inclusion criteria: a subject must meet all of the following criteria to be eligible to participate in this study: 1. provides written informed consent prior to initiation of any study procedures. 2. be able to understand and agrees to comply with planned study procedures and be available for all study visits. 3. agrees to the collection of venous blood per protocol. 4. male or non-pregnant female, \>/= to 18 years of age at time of enrollment. 5. body mass index (bmi) 18.0-35.0 kg/m\^2, inclusive (\< 56 years of age), at screening; bmi 18.0-30.0 kg/m\^2, inclusive (\>/= 56 years of age), at screening. 6. women of childbearing potential\* must agree to use or have practiced true abstinence\*\* or use at least one acceptable primary form of contraception.\*\*\*, \*\*\*\* note: these criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship). * not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure(r) placement). * true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence \[e.g., calendar, ovulation, symptothermal, post-ovulation methods\] and withdrawal are not acceptable methods of contraception). * acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. * must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination. 7. women of childbearing potential must have a negative urine or serum pregnancy test within 24 hours prior to each vaccination. 8. male subjects of childbearing potential\*: use of condoms to ensure effective contraception with a female partner of childbearing potential from first vaccination until 60 days after the last vaccination. \*biological males who are post-pubertal and considered fertile until permanently sterile by bilateral orchiectomy or vasectomy. 9. male subjects agree to refrain from sperm donation from the time of first vaccination until 60 days after the last vaccination. 10. in good health.\* \*as determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room (er), or urgent care for condition or need for supplemental oxygen). this includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. any change in prescription medication due to improvement of a disease outcome, as determined by the participating site principal investigator (pi) or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site pi or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination. 11. oral temperature is less than 100.0 degrees fahrenheit (37.8 degrees celsius). 12. pulse no greater than 100 beats per minute. 13. systolic blood pressure (bp) is 85 to 150 mm hg, inclusive. 14. clinical screening laboratory evaluations (white blood cell (wbc), hemoglobin (hgb), platelets (plts), alanine transaminase (alt), aspartate transaminase (ast), creatinine (cr), alkaline phosphatase (alp), total bilirubin (t. bili), lipase, prothrombin time (pt), and partial thromboplastin time (ptt)) are within acceptable normal reference ranges at the clinical laboratory being used. 15. must agree to have samples stored for secondary research. 16. agrees to adhere to lifestyle considerations throughout study duration. 17. must agree to refrain from donating blood or plasma during the study (outside of this study). leukapheresis inclusion criteria: a subject must meet all of the following criteria to be eligible for leukapheresis: 1. written informed consent for leukapheresis is provided. 2. weight \>/= 110 pounds. 3. screening laboratory evaluations are within acceptable ranges at the site where the leukapheresis procedure will be performed. 4. negative urine or serum pregnancy test within 48 hours of the leukapheresis procedure for women of childbearing potential. 5. adequate bilateral antecubital venous access. 6. no use of blood thinners, aspirin or nonsteroidal anti-inflammatory drugs (nsaids) at least 5 days before the leukapheresis procedure. 7. enrolled in cohorts 2, 3, 5, 10, or 11, and possibly cohort 6, if enrolled, and completed the two-dose vaccination series. optional substudy inclusion criteria: 1. enrolled in the main study and received both the first and second mrna-1273 vaccinations. 2. provides written informed consent for the third mrna-1273 vaccination. 3. agrees to the collection of venous blood per substudy. 4. must agree to have samples stored for secondary research. 5. women of childbearing potential have had a negative urine pregnancy test within 24 hours before the third mrna-1273 vaccination. 6. women of childbearing potential\* must agree to use or have practiced true abstinence\*\* or use at least one acceptable primary form of contraception.\*\*\*/\*\*\*\* * not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure® placement). * true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence \[e.g., calendar, ovulation, symptothermal, post-ovulation methods\] and withdrawal are not acceptable methods of contraception). * acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. \*\*\*\*must use at least one acceptable primary form of contraception for at least 30 days prior to the third mrna-1273 vaccination and for at least 30 days after the third mrna-1273 vaccination.

