inclusion criteria: 1. aged 18 to 65 years (included). 2. able and willing to comply with all study requirements. 3. willing to allow the investigators to discuss the volunteers' medical history with his/her general practitioner/personal doctors and access all medical records which are relevant to study procedures. 4. healthy adults, or adults with stable medical condition who have a pre-existing medical condition that does not meet any exclusion criteria. a stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 3 months before enrollment. 5. have completed two doses vaccination by inactivated covid-19 vaccine prior to the study vaccination. the duration since the last primary vaccination is 90\~365 days (included) for both phase ii and phase iii stages. 6. for females of childbearing potential only, willing to practice continuous effective contraception till 90 days after the study vaccination, and have negative pregnancy tests before study vaccination. * nonchildbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥ 12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of investigator to confirm postmenopausal status. * the effective contraceptive methods include sexual abstinence or adequate contraceptive measures such as intrauterine or implanted contraceptive device, oral contraceptives, injected or implanted contraceptives, sustained-release topical contraceptives, condoms (male), diaphragm, and cervical cap, etc. 7. males participating in this study who are involved in heterosexual activity must agree to practice adequate contraception (as described above) and refrain from donating sperm till 90 days after receiving the study vaccination. 8. agreement to refrain from blood donation during the study. 9. provide written informed consent form (icf) prior to study enrollment.

inclusion criteria: 1. aged 18 to 65 years (included). 2. able and willing to comply with all study requirements. 3. willing to allow the investigators to discuss the volunteers' medical history with his/her general practitioner/personal doctors and access all medical records which are relevant to study procedures. 4. healthy adults, or adults with stable medical condition who have a pre-existing medical condition that does not meet any exclusion criteria. a stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 3 months before enrollment. 5. have completed two doses vaccination by inactivated covid-19 vaccine prior to the study vaccination. the duration since the last primary vaccination is 90\~365 days (included) for both phase ii and phase iii stages. 6. for females of childbearing potential only, willing to practice continuous effective contraception till 90 days after the study vaccination, and have negative pregnancy tests before study vaccination. * nonchildbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥ 12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of investigator to confirm postmenopausal status. * the effective contraceptive methods include sexual abstinence or adequate contraceptive measures such as intrauterine or implanted contraceptive device, oral contraceptives, injected or implanted contraceptives, sustained-release topical contraceptives, condoms (male), diaphragm, and cervical cap, etc. 7. males participating in this study who are involved in heterosexual activity must agree to practice adequate contraception (as described above) and refrain from donating sperm till 90 days after receiving the study vaccination. 8. agreement to refrain from blood donation during the study. 9. provide written informed consent form (icf) prior to study enrollment.

inclusion criteria: aged 18 to 65 years (included). able and willing to comply with all study requirements. willing to allow the investigators to discuss the volunteers' medical history with his/her general practitioner/personal doctors and access all medical records which are relevant to study procedures. healthy adults, or adults with stable medical condition who have a pre-existing medical condition that does not meet any exclusion criteria. a stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 3 months before enrollment. have completed two doses vaccination by inactivated covid-19 vaccine prior to the study vaccination. the duration since the last primary vaccination is 90~365 days (included) for both phase ii and phase iii stages. for females of childbearing potential only, willing to practice continuous effective contraception till 90 days after the study vaccination, and have negative pregnancy tests before study vaccination. nonchildbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥ 12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of investigator to confirm postmenopausal status. the effective contraceptive methods include sexual abstinence or adequate contraceptive measures such as intrauterine or implanted contraceptive device, oral contraceptives, injected or implanted contraceptives, sustained-release topical contraceptives, condoms (male), diaphragm, and cervical cap, etc. males participating in this study who are involved in heterosexual activity must agree to practice adequate contraception (as described above) and refrain from donating sperm till 90 days after receiving the study vaccination. agreement to refrain from blood donation during the study. provide written informed consent form (icf) prior to study enrollment.

inclusion criteria: aged 18 to 65 years (included). able and willing to comply with all study requirements. willing to allow the investigators to discuss the volunteers' medical history with his/her general practitioner/personal doctors and access all medical records which are relevant to study procedures. healthy adults, or adults with stable medical condition who have a pre-existing medical condition that does not meet any exclusion criteria. a stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 3 months before enrollment. have completed two doses vaccination by inactivated covid-19 vaccine prior to the study vaccination. the duration since the last primary vaccination is 90~365 days (included) for both phase ii and phase iii stages. for females of childbearing potential only, willing to practice continuous effective contraception till 90 days after the study vaccination, and have negative pregnancy tests before study vaccination. nonchildbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥ 12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of investigator to confirm postmenopausal status. the effective contraceptive methods include sexual abstinence or adequate contraceptive measures such as intrauterine or implanted contraceptive device, oral contraceptives, injected or implanted contraceptives, sustained-release topical contraceptives, condoms (male), diaphragm, and cervical cap, etc. males participating in this study who are involved in heterosexual activity must agree to practice adequate contraception (as described above) and refrain from donating sperm till 90 days after receiving the study vaccination. agreement to refrain from blood donation during the study. provide written informed consent form (icf) prior to study enrollment.