inclusion criteria: healthy adults aged 18 years and above, who have received 2 prior doses of inactivated covid-19 vaccine (coronavac®) or mrna covid-19 vaccine (comirnaty), with the second dose given 3-8 months (for coronavac® recipients) and 6-8 months (for comirnaty recipients) prior to the day of booster vaccination in the present study; antigen rapid testing negative at the day of enrollment on site or with a pcr negative result within 72 hours before enrollment; the participants are able to understand and sign the informed consent voluntarily; female participants who have a negative pregnancy test on the day of vaccination, and not currently breastfeeding; the participants are planning to be living in turkey for the following 1 year after attending this study; 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 2 prior doses of inactivated covid-19 vaccine (coronavac®) or mrna covid-19 vaccine (comirnaty), with the second dose given 3-8 months (for coronavac® recipients) and 6-8 months (for comirnaty recipients) prior to the day of booster vaccination in the present study; antigen rapid testing negative at the day of enrollment on site or with a pcr negative result within 72 hours before enrollment; the participants are able to understand and sign the informed consent voluntarily; female participants who have a negative pregnancy test on the day of vaccination, and not currently breastfeeding; the participants are planning to be living in turkey for the following 1 year after attending this study; 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.