inclusion criteria: a subject must meet all of the following criteria to be eligible to participate in this study: 1. provides written informed consent prior to initiation of any study procedures. 2. be able to understand and agrees to comply with planned study procedures and be available for all study visits. 3. agrees to the collection of venous blood per protocol. 4. male or non-pregnant female, \>/= to 18 years of age at time of enrollment. 5. body mass index (bmi) 18.0-35.0 kg/m\^2, inclusive (\< 56 years of age), at screening; bmi 18.0-30.0 kg/m\^2, inclusive (\>/= 56 years of age), at screening. 6. women of childbearing potential\* must agree to use or have practiced true abstinence\*\* or use at least one acceptable primary form of contraception.\*\*\*, \*\*\*\* note: these criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship). * not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure(r) placement). * true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence \[e.g., calendar, ovulation, symptothermal, post-ovulation methods\] and withdrawal are not acceptable methods of contraception). * acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. * must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination. 7. women of childbearing potential must have a negative urine or serum pregnancy test within 24 hours prior to each vaccination. 8. male subjects of childbearing potential\*: use of condoms to ensure effective contraception with a female partner of childbearing potential from first vaccination until 60 days after the last vaccination. \*biological males who are post-pubertal and considered fertile until permanently sterile by bilateral orchiectomy or vasectomy. 9. male subjects agree to refrain from sperm donation from the time of first vaccination until 60 days after the last vaccination. 10. in good health.\* \*as determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room (er), or urgent care for condition or need for supplemental oxygen). this includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. any change in prescription medication due to improvement of a disease outcome, as determined by the participating site principal investigator (pi) or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site pi or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination. 11. oral temperature is less than 100.0 degrees fahrenheit (37.8 degrees celsius). 12. pulse no greater than 100 beats per minute. 13. systolic blood pressure (bp) is 85 to 150 mm hg, inclusive. 14. clinical screening laboratory evaluations (white blood cell (wbc), hemoglobin (hgb), platelets (plts), alanine transaminase (alt), aspartate transaminase (ast), creatinine (cr), alkaline phosphatase (alp), total bilirubin (t. bili), lipase, prothrombin time (pt), and partial thromboplastin time (ptt)) are within acceptable normal reference ranges at the clinical laboratory being used. 15. must agree to have samples stored for secondary research. 16. agrees to adhere to lifestyle considerations throughout study duration. 17. must agree to refrain from donating blood or plasma during the study (outside of this study). leukapheresis inclusion criteria: a subject must meet all of the following criteria to be eligible for leukapheresis: 1. written informed consent for leukapheresis is provided. 2. weight \>/= 110 pounds. 3. screening laboratory evaluations are within acceptable ranges at the site where the leukapheresis procedure will be performed. 4. negative urine or serum pregnancy test within 48 hours of the leukapheresis procedure for women of childbearing potential. 5. adequate bilateral antecubital venous access. 6. no use of blood thinners, aspirin or nonsteroidal anti-inflammatory drugs (nsaids) at least 5 days before the leukapheresis procedure. 7. enrolled in cohorts 2, 3, 5, 10, or 11, and possibly cohort 6, if enrolled, and completed the two-dose vaccination series. optional substudy inclusion criteria: 1. enrolled in the main study and received both the first and second mrna-1273 vaccinations. 2. provides written informed consent for the third mrna-1273 vaccination. 3. agrees to the collection of venous blood per substudy. 4. must agree to have samples stored for secondary research. 5. women of childbearing potential have had a negative urine pregnancy test within 24 hours before the third mrna-1273 vaccination. 6. women of childbearing potential\* must agree to use or have practiced true abstinence\*\* or use at least one acceptable primary form of contraception.\*\*\*/\*\*\*\* * not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure® placement). * true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence \[e.g., calendar, ovulation, symptothermal, post-ovulation methods\] and withdrawal are not acceptable methods of contraception). * acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. \*\*\*\*must use at least one acceptable primary form of contraception for at least 30 days prior to the third mrna-1273 vaccination and for at least 30 days after the third mrna-1273 vaccination.

