1. history of severe acute respiratory syndrome (sars), middle east respiratory syndrome (mers), or other coronavirus infections. 2. individuals using prescription medications for prophylaxis or treatment of sars-cov-2 (including vaccination of licensed covid-19 vaccines). 3. history of a previous laboratory-confirmed diagnosis of sars-cov-2 infection or covid-19. 4. history of allergy to any component of the study vaccine or history of severe allergic reaction to vaccine or drug (including but not limited to anaphylaxis, allergic laryngeal edema, anaphylactic purpura, thrombocytopenic purpura, or localized allergic necrosis (arthus reaction)). 5. positive nucleic acid for sars-cov-2 in nasopharyngeal/oropharyngeal swab specimens. 6. positive hiv test results. 7. a history or family history of convulsions, epilepsy, encephalopathy and psychosis. 8. malignant tumors in the active phase, malignant tumors not receiving adequate treatment, malignant tumors at potential risk of recurrence during the study period. 9. asplenia or functional asplenia, complete or partial splenectomy from any cause. 10. prolonged (defined as more than 14 days) use of immunosuppressive or other immunomodulatory drugs (e.g., corticosteroids, ≥20 mg/d prednisone or equivalent; however, inhaled and topical steroids are permitted) within 6 months prior to the 1st dose of investigational vaccine. 11. any other licensed vaccines given within 28 days prior to the study vaccination, or planned administration of vaccine(s) within 28 days after the 2nd dose in the blinded crossover vaccination. 12. have received immunoglobulin or other blood products within 3 months prior to enrollment or plan to receive them during the study period. 13. blood donation or blood loss ≥ 450 ml within 1 month prior to enrollment or planned to donate blood during the study period. 14. participants who have received any other investigational product within 1 month prior to enrollment or intent to participate in another clinical study at any time during the conduct of this study. 15. women who are pregnant or breastfeeding. 16. participants deemed unsuitable for participation in this study based on the investigator's assessment.

1. history of severe acute respiratory syndrome (sars), middle east respiratory syndrome (mers), or other coronavirus infections. 2. individuals using prescription medications for prophylaxis or treatment of sars-cov-2 (including vaccination of licensed covid-19 vaccines). 3. history of a previous laboratory-confirmed diagnosis of sars-cov-2 infection or covid-19. 4. history of allergy to any component of the study vaccine or history of severe allergic reaction to vaccine or drug (including but not limited to anaphylaxis, allergic laryngeal edema, anaphylactic purpura, thrombocytopenic purpura, or localized allergic necrosis (arthus reaction)). 5. positive nucleic acid for sars-cov-2 in nasopharyngeal/oropharyngeal swab specimens. 6. positive hiv test results. 7. a history or family history of convulsions, epilepsy, encephalopathy and psychosis. 8. malignant tumors in the active phase, malignant tumors not receiving adequate treatment, malignant tumors at potential risk of recurrence during the study period. 9. asplenia or functional asplenia, complete or partial splenectomy from any cause. 10. prolonged (defined as more than 14 days) use of immunosuppressive or other immunomodulatory drugs (e.g., corticosteroids, ≥20 mg/d prednisone or equivalent; however, inhaled and topical steroids are permitted) within 6 months prior to the 1st dose of investigational vaccine. 11. any other licensed vaccines given within 28 days prior to the study vaccination, or planned administration of vaccine(s) within 28 days after the 2nd dose in the blinded crossover vaccination. 12. have received immunoglobulin or other blood products within 3 months prior to enrollment or plan to receive them during the study period. 13. blood donation or blood loss ≥ 450 ml within 1 month prior to enrollment or planned to donate blood during the study period. 14. participants who have received any other investigational product within 1 month prior to enrollment or intent to participate in another clinical study at any time during the conduct of this study. 15. women who are pregnant or breastfeeding. 16. participants deemed unsuitable for participation in this study based on the investigator's assessment.

history of severe acute respiratory syndrome (sars), middle east respiratory syndrome (mers), or other coronavirus infections. individuals using prescription medications for prophylaxis or treatment of sars-cov-2 (including vaccination of licensed covid-19 vaccines). history of a previous laboratory-confirmed diagnosis of sars-cov-2 infection or covid-19. history of allergy to any component of the study vaccine or history of severe allergic reaction to vaccine or drug (including but not limited to anaphylaxis, allergic laryngeal edema, anaphylactic purpura, thrombocytopenic purpura, or localized allergic necrosis (arthus reaction)). positive nucleic acid for sars-cov-2 in nasopharyngeal/oropharyngeal swab specimens. positive hiv test results. a history or family history of convulsions, epilepsy, encephalopathy and psychosis. malignant tumors in the active phase, malignant tumors not receiving adequate treatment, malignant tumors at potential risk of recurrence during the study period. asplenia or functional asplenia, complete or partial splenectomy from any cause. prolonged (defined as more than 14 days) use of immunosuppressive or other immunomodulatory drugs (e.g., corticosteroids, ≥20 mg/d prednisone or equivalent; however, inhaled and topical steroids are permitted) within 6 months prior to the 1st dose of investigational vaccine. any other licensed vaccines given within 28 days prior to the study vaccination, or planned administration of vaccine(s) within 28 days after the 2nd dose in the blinded crossover vaccination. have received immunoglobulin or other blood products within 3 months prior to enrollment or plan to receive them during the study period. blood donation or blood loss ≥ 450 ml within 1 month prior to enrollment or planned to donate blood during the study period. participants who have received any other investigational product within 1 month prior to enrollment or intent to participate in another clinical study at any time during the conduct of this study. women who are pregnant or breastfeeding. participants deemed unsuitable for participation in this study based on the investigator's assessment.

history of severe acute respiratory syndrome (sars), middle east respiratory syndrome (mers), or other coronavirus infections. individuals using prescription medications for prophylaxis or treatment of sars-cov-2 (including vaccination of licensed covid-19 vaccines). history of a previous laboratory-confirmed diagnosis of sars-cov-2 infection or covid-19. history of allergy to any component of the study vaccine or history of severe allergic reaction to vaccine or drug (including but not limited to anaphylaxis, allergic laryngeal edema, anaphylactic purpura, thrombocytopenic purpura, or localized allergic necrosis (arthus reaction)). positive nucleic acid for sars-cov-2 in nasopharyngeal/oropharyngeal swab specimens. positive hiv test results. a history or family history of convulsions, epilepsy, encephalopathy and psychosis. malignant tumors in the active phase, malignant tumors not receiving adequate treatment, malignant tumors at potential risk of recurrence during the study period. asplenia or functional asplenia, complete or partial splenectomy from any cause. prolonged (defined as more than 14 days) use of immunosuppressive or other immunomodulatory drugs (e.g., corticosteroids, ≥20 mg/d prednisone or equivalent; however, inhaled and topical steroids are permitted) within 6 months prior to the 1st dose of investigational vaccine. any other licensed vaccines given within 28 days prior to the study vaccination, or planned administration of vaccine(s) within 28 days after the 2nd dose in the blinded crossover vaccination. have received immunoglobulin or other blood products within 3 months prior to enrollment or plan to receive them during the study period. blood donation or blood loss ≥ 450 ml within 1 month prior to enrollment or planned to donate blood during the study period. participants who have received any other investigational product within 1 month prior to enrollment or intent to participate in another clinical study at any time during the conduct of this study. women who are pregnant or breastfeeding. participants deemed unsuitable for participation in this study based on the investigator's assessment.