inclusion criteria: for part 1a and part 2: participants 12 to <18 years of age at the time of consent (screening visit, day 0) who, in the opinion of the investigator, are in good general health based on review of medical history and screening physical examination. investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) [lar(s)] understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. body mass index (bmi) at or above the third percentile according to world health organization (who) child growth standards at the screening visit (day 0) female participants of nonchildbearing potential may be enrolled in the study. nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at screening (day 0), on the day of the first injection (day 1), on the day of the second injection (day 29), and on the day of the bd (day 149) in part 2; has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (day 1); and has agreed to continue adequate contraception through 3 months following the second injection (day 29) and bd (day 149) in part 2. for part 1c-1 homologous booster dose: participants must have been previously enrolled in the mrna-1273-p203 study, are actively participating in part 1a or part 1b and are least 5 months from the last dose. female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test on the day of the first injection (bd-day 1). part 1c-2 heterologous booster dose: male or female, 12 to < 18 years of age at the time of consent who, in the opinion of the investigator, is in good general health based on review of medical history and screening physical examination and has completed non-moderna primary covid-19 vaccination series under eua (for example, pfizer) at least 3 months from consent.

inclusion criteria: for part 1a and part 2: participants 12 to <18 years of age at the time of consent (screening visit, day 0) who, in the opinion of the investigator, are in good general health based on review of medical history and screening physical examination. investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) [lar(s)] understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. body mass index (bmi) at or above the third percentile according to world health organization (who) child growth standards at the screening visit (day 0) female participants of nonchildbearing potential may be enrolled in the study. nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at screening (day 0), on the day of the first injection (day 1), on the day of the second injection (day 29), and on the day of the bd (day 149) in part 2; has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (day 1); and has agreed to continue adequate contraception through 3 months following the second injection (day 29) and bd (day 149) in part 2. for part 1c-1 homologous booster dose: participants must have been previously enrolled in the mrna-1273-p203 study, are actively participating in part 1a or part 1b and are least 5 months from the last dose. female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test on the day of the first injection (bd-day 1). part 1c-2 heterologous booster dose: male or female, 12 to < 18 years of age at the time of consent who, in the opinion of the investigator, is in good general health based on review of medical history and screening physical examination and has completed non-moderna primary covid-19 vaccination series under eua (for example, pfizer) at least 3 months from consent.

inclusion criteria: for part a: participants 12 to <18 years of age at the time of consent (screening visit, day 0) who, in the opinion of the investigator, are in good general health based on review of medical history and screening physical examination. investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) [lar(s)] understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. body mass index (bmi) at or above the third percentile according to world health organization (who) child growth standards at the screening visit (day 0) female participants of nonchildbearing potential may be enrolled in the study. nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at screening (day 0), on the day of the first injection (day 1), and on the day of the second injection (day 29); has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (day 1); and has agreed to continue adequate contraception through 3 months following the second injection (day 29). for part b: participants who were previously enrolled in the mrna-1273 p203 study and chose to be unblinded. female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test on the day of the first injection (open-label-day 1) and on the day of the second injection (open-label-day 29). for part c: participants must have been previously enrolled in the mrna-1273-p203 study, are actively participating in part a or part b and are least 6 months from the last dose. female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test on the day of the first injection (bd-day 1).

inclusion criteria: for part a: participants 12 to <18 years of age at the time of consent (screening visit, day 0) who, in the opinion of the investigator, are in good general health based on review of medical history and screening physical examination. investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) [lar(s)] understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. body mass index (bmi) at or above the third percentile according to world health organization (who) child growth standards at the screening visit (day 0) female participants of nonchildbearing potential may be enrolled in the study. nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at screening (day 0), on the day of the first injection (day 1), and on the day of the second injection (day 29); has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (day 1); and has agreed to continue adequate contraception through 3 months following the second injection (day 29). for part b: participants who were previously enrolled in the mrna-1273 p203 study and chose to be unblinded. female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test on the day of the first injection (open-label-day 1) and on the day of the second injection (open-label-day 29). for part c: participants must have been previously enrolled in the mrna-1273-p203 study, are actively participating in part a or part b and are least 6 months from the last dose. female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test on the day of the first injection (bd-day 1).

inclusion criteria: for part a: - participants 12 to <18 years of age at the time of consent (screening visit, day 0) who, in the opinion of the investigator, are in good general health based on review of medical history and screening physical examination. - investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) [lar(s)] understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. - body mass index (bmi) at or above the third percentile according to world health organization (who) child growth standards at the screening visit (day 0) - female participants of nonchildbearing potential may be enrolled in the study. nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). - female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at screening (day 0), on the day of the first injection (day 1), and on the day of the second injection (day 29); has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (day 1); and has agreed to continue adequate contraception through 3 months following the second injection (day 29). for part b: - participants who were previously enrolled in the mrna-1273 p203 study and chose to be unblinded. - female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test on the day of the first injection (open-label-day 1) and on the day of the second injection (open-label-day 29).

inclusion criteria: for part a: - participants 12 to <18 years of age at the time of consent (screening visit, day 0) who, in the opinion of the investigator, are in good general health based on review of medical history and screening physical examination. - investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) [lar(s)] understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. - body mass index (bmi) at or above the third percentile according to world health organization (who) child growth standards at the screening visit (day 0) - female participants of nonchildbearing potential may be enrolled in the study. nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). - female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at screening (day 0), on the day of the first injection (day 1), and on the day of the second injection (day 29); has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (day 1); and has agreed to continue adequate contraception through 3 months following the second injection (day 29). for part b: - participants who were previously enrolled in the mrna-1273 p203 study and chose to be unblinded. - female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test on the day of the first injection (open-label-day 1) and on the day of the second injection (open-label-day 29).

inclusion criteria: - participants 12 to <18 years of age at the time of consent (screening visit, day 0) who, in the opinion of the investigator, are in good general health based on review of medical history and screening physical examination. - investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) [lar(s)] understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. - body mass index (bmi) at or above the third percentile according to world health organization (who) child growth standards at the screening visit (day 0) - female participants of nonchildbearing potential may be enrolled in the study. nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). - female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at screening (day 0), on the day of the first injection (day 1), and on the day of the second injection (day 29); has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (day 1); has agreed to continue adequate contraception through 3 months following the second injection (day 29); and is not currently breastfeeding.

inclusion criteria: - participants 12 to <18 years of age at the time of consent (screening visit, day 0) who, in the opinion of the investigator, are in good general health based on review of medical history and screening physical examination. - investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) [lar(s)] understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. - body mass index (bmi) at or above the third percentile according to world health organization (who) child growth standards at the screening visit (day 0) - female participants of nonchildbearing potential may be enrolled in the study. nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). - female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at screening (day 0), on the day of the first injection (day 1), and on the day of the second injection (day 29); has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (day 1); has agreed to continue adequate contraception through 3 months following the second injection (day 29); and is not currently breastfeeding.