inclusion criteria: a subject must meet all of the following criteria to be eligible to participate in this study: provides written informed consent prior to initiation of any study procedures. be able to understand and agrees to comply with planned study procedures and be available for all study visits. agrees to the collection of venous blood per protocol. male or non-pregnant female, >/= to 18 years of age at time of enrollment. body mass index (bmi) 18.0-35.0 kg/m^2, inclusive (< 56 years of age), at screening; bmi 18.0-30.0 kg/m^2, inclusive (>/= 56 years of age), at screening. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***, **** note: these criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship). not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure(r) placement). true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination. women of childbearing potential must have a negative urine or serum pregnancy test within 24 hours prior to each vaccination. male subjects of childbearing potential*: use of condoms to ensure effective contraception with a female partner of childbearing potential from first vaccination until 60 days after the last vaccination. *biological males who are post-pubertal and considered fertile until permanently sterile by bilateral orchiectomy or vasectomy. male subjects agree to refrain from sperm donation from the time of first vaccination until 60 days after the last vaccination. in good health.* *as determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room (er), or urgent care for condition or need for supplemental oxygen). this includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. any change in prescription medication due to improvement of a disease outcome, as determined by the participating site principal investigator (pi) or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site pi or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination. oral temperature is less than 100.0 degrees fahrenheit (37.8 degrees celsius). pulse no greater than 100 beats per minute. systolic blood pressure (bp) is 85 to 150 mm hg, inclusive. clinical screening laboratory evaluations (white blood cell (wbc), hemoglobin (hgb), platelets (plts), alanine transaminase (alt), aspartate transaminase (ast), creatinine (cr), alkaline phosphatase (alp), total bilirubin (t. bili), lipase, prothrombin time (pt), and partial thromboplastin time (ptt)) are within acceptable normal reference ranges at the clinical laboratory being used. must agree to have samples stored for secondary research. agrees to adhere to lifestyle considerations throughout study duration. must agree to refrain from donating blood or plasma during the study (outside of this study). leukapheresis inclusion criteria: a subject must meet all of the following criteria to be eligible for leukapheresis: written informed consent for leukapheresis is provided. weight >/= 110 pounds. screening laboratory evaluations are within acceptable ranges at the site where the leukapheresis procedure will be performed. negative urine or serum pregnancy test within 48 hours of the leukapheresis procedure for women of childbearing potential. adequate bilateral antecubital venous access. no use of blood thinners, aspirin or nonsteroidal anti-inflammatory drugs (nsaids) at least 5 days before the leukapheresis procedure. enrolled in cohorts 2, 3, 5, 10, or 11, and possibly cohort 6, if enrolled, and completed the two-dose vaccination series. optional substudy inclusion criteria: enrolled in the main study and received both the first and second mrna-1273 vaccinations. provides written informed consent for the third mrna-1273 vaccination. agrees to the collection of venous blood per substudy. must agree to have samples stored for secondary research. women of childbearing potential have had a negative urine pregnancy test within 24 hours before the third mrna-1273 vaccination. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***/**** not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure® placement). true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. ****must use at least one acceptable primary form of contraception for at least 30 days prior to the third mrna-1273 vaccination and for at least 30 days after the third mrna-1273 vaccination.

