inclusion criteria: 1. male or female participants aged ≥ 18 years at randomization. 2. healthy adults or adults with pre-existing medical conditions who are in stable condition. a stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease 3 months before enrollment and expected to remain stable for the duration of the study. 3. documented to have received two homologous doses of azd1222, mrna-1273, or mvc-cov1901 vaccine, with the latest dose between 150 and 365 days prior to randomization, with an interval between the two homologous doses of ≥ 4 weeks to ≤ 12 weeks, and did not receive any other investigational or approved covid-19 vaccines 4. female participants must: 1. be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal; 2. or, if of childbearing potential, be abstinent or agree to use medically effective contraception from 14 days before screening to 30 days following the injection of study intervention. acceptable forms include: i.implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii.established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c.have a negative pregnancy test 5. participant is willing and able to comply with all required study visits and follow-up required by this protocol. 6. participant, and the participant's legal representative if applicable, must understand the procedures of the study and provide written informed consent.

inclusion criteria: 1. male or female participants aged ≥ 18 years at randomization. 2. healthy adults or adults with pre-existing medical conditions who are in stable condition. a stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease 3 months before enrollment and expected to remain stable for the duration of the study. 3. documented to have received two homologous doses of azd1222, mrna-1273, or mvc-cov1901 vaccine, with the latest dose between 150 and 365 days prior to randomization, with an interval between the two homologous doses of ≥ 4 weeks to ≤ 12 weeks, and did not receive any other investigational or approved covid-19 vaccines 4. female participants must: 1. be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal; 2. or, if of childbearing potential, be abstinent or agree to use medically effective contraception from 14 days before screening to 30 days following the injection of study intervention. acceptable forms include: i.implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii.established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c.have a negative pregnancy test 5. participant is willing and able to comply with all required study visits and follow-up required by this protocol. 6. participant, and the participant's legal representative if applicable, must understand the procedures of the study and provide written informed consent.

inclusion criteria: male or female participants aged ≥ 18 years at randomization. healthy adults or adults with pre-existing medical conditions who are in stable condition. a stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease 3 months before enrollment and expected to remain stable for the duration of the study. documented to have received two homologous doses of azd1222, mrna-1273, or mvc-cov1901 vaccine, with the latest dose between 150 and 365 days prior to randomization, with an interval between the two homologous doses of ≥ 4 weeks to ≤ 12 weeks, and did not receive any other investigational or approved covid-19 vaccines female participants must: be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal; or, if of childbearing potential, be abstinent or agree to use medically effective contraception from 14 days before screening to 30 days following the injection of study intervention. acceptable forms include: i.implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii.established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c.have a negative pregnancy test participant is willing and able to comply with all required study visits and follow-up required by this protocol. participant, and the participant's legal representative if applicable, must understand the procedures of the study and provide written informed consent.

inclusion criteria: male or female participants aged ≥ 18 years at randomization. healthy adults or adults with pre-existing medical conditions who are in stable condition. a stable medical condition is defined as disease not requiring significant change in therapy or hospitalization for worsening disease 3 months before enrollment and expected to remain stable for the duration of the study. documented to have received two homologous doses of azd1222, mrna-1273, or mvc-cov1901 vaccine, with the latest dose between 150 and 365 days prior to randomization, with an interval between the two homologous doses of ≥ 4 weeks to ≤ 12 weeks, and did not receive any other investigational or approved covid-19 vaccines female participants must: be either of non-childbearing potential, i.e. surgically sterilized (defined as having undergone hysterectomy and/or bilateral oophorectomy and/or bilateral salpingectomy; tubal ligation alone is not considered sufficient) or one year post-menopausal; or, if of childbearing potential, be abstinent or agree to use medically effective contraception from 14 days before screening to 30 days following the injection of study intervention. acceptable forms include: i.implanted hormonal methods of contraception or placement of an intrauterine device or intrauterine system ii.established use of hormonal methods (injectable, pill, patch or ring) combined with barrier methods of contraception: condom or occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/suppository c.have a negative pregnancy test participant is willing and able to comply with all required study visits and follow-up required by this protocol. participant, and the participant's legal representative if applicable, must understand the procedures of the study and provide written informed consent.