A Phase 1 Study to Evaluate the Safety, Reactogenicity and Immunogenicity of YKYY025 Injection

Inclusion Criteria:

1. Male or female participants aged ≥18 to 49 years of age (Part 1) or

   ≥ 50 years of age (Part 2)，can provide legal proof of identity.

2. Can understand the study procedures, voluntarily sign the informed consent form after informed consent.
3. Able and willing to complete the study procedures during the entire study Follow-up period.
4. Body temperature < 37.0°C (Axillary temperature) on the day of enrollment (before vaccination).
5. For all women of childbearing potential (WOCBP) females must use an effective contraceptive method from at least 2 weeks prior to study vaccination. All male and female subjects of childbearing potential agree to use a reliable method of contraception (Oral contraceptives (excluding emergency contraceptives), injectable or buried contraceptives, slow-release local contraceptives, hormone patches, intrauterine devices (IUDs), sterilization, abstinence, condoms (male), diaphragm, cervical cap) until at least 6 months after study vaccination.

Exclusion Criteria:

1. Previously received RSV vaccine or have participated or are currently participating in RSV vaccine clinical trials and have received the trial vaccine.
2. A history of confirmed RSV infection or respiratory diseases related to RSV infection within 6 months prior to vaccination.
3. Long term (defined as 14 days or more) treatment with immunosuppressants or other immunomodulatory drugs within 6 months prior to vaccination, or planned to receive such treatment within 30 days after vaccination, such as long-term use of systemic corticosteroids (continuous use for 2 weeks or more, dose ≥ 20mg/day prednisone or equivalent prednisone dose, excluding topical medications such as ointments, eye drops, inhalants, or nasal sprays);
4. Received immunoglobulin and/or any blood products （such as blood, plasma） within 4 months of vaccination, or planning to receive during the study.
5. Have used other experimental or unregistered products (drugs or vaccines) within 30 days before vaccination, or are currently participating in other clinical trials, or planning to participate in other clinical trials during the study.
6. Received any live attenuated vaccines/ mRNA vaccine ≤ 28 days prior to vaccination or received any other non live vaccines (recombinant vaccines, subunit vaccines, inactivated vaccines, and other craft vaccines) within 14 days prior to vaccination.
7. Donate blood or lose a large amount of blood (>450 ml) within 30 days prior to vaccination, or plan to donate blood within 30 days after vaccination.
8. Within 3 days prior to vaccination, suffering from acute illness or being in the acute phase of chronic illness, or using antipyretic, analgesic, anti-inflammatory, and/or anti allergic drugs.
9. History of severe hypersensitivity reactions to any vaccine or medication, or known to be allergic to any component of the test vaccine, including rapid onset severe allergic reactions or other serious adverse reactions to mRNA vaccine administration.
10. Previous or current congenital or acquired immunodeficiency disease (such as AIDS), systemic lupus erythematosus or other autoimmune diseases (such as Guillain Barre syndrome, multiple sclerosis, neuromyelitis optica and other inflammatory demyelinating neuropathy history); Other diseases that affect immune function (such as splenomegaly or functional splenomegaly).
11. History of heart diseases such as myocarditis and pericarditis, or having any diseases that increase the risk of myocarditis or pericarditis (including cardiomyopathy, endocardial fibrosis, eosinophilic syndrome, hypersensitivity myocarditis, eosinophilic granulomas with polyangitis, persistent myocardial viral (such as enterovirus or adenovirus) infection, etc.).
12. Previous or current serious cardiovascular and cerebrovascular diseases (such as congenital heart disease (except for cured atrial septal defect, patent foramen ovale, etc.), supraventricular tachycardia, atrial fibrillation, stroke, cerebral hemorrhage, etc.), liver and kidney diseases (such as chronic hepatitis active or suspected active hepatitis, nephrotic syndrome, uremia, etc.), respiratory diseases (such as chronic obstructive pulmonary disease, asthma, etc.), diabetes with serious complications, malignant tumors, etc.
13. Currently or previously suffering from neurological disorders such as epilepsy (if there is a history of seizures), acute disseminated encephalomyelitis, or mental illnesses that affect normal behavior
14. Suffering from thrombocytopenia, coagulation dysfunction, or any condition that can be a contraindication for intramuscular injection
15. Participants aged 18-59: those with hypertension that cannot be controlled by medication or with systolic blood pressure ≥ 140mmHg and/or diastolic blood pressure ≥ 90mmHg during pre enrollment blood pressure measurement; Participants aged ≥ 60: those with hypertension that cannot be controlled by medication or with systolic blood pressure ≥ 150mmHg and/or diastolic blood pressure ≥ 100mmHg measured before enrollment.
16. Planning to become pregnant during the study or Women of Childbearing Potential who have a positive pregnancy test before vaccination or are pregnant or lactating at screening or prior to vaccination.
17. Prior to vaccination, if the laboratory testing indicators and/or electrocardiogram examination specified in the protocol show abnormalities that meet the grading standards of level 2 or above, or if the abnormal results are clinically significant and deemed unsuitable for vaccination with the experimental vaccine by the investigator.
18. Participants have a positive result for hepatitis B surface antigen, hepatitis C antibody , HIV antibody and anti-Treponema pallidum antibody.
19. Any other reason to be excluded in the opinion of the Investigator