inclusion criteria: a subject must meet all of the following criteria to be eligible to participate in this study: provides written informed consent prior to initiation of any study procedures. be able to understand and agrees to comply with planned study procedures and be available for all study visits. agrees to the collection of venous blood per protocol. male or non-pregnant female, >/= to 18 years of age at time of enrollment. body mass index (bmi) 18.0-35.0 kg/m^2, inclusive (< 56 years of age), at screening; bmi 18.0-30.0 kg/m^2, inclusive (>/= 56 years of age), at screening. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***, **** note: these criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship). not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure(r) placement). true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination. women of childbearing potential must have a negative urine or serum pregnancy test within 24 hours prior to each vaccination. male subjects of childbearing potential*: use of condoms to ensure effective contraception with a female partner of childbearing potential from first vaccination until 60 days after the last vaccination. *biological males who are post-pubertal and considered fertile until permanently sterile by bilateral orchiectomy or vasectomy. male subjects agree to refrain from sperm donation from the time of first vaccination until 60 days after the last vaccination. in good health.* *as determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room (er), or urgent care for condition or need for supplemental oxygen). this includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. any change in prescription medication due to improvement of a disease outcome, as determined by the participating site principal investigator (pi) or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site pi or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination. oral temperature is less than 100.0 degrees fahrenheit (37.8 degrees celsius). pulse no greater than 100 beats per minute. systolic blood pressure (bp) is 85 to 150 mm hg, inclusive. clinical screening laboratory evaluations (white blood cell (wbc), hemoglobin (hgb), platelets (plts), alanine transaminase (alt), aspartate transaminase (ast), creatinine (cr), alkaline phosphatase (alp), total bilirubin (t. bili), lipase, prothrombin time (pt), and partial thromboplastin time (ptt)) are within acceptable normal reference ranges at the clinical laboratory being used. must agree to have samples stored for secondary research. agrees to adhere to lifestyle considerations throughout study duration. must agree to refrain from donating blood or plasma during the study (outside of this study). leukapheresis inclusion criteria: a subject must meet all of the following criteria to be eligible for leukapheresis: written informed consent for leukapheresis is provided. weight >/= 110 pounds. screening laboratory evaluations are within acceptable ranges at the site where the leukapheresis procedure will be performed. negative urine or serum pregnancy test within 48 hours of the leukapheresis procedure for women of childbearing potential. adequate bilateral antecubital venous access. no use of blood thinners, aspirin or nonsteroidal anti-inflammatory drugs (nsaids) at least 5 days before the leukapheresis procedure. enrolled in cohorts 2, 3, 5, 10, or 11, and possibly cohort 6, if enrolled, and completed the two-dose vaccination series. optional substudy inclusion criteria: enrolled in the main study and received both the first and second mrna-1273 vaccinations. provides written informed consent for the third mrna-1273 vaccination. agrees to the collection of venous blood per substudy. must agree to have samples stored for secondary research. women of childbearing potential have had a negative urine pregnancy test within 24 hours before the third mrna-1273 vaccination. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***/**** not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure® placement). true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. ****must use at least one acceptable primary form of contraception for at least 30 days prior to the third mrna-1273 vaccination and for at least 30 days after the third mrna-1273 vaccination.

inclusion criteria: a subject must meet all of the following criteria to be eligible to participate in this study: 1. provides written informed consent prior to initiation of any study procedures. 2. be able to understand and agrees to comply with planned study procedures and be available for all study visits. 3. agrees to the collection of venous blood per protocol. 4. male or non-pregnant female, >/= to 18 years of age at time of enrollment. 5. body mass index (bmi) 18.0-35.0 kg/m^2, inclusive (< 56 years of age), at screening; bmi 18.0-30.0 kg/m^2, inclusive (>/= 56 years of age), at screening. 6. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***, **** note: these criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship). - not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure(r) placement). - true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). - acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. - must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination. 7. women of childbearing potential must have a negative urine or serum pregnancy test within 24 hours prior to each vaccination. 8. male subjects of childbearing potential*: use of condoms to ensure effective contraception with a female partner of childbearing potential from first vaccination until 60 days after the last vaccination. *biological males who are post-pubertal and considered fertile until permanently sterile by bilateral orchiectomy or vasectomy. 9. male subjects agree to refrain from sperm donation from the time of first vaccination until 60 days after the last vaccination. 10. in good health.* *as determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room (er), or urgent care for condition or need for supplemental oxygen). this includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. any change in prescription medication due to improvement of a disease outcome, as determined by the participating site principal investigator (pi) or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site pi or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination. 11. oral temperature is less than 100.0 degrees fahrenheit (37.8 degrees celsius). 12. pulse no greater than 100 beats per minute. 13. systolic blood pressure (bp) is 85 to 150 mm hg, inclusive. 