inclusion criteria: * healthy adults aged 18 years and above, who have received two or three prior doses of mrna covid-19 vaccine (comirnaty®, made by biontech/pfizer) or coronavac®, with the last dose given at least 90 days prior to the day of booster vaccination in the present study; * evidence of a deep throat saliva (dts) pcr negative for sars-cov-2 within 24 hours before enrollment; * female participants who have a negative pregnancy test on the day of the first dose of the booster vaccine in the present study, and are not currently breastfeeding; * the participants are able to understand and sign the informed consent voluntarily before the first dose of booster vaccine in the present study; * the participants are willing and able to comply with all schedule visits, sample collection, vaccine plan, and other study procedures; * the participants must be willing to provide verifiable identification (in accordance with the local regulations), has means to be contacted and to contact the investigator during the study.

inclusion criteria: * healthy adults aged 18 years and above, who have received two or three prior doses of mrna covid-19 vaccine (comirnaty®, made by biontech/pfizer) or coronavac®, with the last dose given at least 90 days prior to the day of booster vaccination in the present study; * evidence of a deep throat saliva (dts) pcr negative for sars-cov-2 within 24 hours before enrollment; * female participants who have a negative pregnancy test on the day of the first dose of the booster vaccine in the present study, and are not currently breastfeeding; * the participants are able to understand and sign the informed consent voluntarily before the first dose of booster vaccine in the present study; * the participants are willing and able to comply with all schedule visits, sample collection, vaccine plan, and other study procedures; * the participants must be willing to provide verifiable identification (in accordance with the local regulations), has means to be contacted and to contact the investigator during the study.

inclusion criteria: healthy adults aged 18 years and above, who have received two or three prior doses of mrna covid-19 vaccine (comirnaty®, made by biontech/pfizer) or coronavac®, with the last dose given at least 90 days prior to the day of booster vaccination in the present study; evidence of a deep throat saliva (dts) pcr negative for sars-cov-2 within 24 hours before enrollment; female participants who have a negative pregnancy test on the day of the first dose of the booster vaccine in the present study, and are not currently breastfeeding; the participants are able to understand and sign the informed consent voluntarily before the first dose of booster vaccine in the present study; the participants are willing and able to comply with all schedule visits, sample collection, vaccine plan, and other study procedures; the participants must be willing to provide verifiable identification (in accordance with the local regulations), has means to be contacted and to contact the investigator during the study.

inclusion criteria: healthy adults aged 18 years and above, who have received two or three prior doses of mrna covid-19 vaccine (comirnaty®, made by biontech/pfizer) or coronavac®, with the last dose given at least 90 days prior to the day of booster vaccination in the present study; evidence of a deep throat saliva (dts) pcr negative for sars-cov-2 within 24 hours before enrollment; female participants who have a negative pregnancy test on the day of the first dose of the booster vaccine in the present study, and are not currently breastfeeding; the participants are able to understand and sign the informed consent voluntarily before the first dose of booster vaccine in the present study; the participants are willing and able to comply with all schedule visits, sample collection, vaccine plan, and other study procedures; the participants must be willing to provide verifiable identification (in accordance with the local regulations), has means to be contacted and to contact the investigator during the study.