inclusion criteria: * (part a only) participants who are at high risk of sars-cov-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to sars-cov-2 and covid-19. * understands and agrees to comply with the study procedures and provides written informed consent. * able to comply with study procedures based on the assessment of the investigator. * female participants of non-childbearing potential may be enrolled in the study. non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of the investigator to confirm postmenopausal status. * female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: * has a negative pregnancy test at screening and on the day of the first dose (day 1, open-label day 1, and booster dose day 1). * has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1). * has agreed to continue adequate contraception through 3 months following the last dose (day 29, open-label day 29, and booster dose day 1). * is not currently breastfeeding. * 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 during the 3 months before enrollment. * (part c only) is currently enrolled in part b of the current study (mrna-1273-p301). * (part c only) has received at least 1 dose of mrna-1273 in the current study (mrna-1273-p301).

inclusion criteria: * (part a only) participants who are at high risk of sars-cov-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to sars-cov-2 and covid-19. * understands and agrees to comply with the study procedures and provides written informed consent. * able to comply with study procedures based on the assessment of the investigator. * female participants of non-childbearing potential may be enrolled in the study. non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of the investigator to confirm postmenopausal status. * female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: * has a negative pregnancy test at screening and on the day of the first dose (day 1, open-label day 1, and booster dose day 1). * has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1). * has agreed to continue adequate contraception through 3 months following the last dose (day 29, open-label day 29, and booster dose day 1). * is not currently breastfeeding. * 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 during the 3 months before enrollment. * (part c only) is currently enrolled in part b of the current study (mrna-1273-p301). * (part c only) has received at least 1 dose of mrna-1273 in the current study (mrna-1273-p301).

inclusion criteria: (part a only) participants who are at high risk of sars-cov-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to sars-cov-2 and covid-19. understands and agrees to comply with the study procedures and provides written informed consent. able to comply with study procedures based on the assessment of the investigator. female participants of non-childbearing potential may be enrolled in the study. non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of the investigator to confirm postmenopausal status. female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: has a negative pregnancy test at screening and on the day of the first dose (day 1, open-label day 1, and booster dose day 1). has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1). has agreed to continue adequate contraception through 3 months following the last dose (day 29, open-label day 29, and booster dose day 1). is not currently breastfeeding. 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 during the 3 months before enrollment. (part c only) is currently enrolled in part b of the current study (mrna-1273-p301). (part c only) has received at least 1 dose of mrna-1273 in the current study (mrna-1273-p301).

inclusion criteria: (part a only) participants who are at high risk of sars-cov-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to sars-cov-2 and covid-19. understands and agrees to comply with the study procedures and provides written informed consent. able to comply with study procedures based on the assessment of the investigator. female participants of non-childbearing potential may be enrolled in the study. non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of the investigator to confirm postmenopausal status. female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: has a negative pregnancy test at screening and on the day of the first dose (day 1, open-label day 1, and booster dose day 1). has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1). has agreed to continue adequate contraception through 3 months following the last dose (day 29, open-label day 29, and booster dose day 1). is not currently breastfeeding. 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 during the 3 months before enrollment. (part c only) is currently enrolled in part b of the current study (mrna-1273-p301). (part c only) has received at least 1 dose of mrna-1273 in the current study (mrna-1273-p301).

inclusion criteria: - participants who are at high risk of sars-cov-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to sars-cov-2 and covid-19. - understands and agrees to comply with the study procedures and provides written informed consent. - able to comply with study procedures based on the assessment of the investigator. - female participants of non-childbearing potential may be enrolled in the study. non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of the investigator to confirm postmenopausal status. - female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: - has a negative pregnancy test at screening and on the day of the first dose (day 1). - has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1). - has agreed to continue adequate contraception through 3 months following the second dose on day 29. - is not currently breastfeeding. - 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 during the 3 months before enrollment. additional inclusion criteria for part b: participants who were previously enrolled in the mrna-1273-p301 study and chose to be unblinded.

inclusion criteria: - participants who are at high risk of sars-cov-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to sars-cov-2 and covid-19. - understands and agrees to comply with the study procedures and provides written informed consent. - able to comply with study procedures based on the assessment of the investigator. - female participants of non-childbearing potential may be enrolled in the study. non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of the investigator to confirm postmenopausal status. - female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: - has a negative pregnancy test at screening and on the day of the first dose (day 1). - has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1). - has agreed to continue adequate contraception through 3 months following the second dose on day 29. - is not currently breastfeeding. - 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 during the 3 months before enrollment. additional inclusion criteria for part b: participants who were previously enrolled in the mrna-1273-p301 study and chose to be unblinded.

inclusion criteria: - participants who are at high risk of sars-cov-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to sars-cov-2 and covid-19. - understands and agrees to comply with the study procedures and provides written informed consent. - able to comply with study procedures based on the assessment of the investigator. - female participants of non-childbearing potential may be enrolled in the study. non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of the investigator to confirm postmenopausal status. - female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: - has a negative pregnancy test at screening and on the day of the first dose (day 1). - has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1). - has agreed to continue adequate contraception through 3 months following the second dose on day 29. - is not currently breastfeeding. - male participants engaging in activity that could result in pregnancy of sexual partners must agree to practice adequate contraception and refrain from sperm donation from the time of the first dose and through 3 months after the second dose. - 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 during the 3 months before enrollment.

inclusion criteria: - participants who are at high risk of sars-cov-2 infection, defined as adults whose locations or circumstances put them at appreciable risk of exposure to sars-cov-2 and covid-19. - understands and agrees to comply with the study procedures and provides written informed consent. - able to comply with study procedures based on the assessment of the investigator. - female participants of non-childbearing potential may be enrolled in the study. non-childbearing potential is defined as surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy) or postmenopausal (defined as amenorrhea for ≥12 consecutive months prior to screening without an alternative medical cause). a follicle-stimulating hormone (fsh) level may be measured at the discretion of the investigator to confirm postmenopausal status. - female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: - has a negative pregnancy test at screening and on the day of the first dose (day 1). - has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (day 1). - has agreed to continue adequate contraception through 3 months following the second dose on day 29. - is not currently breastfeeding. - male participants engaging in activity that could result in pregnancy of sexual partners must agree to practice adequate contraception and refrain from sperm donation from the time of the first dose and through 3 months after the second dose. - 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 during the 3 months before enrollment.