This is a phase 2, randomized, observer-blinded, placebo-controlled clinical study to evaluate the immunogenicity and safety of SARS-CoV-2 mRNA Vaccine (SYS6006）in healthy participants aged 18 or more.

Inclusion Criteria:

1. Age at the time of the first dose of vaccine: 18 and above;
2. Axillary body temperature is less than 37.3 degree centigrade on the day of enrollment;
3. Based on the medical history, physical examination and laboratory examination results, the investigator clinically determined the qualified person.
4. Has independent judgment, and participate voluntarily and sign an informed consent form.

Exclusion Criteria:

• Those who meet any of the following criteria must be excluded from this study:

  1. Has a history of SARS or SARS-CoV-2 infection, or close contact with SARS-CoV-2 infected person( nucleic acid test positive), or living abroad within 30 days before screening;
  2. Positive for SARS-CoV-2 antibody test;
  3. History of allergy to paracetamol or vaccination (such as acute allergic reaction, urticaria, dyspnea, angioedema, or abdominal pain);
  4. Has a history of vaccination with SARS-CoV-2 vaccine, or vaccination with other inactivated or recombinant vaccine within 7 days or received live attenuated vaccine within 14 days before the first dose;
  5. Has a medical or family history of epilepsy or convulsion, neurological disorders, mental disorders, etc.;
  6. Is contraindications for intramuscular injection, such as diagnosed thrombocytopenia, any coagulation disorder or receiving anticoagulant treatment, etc.;
  7. Has known or suspected serious disorders judged by the Investigator, including but not limited to respiratory disorders (pulmonary failure, etc.), liver and kidney disorders, cardiovascular disorders (cardiac failure, severe hypertension, etc.), malignant tumors, infections or allergic skin disorders, HIV infection, or in the acute infection or chronic disease activity period (within 3 days before vaccination);
  8. Has chronic diseases (such as Down syndrome, sickle cell anemia, neurological disorders and Guillain-Barre syndrome, excluding chronic diseases such as controlled stable diabetes mellitus/hypertension/hyperlipidemia) that are not suitable for this study at the discretion of the Investigator;
  9. Has known immunological impairment or immunodeficiency diagnosed by hospital before enrollment, or functional asplenia or splenectomy caused by any condition;
  10. For women of childbearing potential: positive pregnancy test before the enrollment, being in pregnant, breastfeeding, or have a pregnancy plan within 1 year; men: whose spouses is of childbearing potential and has a pregnancy plan within 1 year;
  11. Is participating or plan to participate in other clinical studies during the study;
  12. Has received immune enhancement or immunosuppressive therapy within 3 months (continuous oral or instillation for more than 14 days); or received treatment with whole blood, plasma and immunoglobulin within 1 month;
  13. Is unlikely to adhere to the study procedures or keep appointments at the discretion of the investigators;or has other conditions that are not suitable for participating in this clinical trial.