key inclusion criteria: * for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. * participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205; or have received a 2 dose primary series of mrna-1273 followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua at least 3 months prior to enrolment in mrna-1273-p205; and able to provide proof of vaccination status at the time of screening (day 1); or for enrollment in part a.2, participant must be currently enrolled and compliant in part a.1 of the mrna 1273 p205 study and must have received their first booster dose of mrna 1273.211 50 μg; or for enrollment in part j, participant must meet at least 1 of the following criteria: completed enrollment in part h of the mrna-1273-p205 study; or received a 2-dose primary series of mrna-1273 (100 μg) followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua, followed by a 50 μg booster dose of mrna-1273.222 under eua at least 3 months prior to enrollment in part j of mrna-1273-p205; or previously received a 2-dose primary series of mrna vaccine against sars-cov-2 followed by a booster dose of a monovalent mrna vaccine, followed by a second booster dose of a bivalent mrna vaccine. participants in part j must also provide proof of vaccination status at the time of screening (day 0 or day 1). key

key inclusion criteria: * for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. * participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205; or have received a 2 dose primary series of mrna-1273 followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua at least 3 months prior to enrolment in mrna-1273-p205; and able to provide proof of vaccination status at the time of screening (day 1); or for enrollment in part a.2, participant must be currently enrolled and compliant in part a.1 of the mrna 1273 p205 study and must have received their first booster dose of mrna 1273.211 50 μg; or for enrollment in part j, participant must meet at least 1 of the following criteria: completed enrollment in part h of the mrna-1273-p205 study; or received a 2-dose primary series of mrna-1273 (100 μg) followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua, followed by a 50 μg booster dose of mrna-1273.222 under eua at least 3 months prior to enrollment in part j of mrna-1273-p205; or previously received a 2-dose primary series of mrna vaccine against sars-cov-2 followed by a booster dose of a monovalent mrna vaccine, followed by a second booster dose of a bivalent mrna vaccine. participants in part j must also provide proof of vaccination status at the time of screening (day 0 or day 1). key

key inclusion criteria: for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205; or have received a 2 dose primary series of mrna-1273 followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua at least 3 months prior to enrolment in mrna-1273-p205; and able to provide proof of vaccination status at the time of screening (day 1); or for enrollment in part a.2, participant must be currently enrolled and compliant in part a.1 of the mrna 1273 p205 study and must have received their first booster dose of mrna 1273.211 50 μg; or for enrollment in part j, participant must meet at least 1 of the following criteria: completed enrollment in part h of the mrna-1273-p205 study; or received a 2-dose primary series of mrna-1273 (100 μg) followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua, followed by a 50 μg booster dose of mrna-1273.222 under eua at least 3 months prior to enrollment in part j of mrna-1273-p205; or previously received a 2-dose primary series of mrna vaccine against sars-cov-2 followed by a booster dose of a monovalent mrna vaccine, followed by a second booster dose of a bivalent mrna vaccine. participants in part j must also provide proof of vaccination status at the time of screening (day 0 or day 1). key

key inclusion criteria: for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205; or have received a 2 dose primary series of mrna-1273 followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua at least 3 months prior to enrolment in mrna-1273-p205; and able to provide proof of vaccination status at the time of screening (day 1); or for enrollment in part a.2, participant must be currently enrolled and compliant in part a.1 of the mrna 1273 p205 study and must have received their first booster dose of mrna 1273.211 50 μg; or for enrollment in part j, participant must meet at least 1 of the following criteria: completed enrollment in part h of the mrna-1273-p205 study; or received a 2-dose primary series of mrna-1273 (100 μg) followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua, followed by a 50 μg booster dose of mrna-1273.222 under eua at least 3 months prior to enrollment in part j of mrna-1273-p205; or previously received a 2-dose primary series of mrna vaccine against sars-cov-2 followed by a booster dose of a monovalent mrna vaccine, followed by a second booster dose of a bivalent mrna vaccine. participants in part j must also provide proof of vaccination status at the time of screening (day 0 or day 1). key

