To evaluate the safety of Sinovac RSV mRNA among participants aged ≥18 years

This is a randomized, double-blind, placebo-controlled clinical trial, enrolling a total of 60 participants aged ≥18 years. Eligible participants will be enrolled sequentially by age (18~49 → ≥50 years old) and dosage (low → High), and then be randomized to the study group and the control group in a ratio of 2:1. Sentinel participant will be set. All participants were required to complete immediate reaction observation within 30 minutes after each vaccine dose, and adverse events until 30 days after vaccination, as well as serious adverse events and adverse event of special interest throughout the whole study period.

Inclusion Criteria:

1. Participants can understand and voluntarily sign the informed consent form;
2. Able to provide legal identity documents;
3. Healthy individuals aged 18 years and above;
4. Participants of childbearing potential and their sexual partners voluntarily adopt effective contraceptive measures (such as correct use of condoms, or female participants using combined oral contraceptives, intrauterine devices) from the time the participant signs the informed consent form until 6 months after the administration of the investigational vaccine, and have no plans to donate sperm or ovums; Female participants of childbearing potential must also have adopted effective contraceptive measures within 2 weeks before enrollment.

Exclusion Criteria:

Participants who meet any of the following criteria will be excluded from this trial:

1. Those with fever on the day of vaccination with the trial vaccine, with an axillary temperature > 37.0℃ before vaccination;
2. Those with a history of previous RSV vaccination, or a history of RSV infection within 6 months before enrollment.
3. Known to be allergic to vaccines or vaccine components;
4. Before vaccination, the electrocardiogram examination and clinical laboratory test indicators specified in the protocol are abnormal and clinically significant;
5. Female participants who are breastfeeding or pregnant;
6. Having autoimmune diseases or immunodeficiency diseases (including but not limited to systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid diseases, asplenia, functional asplenia, splenectomy, HIV infection);
7. History of poorly controlled chronic diseases or severe diseases, including but not limited to cardiovascular diseases (such as hypertension uncontrolled by drugs, i.e., systolic blood pressure ≥ 140 mmHg and/or diastolic blood pressure ≥ 90 mmHg for participants aged 18-59 years old, systolic blood pressure ≥ 150 mmHg and/or diastolic blood pressure ≥ 100 mmHg for participants aged 60 years old and above, as well as congenital heart disease, atrial fibrillation, myocarditis, pericarditis), hematological system diseases, liver and kidney diseases, respiratory system diseases, malignant tumors, history of transplantation of major functional organs, or any other diseases or physiological conditions that the researcher believes can interfere with the trial results;
8. Current encephalopathy (such as damage to cerebral nerve tissue caused by congenital hypoplasia of the brain, brain trauma, brain tumors, cerebral hemorrhage, brain infection, chemical drug poisoning, etc.);
9. Severe neurological diseases, such as a history of demyelinating diseases including but not limited to Guillain-Barré syndrome, multiple sclerosis, neuromyelitis optica, acute disseminated encephalomyelitis, etc., or a history of convulsions, epilepsy, or a family history of mental illness;
10. Having a doctor-diagnosed coagulation dysfunction (such as coagulation factor deficiency, coagulative diseases, abnormal platelets);
11. Having received systemic immunosuppressive agents or other immunomodulatory treatments (prednisone [≥ 20 mg/day] or its equivalent) for ≥ 14 days within the past 3 months, cytotoxic therapy, or planning to receive such treatments during the study period;
12. Having received immunoglobulin or other blood products within 3 months before receiving the trial vaccine, or planning to receive such treatments during the study period;
13. Currently participating in other clinical trials, or planning to receive such drugs or vaccines during the study period;
14. Having received live attenuated vaccines within the past 14 days or other vaccines with processes such as subunit or inactivated vaccines within 7 days;
15. Having various acute diseases or acute exacerbation of chronic diseases within the past 7 days, or known or suspected active infection;
16. Axillary temperature > 37.0℃ within the past 3 days, or use of anti-allergic drugs, non-steroidal antipyretic analgesics, etc.;
17. Those with skin damage, inflammation, ulceration, rash, scarring, or other conditions at the target vaccination site that may interfere with vaccination or observation of local reactions;
18. According to the researcher's judgment, participants have any other factors that make them unsuitable for participating in the clinical trial.