inclusion criteria: 1. male or female participant aged ≥20 to \<70 years at randomization. 2. has received one dose of the mrna-1273 8 to 12 weeks before randomization. 3. female participant 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 last administration 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 4. participant is willing and able to comply with all required study visits and follow-up required by this protocol. 5. participant or the participant's legal representative must understand the procedures of the study and provide written informed consent.

inclusion criteria: 1. male or female participant aged ≥20 to \<70 years at randomization. 2. has received one dose of the mrna-1273 8 to 12 weeks before randomization. 3. female participant 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 last administration 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 4. participant is willing and able to comply with all required study visits and follow-up required by this protocol. 5. participant or the participant's legal representative must understand the procedures of the study and provide written informed consent.

inclusion criteria: male or female participant aged ≥20 to <70 years at randomization. has received one dose of the mrna-1273 8 to 12 weeks before randomization. female participant 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 last administration 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 or the participant's legal representative must understand the procedures of the study and provide written informed consent.

inclusion criteria: male or female participant aged ≥20 to <70 years at randomization. has received one dose of the mrna-1273 8 to 12 weeks before randomization. female participant 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 last administration 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 or the participant's legal representative must understand the procedures of the study and provide written informed consent.

inclusion criteria: 1. male or female participant aged ≥20 to <70 years at randomization. 2. has received one dose of the mrna-1273 8 to 12 weeks before randomization. 3. female participant 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 last administration 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 4. participant is willing and able to comply with all required study visits and follow-up required by this protocol. 5. participant or the participant's legal representative must understand the procedures of the study and provide written informed consent.

inclusion criteria: 1. male or female participant aged ≥20 to <70 years at randomization. 2. has received one dose of the mrna-1273 8 to 12 weeks before randomization. 3. female participant 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 last administration 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 4. participant is willing and able to comply with all required study visits and follow-up required by this protocol. 5. participant or the participant's legal representative must understand the procedures of the study and provide written informed consent.