key inclusion criteria: for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205; or have received a 2 dose primary series of mrna-1273 followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua at least 3 months prior to enrolment in mrna-1273-p205; and able to provide proof of vaccination status at the time of screening (day 1); or for enrollment in part a.2, participant must be currently enrolled and compliant in part a.1 of the mrna 1273 p205 study and must have received their first booster dose of mrna 1273.211 50 μg. key

key inclusion criteria: for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205; or have received a 2 dose primary series of mrna-1273 followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua at least 3 months prior to enrolment in mrna-1273-p205; and able to provide proof of vaccination status at the time of screening (day 1); or for enrollment in part a.2, participant must be currently enrolled and compliant in part a.1 of the mrna 1273 p205 study and must have received their first booster dose of mrna 1273.211 50 μg. key

key inclusion criteria: for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205; or have received a 2 dose primary series of mrna-1273 followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua at least 3 months prior to enrolment in mrna-1273-p205; and able to provide proof of vaccination status at the time of screening (day 1). key

key inclusion criteria: for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205; or have received a 2 dose primary series of mrna-1273 followed by a 50 μg booster dose of mrna-1273 in the mrna-1273-p301 (cove) study or under eua at least 3 months prior to enrolment in mrna-1273-p205; and able to provide proof of vaccination status at the time of screening (day 1). key

key inclusion criteria: for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. in parts c and d, study participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205, and able to provide proof of vaccination status at the time of screening (day 1). in part e, study participants must have previously received an mrna vaccine for the prevention of covid-19 in another clinical study or under eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205 and must be able to provide proof of vaccination status at the time of screening (day 1). in part f, study participants must have previously received an mrna-1273 vaccine followed by a booster dose of mrna-1273 at least 3 months prior to enrollment in mrna-1273-p205. key

key inclusion criteria: for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. in parts c and d, study participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205, and able to provide proof of vaccination status at the time of screening (day 1). in part e, study participants must have previously received an mrna vaccine for the prevention of covid-19 in another clinical study or under eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205 and must be able to provide proof of vaccination status at the time of screening (day 1). in part f, study participants must have previously received an mrna-1273 vaccine followed by a booster dose of mrna-1273 at least 3 months prior to enrollment in mrna-1273-p205. key

key inclusion criteria: - for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. - in parts c and d, study participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the emergency use authorization (eua) with their second dose at least 6 months prior to enrollment in mrna-1273-p205, and able to provide proof of vaccination status at the time of screening (day 1). in part e, study participants must have previously received an mrna vaccine for the prevention of covid-19 in another clinical study or under eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205 and must be able to provide proof of vaccination status at the time of screening (day 1). key

key inclusion criteria: - for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. - in parts c and d, study participants must have been either previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in that study, with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early); or participant must have received 2 doses of mrna-1273 under the emergency use authorization (eua) with their second dose at least 6 months prior to enrollment in mrna-1273-p205, and able to provide proof of vaccination status at the time of screening (day 1). in part e, study participants must have previously received an mrna vaccine for the prevention of covid-19 in another clinical study or under eua with their second dose at least 6 months prior to enrollment in mrna-1273-p205 and must be able to provide proof of vaccination status at the time of screening (day 1). key

key inclusion criteria: - for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. - participant must have been previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in part a of that study (that is, already unblinded and aware of their actual treatment), with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early). key

key inclusion criteria: - for female participants of childbearing potential: negative pregnancy test, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to day 1, agreement to continue adequate contraception or abstinence through 3 months following the vaccination, and not currently breastfeeding. - participant must have been previously enrolled in the mrna-1273-p301 (cove) study, must have received 2 doses of mrna-1273 in part a of that study (that is, already unblinded and aware of their actual treatment), with their second dose at least 6 months prior to enrollment in this study (mrna-1273-p205), and must be currently enrolled and compliant in that study (that is, has not withdrawn or discontinued early). key