14. clinical screening laboratory evaluations (white blood cell (wbc), hemoglobin (hgb), platelets (plts), alanine transaminase (alt), aspartate transaminase (ast), creatinine (cr), alkaline phosphatase (alp), total bilirubin (t. bili), lipase, prothrombin time (pt), and partial thromboplastin time (ptt)) are within acceptable normal reference ranges at the clinical laboratory being used. 15. must agree to have samples stored for secondary research. 16. agrees to adhere to lifestyle considerations throughout study duration. 17. must agree to refrain from donating blood or plasma during the study (outside of this study). leukapheresis inclusion criteria: a subject must meet all of the following criteria to be eligible for leukapheresis: 1. written informed consent for leukapheresis is provided. 2. weight >/= 110 pounds. 3. screening laboratory evaluations are within acceptable ranges at the site where the leukapheresis procedure will be performed. 4. negative urine or serum pregnancy test within 48 hours of the leukapheresis procedure for women of childbearing potential. 5. adequate bilateral antecubital venous access. 6. no use of blood thinners, aspirin or nonsteroidal anti-inflammatory drugs (nsaids) at least 5 days before the leukapheresis procedure. 7. enrolled in cohorts 2, 3, 5, 10, or 11, and possibly cohort 6, if enrolled, and completed the two-dose vaccination series. optional substudy inclusion criteria: 1. enrolled in the main study and received both the first and second mrna-1273 vaccinations. 2. provides written informed consent for the third mrna-1273 vaccination. 3. agrees to the collection of venous blood per substudy. 4. must agree to have samples stored for secondary research. 5. women of childbearing potential have had a negative urine pregnancy test within 24 hours before the third mrna-1273 vaccination. 6. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***/**** - not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure® placement). - true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). - acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. ****must use at least one acceptable primary form of contraception for at least 30 days prior to the third mrna-1273 vaccination and for at least 30 days after the third mrna-1273 vaccination.

inclusion criteria: a subject must meet all of the following criteria to be eligible to participate in this study: 1. provides written informed consent prior to initiation of any study procedures. 2. be able to understand and agrees to comply with planned study procedures and be available for all study visits. 3. agrees to the collection of venous blood per protocol. 4. male or non-pregnant female, >/= to 18 years of age at time of enrollment. 5. body mass index (bmi) 18.0-35.0 kg/m^2, inclusive (< 56 years of age), at screening; bmi 18.0-30.0 kg/m^2, inclusive (>/= 56 years of age), at screening. 6. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***, **** note: these criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship). - not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure(r) placement). - true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). - acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. - must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination. 7. women of childbearing potential must have a negative urine or serum pregnancy test within 24 hours prior to each vaccination. 8. male subjects of childbearing potential*: use of condoms to ensure effective contraception with a female partner of childbearing potential from first vaccination until 60 days after the last vaccination. *biological males who are post-pubertal and considered fertile until permanently sterile by bilateral orchiectomy or vasectomy. 9. male subjects agree to refrain from sperm donation from the time of first vaccination until 60 days after the last vaccination. 10. in good health.* *as determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room (er), or urgent care for condition or need for supplemental oxygen). this includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. any change in prescription medication due to improvement of a disease outcome, as determined by the participating site principal investigator (pi) or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site pi or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination. 11. oral temperature is less than 100.0 degrees fahrenheit (37.8 degrees celsius). 12. pulse no greater than 100 beats per minute. 13. systolic blood pressure (bp) is 85 to 150 mm hg, inclusive. 14. clinical screening laboratory evaluations (white blood cell (wbc), hemoglobin (hgb), platelets (plts), alanine transaminase (alt), aspartate transaminase (ast), creatinine (cr), alkaline phosphatase (alp), total bilirubin (t. bili), lipase, prothrombin time (pt), and partial thromboplastin time (ptt)) are within acceptable normal reference ranges at the clinical laboratory being used. 15. must agree to have samples stored for secondary research. 16. agrees to adhere to lifestyle considerations throughout study duration. 17. must agree to refrain from donating blood or plasma during the study (outside of this study). leukapheresis inclusion criteria: a subject must meet all of the following criteria to be eligible for leukapheresis: 1. written informed consent for leukapheresis is provided. 2. weight >/= 110 pounds. 3. screening laboratory evaluations are within acceptable ranges at the site where the leukapheresis procedure will be performed. 4. negative urine or serum pregnancy test within 48 hours of the leukapheresis procedure for women of childbearing potential. 5. adequate bilateral antecubital venous access. 6. no use of blood thinners, aspirin or nonsteroidal anti-inflammatory drugs (nsaids) at least 5 days before the leukapheresis procedure. 7. enrolled in cohorts 2, 3, 5, 10, or 11, and possibly cohort 6, if enrolled, and completed the two-dose vaccination series. optional substudy inclusion criteria: 1. enrolled in the main study and received both the first and second mrna-1273 vaccinations. 2. provides written informed consent for the third mrna-1273 vaccination. 3. agrees to the collection of venous blood per substudy. 4. must agree to have samples stored for secondary research. 5. women of childbearing potential have had a negative urine pregnancy test within 24 hours before the third mrna-1273 vaccination. 6. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***/**** - not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure® placement). - true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). - acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. ****must use at least one acceptable primary form of contraception for at least 30 days prior to the third mrna-1273 vaccination and for at least 30 days after the third mrna-1273 vaccination.

