inclusion criteria: 1. male or female participants ≥ 18 years and \<55 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 a 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 or three homologous doses of mvc-cov1901 vaccine, 1st and 2nd doses within 12 weeks, 2nd and 3rd doses between 12\~24 weeks (group b only), and with the latest dose \> 84 days prior to randomization, 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. female with childbearing potential must have a negative pregnancy test by this protocol 5. participant is willing and able to comply with all required study visits and follow-up required 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 ≥ 18 years and \<55 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 a 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 or three homologous doses of mvc-cov1901 vaccine, 1st and 2nd doses within 12 weeks, 2nd and 3rd doses between 12\~24 weeks (group b only), and with the latest dose \> 84 days prior to randomization, 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. female with childbearing potential must have a negative pregnancy test by this protocol 5. participant is willing and able to comply with all required study visits and follow-up required 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 ≥ 18 years and <55 years at randomization. healthy adults or adults with pre-existing medical conditions who are in stable condition. a stable medical condition is defined as a 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 or three homologous doses of mvc-cov1901 vaccine, 1st and 2nd doses within 12 weeks, 2nd and 3rd doses between 12~24 weeks (group b only), and with the latest dose > 84 days prior to randomization, 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. female with childbearing potential must have a negative pregnancy test by this protocol participant is willing and able to comply with all required study visits and follow-up required 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 ≥ 18 years and <55 years at randomization. healthy adults or adults with pre-existing medical conditions who are in stable condition. a stable medical condition is defined as a 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 or three homologous doses of mvc-cov1901 vaccine, 1st and 2nd doses within 12 weeks, 2nd and 3rd doses between 12~24 weeks (group b only), and with the latest dose > 84 days prior to randomization, 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. female with childbearing potential must have a negative pregnancy test by this protocol participant is willing and able to comply with all required study visits and follow-up required 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 ≥ 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 a 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 mvc-cov1901 vaccine, with the latest dose between 84 and 365 days prior to randomization, with an interval between the two homologous doses of ≥ 25 days, 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. female with childbearing potential must have a negative pregnancy test by this protocol participant is willing and able to comply with all required study visits and follow-up required 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 ≥ 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 a 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 mvc-cov1901 vaccine, with the latest dose between 84 and 365 days prior to randomization, with an interval between the two homologous doses of ≥ 25 days, 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. female with childbearing potential must have a negative pregnancy test by this protocol participant is willing and able to comply with all required study visits and follow-up required participant, and the participant's legal representative if applicable, must understand the procedures of the study and provide written informed consent.