccination or planned administration during the study period, but topical steroids(e.g. ointment, eye drops, inhalants, nasal sprays) that do not exceed the dosage recommended in the instructions or have any systemic signs are acceptable;
19. History of alcohol or drug abuse;
20. Concurrently participating in another clinical study, at any time during the study period, in which the subject has been or will be exposed to an investigational or a non-investigational vaccine/product;
21. Planned move to a location that will prohibit participating in the trial until study end;
22. At screening: Any laboratory test results(hematology, clinical chemistry, coagulation function, or urinalysis) abnormal and clinically significant in the judgment of the investigator (Only for phase 1);
23. At screening: Any electrocardiogram abnormal and clinically significant in the judgment of the investigator (Only for phase 1);
24. Any condition that, in the opinion the investigator, may affect the safety of the subject or the evaluation of the study results.