This is a phase I, randomized, observer-blinded, placebo-controlled and dose-escalation clinical trial to evaluate the safety and preliminary immunogenicity of SARS-CoV-2 mRNA Vaccine (SYS6006) in Chinese healthy adults aged 60 years and over.

Inclusion Criteria:

1. Age 60 years or older;
2. Axillary body temperature is less than 37.3 degree centigrade on the day of enrollment;
3. Based on the medical history and relevant physical examination and laboratory examination results (normal or abnormal but no clinical significance), the investigator clinically determined that the patient was in good health;
4. Has independent judgment, and participate voluntarily and sign an informed consent form.

Exclusion Criteria:

1. Has a history of SARS-CoV or SARS-CoV-2 infection, or close contact with SARS-CoV-2 infected persons (nucleic acid test positive) or living abroad within 30 days before screening;
2. Positive SARS-CoV-2 antibody test;
3. Previous history of allergy to acetaminophen or vaccination(eg, acute allergic reaction, urticaria, dyspnea, angioneurotic edema, or abdominal pain);
4. Has a history of SARS-CoV-2 vaccination, or have received other inactivated or recombinant vaccines within 7 days, or received live attenuated vaccines within 14 days,before the first dose;
5. Has a medical history or family history of epilepsy, convulsions, neurological diseases and psychiatric diseases;
6. Is contraindicative for intramuscular injection, such as having been diagnosed with thrombocytopenia, any coagulation disorder, or receiving anticoagulant therapy;
7. Has known or suspected severe diseases by the judgement of investigators, including but not limited to: respiratory diseases (lung failure, etc.), liver and kidney diseases, cardiovascular diseases (heart failure, severe hypertension, etc.), malignant tumors, infections or allergic skin diseases, HIV infection, or during the acute infection or chronic disease activity period (within 3 days before vaccination).
8. Has chronic diseases which at the discretion of investigator are inappropriate for participation(such as Down 's syndrome, sickle cell anemia or neurological disorders, Guillain-Barre syndrome, etc., not including stable diabetes mellitus/hypertension);
9. Known immunological impairment or dysfunction diagnosed by the hospital before enrollment, or functional asplenia or splenectomy due to any condition;
10. For women of childbearing potential: positive pregnancy test, being in pregnancy or breastfeeding, or have a pregnancy plan within one year;male: their spouse of childbearing age has plans to become pregnant within 1 year;
11. Is participating in other clinical trials or plan to participate in other clinical trials during the study period;
12. Has received immune enhancement or immunosuppressive therapy within 3 months before the first dose of vaccine (continuous oral or instillation for more than 14 days); or received whole blood, plasma and immunoglobulin therapy within one month;
13. Is unlikely to adhere to the study procedures or keep appointments at the discretion of the investigators;or plan to permanently relocate before the end of study; or plan to leave the resident location for a long time at the scheduled visit; or has any other conditions that are inappropriate for participation for this study.