inclusion criteria: - health subjects aged 18-59 years, who have been completed two-dose regimen of inactive sars-cov-2 vaccine in the past 3-6 months, or received one dose of inactive sars-cov-2 vaccine in the past 1-3 months. - the subject can provide with informed consent and sign informed consent form (icf). - the subjects are able to and willing to comply with the requirements of the clinical trial program and could complete the 6-month follow-up of the study. - axillary temperature ≤ 37.0℃. - individuals who are in good health condition at the time of entry into the trial as determined by medical history, physical examination and clinical judgment of the investigator and meet the requirements of immunization.

inclusion criteria: - health subjects aged 18-59 years, who have been completed two-dose regimen of inactive sars-cov-2 vaccine in the past 3-6 months, or received one dose of inactive sars-cov-2 vaccine in the past 1-3 months. - the subject can provide with informed consent and sign informed consent form (icf). - the subjects are able to and willing to comply with the requirements of the clinical trial program and could complete the 6-month follow-up of the study. - axillary temperature ≤ 37.0℃. - individuals who are in good health condition at the time of entry into the trial as determined by medical history, physical examination and clinical judgment of the investigator and meet the requirements of immunization.

inclusion criteria: - health subjects aged 18-59 years, either completed two dose of inactive sars-cov-2 vaccine who will be enrolled in group a or group b, or completed one dose of inactive sars-cov-2 vaccine who will be enrolled in group c or group d. - the subject can provide with informed consent and sign informed consent form (icf) - the subjects are able to and willing to comply with the requirements of the clinical trial program and could complete the 6-month follow-up of the study. - axillary temperature ≤ 37.0℃ - individuals who are in good health condition at the time of entry into the trial as determined by medical history, physical examination and clinical judgment of the investigator and meet the requirements of immunization

inclusion criteria: - health subjects aged 18-59 years, either completed two dose of inactive sars-cov-2 vaccine who will be enrolled in group a or group b, or completed one dose of inactive sars-cov-2 vaccine who will be enrolled in group c or group d. - the subject can provide with informed consent and sign informed consent form (icf) - the subjects are able to and willing to comply with the requirements of the clinical trial program and could complete the 6-month follow-up of the study. - axillary temperature ≤ 37.0℃ - individuals who are in good health condition at the time of entry into the trial as determined by medical history, physical examination and clinical judgment of the investigator and meet the requirements of immunization