inclusion criteria: * male and female healthy volunteers. * aged 21 years and older on day 1 * judged by the investigator to be healthy based on medical history, physical examination, and vital signs evaluation at screening visit. * able to provide informed consent form. * able and willing to comply with all study procedures over follow-up period of approximately 6 months. * have documented completion of a 2-dose primary vaccination series and 1 booster vaccination with bnt162b2 or a 3-dose primary vaccination series, with or without 1 booster dose, with an approved inactivated virus vaccine (bbibp-corv or coronavac). the last dose in all cases should have been administered at least 6 months prior to enrolment in this study. * for female subjects with childbearing potential: must agree to avoid pregnancy from 21 days prior to day 1 until at least 90 days after the last study vaccination. women physically capable of pregnancy (not sterilized and still menstruating or within 1 year of the last menses if menopausal) in sexual relationships with men must use an acceptable method of avoiding pregnancy during this period.

inclusion criteria: * male and female healthy volunteers. * aged 21 years and older on day 1 * judged by the investigator to be healthy based on medical history, physical examination, and vital signs evaluation at screening visit. * able to provide informed consent form. * able and willing to comply with all study procedures over follow-up period of approximately 6 months. * have documented completion of a 2-dose primary vaccination series and 1 booster vaccination with bnt162b2 or a 3-dose primary vaccination series, with or without 1 booster dose, with an approved inactivated virus vaccine (bbibp-corv or coronavac). the last dose in all cases should have been administered at least 6 months prior to enrolment in this study. * for female subjects with childbearing potential: must agree to avoid pregnancy from 21 days prior to day 1 until at least 90 days after the last study vaccination. women physically capable of pregnancy (not sterilized and still menstruating or within 1 year of the last menses if menopausal) in sexual relationships with men must use an acceptable method of avoiding pregnancy during this period.

inclusion criteria: male and female healthy volunteers. aged 21 years and older on day 1 judged by the investigator to be healthy based on medical history, physical examination, and vital signs evaluation at screening visit. able to provide informed consent form. able and willing to comply with all study procedures over follow-up period of approximately 6 months. have documented completion of a 2-dose primary vaccination series and 1 booster vaccination with bnt162b2 or a 3-dose primary vaccination series, with or without 1 booster dose, with an approved inactivated virus vaccine (bbibp-corv or coronavac). the last dose in all cases should have been administered at least 6 months prior to enrolment in this study. for female subjects with childbearing potential: must agree to avoid pregnancy from 21 days prior to day 1 until at least 90 days after the last study vaccination. women physically capable of pregnancy (not sterilized and still menstruating or within 1 year of the last menses if menopausal) in sexual relationships with men must use an acceptable method of avoiding pregnancy during this period.

inclusion criteria: male and female healthy volunteers. aged 21 years and older on day 1 judged by the investigator to be healthy based on medical history, physical examination, and vital signs evaluation at screening visit. able to provide informed consent form. able and willing to comply with all study procedures over follow-up period of approximately 6 months. have documented completion of a 2-dose primary vaccination series and 1 booster vaccination with bnt162b2 or a 3-dose primary vaccination series, with or without 1 booster dose, with an approved inactivated virus vaccine (bbibp-corv or coronavac). the last dose in all cases should have been administered at least 6 months prior to enrolment in this study. for female subjects with childbearing potential: must agree to avoid pregnancy from 21 days prior to day 1 until at least 90 days after the last study vaccination. women physically capable of pregnancy (not sterilized and still menstruating or within 1 year of the last menses if menopausal) in sexual relationships with men must use an acceptable method of avoiding pregnancy during this period.