inclusion criteria: a subject must meet all of the following criteria to be eligible to participate in this study: 1. provides written informed consent prior to initiation of any study procedures. 2. be able to understand and agrees to comply with planned study procedures and be available for all study visits. 3. agrees to the collection of venous blood per protocol. 4. male or non-pregnant female, >/= to 18 years of age at time of enrollment. 5. body mass index (bmi) 18.0-35.0 kg/m^2, inclusive (< 56 years of age), at screening; bmi 18.0-30.0 kg/m^2, inclusive (>/= 56 years of age), at screening. 6. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***, **** note: these criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship). - not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure(r) placement). - true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). - acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. - must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination. 7. women of childbearing potential must have a negative urine or serum pregnancy test within 24 hours prior to each vaccination. 8. male subjects of childbearing potential*: use of condoms to ensure effective contraception with a female partner of childbearing potential from first vaccination until 60 days after the last vaccination. *biological males who are post-pubertal and considered fertile until permanently sterile by bilateral orchiectomy or vasectomy. 9. male subjects agree to refrain from sperm donation from the time of first vaccination until 60 days after the last vaccination. 10. in good health.* *as determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room (er), or urgent care for condition or need for supplemental oxygen). this includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. any change in prescription medication due to improvement of a disease outcome, as determined by the participating site principal investigator (pi) or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site pi or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination. 11. oral temperature is less than 100.0 degrees fahrenheit (37.8 degrees celsius). 12. pulse no greater than 100 beats per minute. 13. systolic blood pressure (bp) is 85 to 150 mm hg, inclusive. 14. clinical screening laboratory evaluations (white blood cell (wbc), hemoglobin (hgb), platelets (plts), alanine transaminase (alt), aspartate transaminase (ast), creatinine (cr), alkaline phosphatase (alp), total bilirubin (t. bili), lipase, prothrombin time (pt), and partial thromboplastin time (ptt)) are within acceptable normal reference ranges at the clinical laboratory being used. 15. must agree to have samples stored for secondary research. 16. agrees to adhere to lifestyle considerations throughout study duration. 17. must agree to refrain from donating blood or plasma during the study (outside of this study). leukapheresis inclusion criteria: a subject must meet all of the following criteria to be eligible for leukapheresis: 1. written informed consent for leukapheresis is provided. 2. weight >/= 110 pounds. 3. screening laboratory evaluations are within acceptable ranges at the site where the leukapheresis procedure will be performed. 4. negative urine or serum pregnancy test within 48 hours of the leukapheresis procedure for women of childbearing potential. 5. adequate bilateral antecubital venous access. 6. no use of blood thinners, aspirin or nonsteroidal anti-inflammatory drugs (nsaids) at least 5 days before the leukapheresis procedure. 7. enrolled in cohorts 2, 3, 5, 10, or 11, and possibly cohort 6, if enrolled, and completed the two-dose vaccination series.

