Phase II Clinical Trial of Lyophilized Respiratory Syncytial Virus mRNA Vaccine in Adults Aged 50 Years or Above

Inclusion criteria

• Voluntarily sign the Informed Consent Form (ICF) approved by the Ethics Committee and agree to participate in the trial before undergoing any trial procedures.
• Healthy adults ≥50 years of age, participants with underlying diseases that are stably controlled may be accepted.
• Willing to and physically able to communicate with the investigators, understand and comply with protocol required follow-up, simple self-observation and recording using the Diary Card.
• Male participants (and their female partners) and female participants of childbearing potential agree to continue effective contraception through 3 months following vaccination.

Exclusion criteria

• Presence of one or more respiratory symptoms within 7 days prior to vaccination, lasting for at least 24 hours.
• Acute illness or fever within 3 days prior to vaccination, or use of anti-inflammatory, anti-allergy, antibiotic, or antiviral medications due to physical discomfort.
• Laboratory-confirmed history of RSV infection within the past 12 months, or previous vaccination with a respiratory syncytial virus vaccine.
• Clinically significant abnormal vital signs, including but not limited to:

• Resting pulse rate <50 beats per minute or >100 beats per minute, participants with resting pulse between 100 to 110 without significant symptoms, such as palpitations, chest tightness, dizziness and fatigue, will be accepted.
• Systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg for participants aged 50-59, or systolic blood pressure ≥160 mmHg or diastolic blood pressure ≥100 mmHg for participants aged ≥60
• Body mass index (BMI) ≥30 kg/m²
• Female participants known to be pregnant or breastfeeding, or positive pregnancy test for women of childbearing potential.
• History of allergy to any ingredients of the investigational products, or severe allergic reactions to other vaccines, foods, or medications.
• Use or planned use of any vaccine other than the investigational products through 30 days prior to and 30 days after vaccination in this trial.
• Current participation in another clinical trial within 30 days prior to vaccination or planned participation before the end of this trial.
• Clinician-diagnosed coagulation abnormalities.
• Known medical history or diagnosis confirming the subject has a condition affecting immune system function.
• History of myocarditis, pericarditis, or idiopathic cardiomyopathy, or presence of any condition that may increase the risk of myocarditis or pericarditis.
• Severe or uncontrolled respiratory, cardiovascular, neurological, hematological, lymphatic, hepatic, renal, metabolic, or skeletal diseases; known severe congenital malformations; developmental disorders; or clinically diagnosed severe chronic conditions that may affect trial outcomes.
• Current infectious period of any communicable disease, acute infection, or acute phase of chronic infection, or ongoing anti-tuberculosis treatment; or prior positive test for hepatitis B surface antigen, hepatitis C virus antibody, or Treponema pallidum antibody.
• Past or current diagnosis of neurological or psychiatric disorders, or family history of neurological/psychiatric disorders; or other neurological conditions deemed unsuitable for trial participation by the investigator.
• Long-term use of immunosuppressants or immunomodulators within 6 months prior to vaccination, excluding topical medications. Topical medications should not exceed recommended doses or induce systemic exposure.
• Treatment with immunoglobulins and/or blood products or blood donation through 3 months prior to and 3 months after vaccination in this trial.
• Suspected or known alcohol dependence or drug abuse, which may affect safety assessment or trial compliance.
• Planned long-term or permanent relocation away from the trial site area before trial completion.
• Investigators, sponsors, and contract research organization (CRO) staff directly involved in the trial.
• Other circumstances deemed unsuitable for trial participation by the investigator.