inclusion criteria: a subject must meet all of the following criteria to be eligible to participate in this study: 1. provides written informed consent prior to initiation of any study procedures. 2. be able to understand and agrees to comply with planned study procedures and be available for all study visits. 3. agrees to the collection of venous blood per protocol. 4. male or non-pregnant female, >/= to 18 years of age at time of enrollment. 5. body mass index (bmi) 18.0-35.0 kg/m^2, inclusive (< 56 years of age), at screening; bmi 18.0-30.0 kg/m^2, inclusive (>/= 56 years of age), at screening. 6. women of childbearing potential* must agree to use or have practiced true abstinence** or use at least one acceptable primary form of contraception.***, **** note: these criteria are applicable to females in a heterosexual relationship and child-bearing potential (i.e., the criteria do not apply to subjects in a same sex relationship). - not of childbearing potential - post-menopausal females (defined as having a history of amenorrhea for at least one year) or a documented status as being surgically sterile (hysterectomy, bilateral oophorectomy, tubal ligation/salpingectomy, or essure(r) placement). - true abstinence is 100% of time no sexual intercourse (male's penis enters the female's vagina). (periodic abstinence [e.g., calendar, ovulation, symptothermal, post-ovulation methods] and withdrawal are not acceptable methods of contraception). - acceptable forms of primary contraception include monogamous relationship with a vasectomized partner who has been vasectomized for 180 days or more prior to the subject's first vaccination, intrauterine devices, birth control pills, and injectable/implantable/insertable hormonal birth control products. - must use at least one acceptable primary form of contraception for at least 30 days prior to the first vaccination and at least one acceptable primary form of contraception for 60 days after the last vaccination. 7. women of childbearing potential must have a negative urine or serum pregnancy test within 24 hours prior to each vaccination. 8. male subjects of childbearing potential*: use of condoms to ensure effective contraception with a female partner of childbearing potential from first vaccination until 60 days after the last vaccination. *biological males who are post-pubertal and considered fertile until permanently sterile by bilateral orchiectomy or vasectomy. 9. male subjects agree to refrain from sperm donation from the time of first vaccination until 60 days after the last vaccination. 10. in good health.* *as determined by medical history and physical examination to evaluate acute or ongoing chronic medical diagnoses/conditions that have been present for at least 90 days, which would affect the assessment of safety of subjects. chronic medical diagnoses/conditions should be stable for the last 60 days (no hospitalizations, emergency room (er), or urgent care for condition or need for supplemental oxygen). this includes no change in chronic prescription medication, dose, or frequency as a result of deterioration of the chronic medical diagnosis/condition in the 60 days before enrollment. any prescription change that is due to change of health care provider, insurance company, etc., or done for financial reasons, and in the same class of medication, will not be considered a deviation of this inclusion criterion. any change in prescription medication due to improvement of a disease outcome, as determined by the participating site principal investigator (pi) or appropriate sub-investigator, will not be considered a deviation of this inclusion criterion. subjects may be on chronic or as needed (prn) medications if, in the opinion of the participating site pi or appropriate sub-investigator, they pose no additional risk to subject safety or assessment of reactogenicity and immunogenicity, and do not indicate a worsening of medical diagnosis/condition. similarly, medication changes subsequent to enrollment and study vaccination are acceptable provided the change was not precipitated by deterioration in the chronic medical condition, and there is no anticipated additional risk to the subject or interference with the evaluation of responses to study vaccination. 11. oral temperature is less than 100.0 degrees fahrenheit (37.8 degrees celsius). 12. pulse no greater than 100 beats per minute. 13. systolic blood pressure (bp) is 85 to 150 mm hg, inclusive. 14. clinical screening laboratory evaluations (white blood cell (wbc), hemoglobin (hgb), platelets (plts), alanine transaminase (alt), aspartate transaminase (ast), creatinine (cr), alkaline phosphatase (alp), total bilirubin (t. bili), lipase, prothrombin time (pt), and partial thromboplastin time (ptt)) are within acceptable normal reference ranges at the clinical laboratory being used. 15. must agree to have samples stored for secondary research. 16. agrees to adhere to lifestyle considerations throughout study duration. 17. must agree to refrain from donating blood or plasma during the study (outside of this study). leukapheresis inclusion criteria: a subject must meet all of the following criteria to be eligible for leukapheresis: 1. written informed consent for leukapheresis is provided. 2. weight >/= 110 pounds. 3. screening laboratory evaluations are within acceptable ranges at the site where the leukapheresis procedure will be performed. 4. negative urine or serum pregnancy test within 48 hours of the leukapheresis procedure for women of childbearing potential. 5. adequate bilateral antecubital venous access. 6. no use of blood thinners, aspirin or nonsteroidal anti-inflammatory drugs (nsaids) at least 5 days before the leukapheresis procedure. 7. enrolled in cohorts 2, 3, 5, 10, or 11, and possibly cohort 6, if enrolled, and completed the two-dose vaccination series.