inclusion criteria: 1. male or female participants 21 years of age, inclusively. 2. protocol versions 1.01-1.06:individuals who previously received mrna primary series vaccination and a booster dose at least 4 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. protocol version 1.07:individuals who had previously received mrna primary series vaccination comprising at least 3 vaccine doses with the last one at least 6 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. note: subjects who had received 4 vaccine doses (i.e., a primary series of 3 doses and an additional boost) may also be enrolled under protocol version 1.07). 3. females and males of childbearing potential must be willing to use effective methods of contraception such as hormones (ocp, oral contraceptive pill), condom or occlusive cap with spermicidal agents, intrauterine device (iud), or intrauterine system (ius) as well as to refrain from donating sperm through 28 days following the last vaccination. 4. participant who is willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. 5. for women of child bearing potential (wocbp): a negative pregnancy test on the day of vaccination (visit 1 and visit 4). 6. participant who is willing and able to operate an electronic diary. 7. healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. note: healthy participants with preexisting stable disease, defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the six (6) weeks before enrollment, can be included. 8. capable of giving signed informed consent as described which includes understanding and compliance with the study procedures, requirements and restrictions listed in the informed consent form (icf) and in this protocol. 9. must agree not to enroll in another study of an investigational agent prior to completion of the study.

inclusion criteria: 1. male or female participants 21 years of age, inclusively. 2. protocol versions 1.01-1.06:individuals who previously received mrna primary series vaccination and a booster dose at least 4 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. protocol version 1.07:individuals who had previously received mrna primary series vaccination comprising at least 3 vaccine doses with the last one at least 6 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. note: subjects who had received 4 vaccine doses (i.e., a primary series of 3 doses and an additional boost) may also be enrolled under protocol version 1.07). 3. females and males of childbearing potential must be willing to use effective methods of contraception such as hormones (ocp, oral contraceptive pill), condom or occlusive cap with spermicidal agents, intrauterine device (iud), or intrauterine system (ius) as well as to refrain from donating sperm through 28 days following the last vaccination. 4. participant who is willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. 5. for women of child bearing potential (wocbp): a negative pregnancy test on the day of vaccination (visit 1 and visit 4). 6. participant who is willing and able to operate an electronic diary. 7. healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. note: healthy participants with preexisting stable disease, defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the six (6) weeks before enrollment, can be included. 8. capable of giving signed informed consent as described which includes understanding and compliance with the study procedures, requirements and restrictions listed in the informed consent form (icf) and in this protocol. 9. must agree not to enroll in another study of an investigational agent prior to completion of the study.

inclusion criteria: male or female participants 21 years of age, inclusively. protocol versions 1.01-1.06:individuals who previously received mrna primary series vaccination and a booster dose at least 4 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. protocol version 1.07:individuals who had previously received mrna primary series vaccination comprising at least 3 vaccine doses with the last one at least 6 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. note: subjects who had received 4 vaccine doses (i.e., a primary series of 3 doses and an additional boost) may also be enrolled under protocol version 1.07). females and males of childbearing potential must be willing to use effective methods of contraception such as hormones (ocp, oral contraceptive pill), condom or occlusive cap with spermicidal agents, intrauterine device (iud), or intrauterine system (ius) as well as to refrain from donating sperm through 28 days following the last vaccination. participant who is willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. for women of child bearing potential (wocbp): a negative pregnancy test on the day of vaccination (visit 1 and visit 4). participant who is willing and able to operate an electronic diary. healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. note: healthy participants with preexisting stable disease, defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the six (6) weeks before enrollment, can be included. capable of giving signed informed consent as described which includes understanding and compliance with the study procedures, requirements and restrictions listed in the informed consent form (icf) and in this protocol. must agree not to enroll in another study of an investigational agent prior to completion of the study.

inclusion criteria: male or female participants 21 years of age, inclusively. protocol versions 1.01-1.06:individuals who previously received mrna primary series vaccination and a booster dose at least 4 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. protocol version 1.07:individuals who had previously received mrna primary series vaccination comprising at least 3 vaccine doses with the last one at least 6 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. note: subjects who had received 4 vaccine doses (i.e., a primary series of 3 doses and an additional boost) may also be enrolled under protocol version 1.07). females and males of childbearing potential must be willing to use effective methods of contraception such as hormones (ocp, oral contraceptive pill), condom or occlusive cap with spermicidal agents, intrauterine device (iud), or intrauterine system (ius) as well as to refrain from donating sperm through 28 days following the last vaccination. participant who is willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. for women of child bearing potential (wocbp): a negative pregnancy test on the day of vaccination (visit 1 and visit 4). participant who is willing and able to operate an electronic diary. healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. note: healthy participants with preexisting stable disease, defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the six (6) weeks before enrollment, can be included. capable of giving signed informed consent as described which includes understanding and compliance with the study procedures, requirements and restrictions listed in the informed consent form (icf) and in this protocol. must agree not to enroll in another study of an investigational agent prior to completion of the study.

inclusion criteria: male or female participants 21 years of age, inclusively. individuals who previously received mrna primary series vaccination and a booster dose at least 4 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. females and males of childbearing potential must be willing to use effective methods of contraception such as hormones (ocp, oral contraceptive pill), condom or occlusive cap with spermicidal agents, intrauterine device (iud), or intrauterine system (ius) as well as to refrain from donating sperm through 28 days following the last vaccination. participant who is willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. for women of child bearing potential (wocbp): a negative pregnancy test on the day of vaccination (visit 1 and visit 4). participant who is willing and able to operate an electronic diary. healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. note: healthy participants with preexisting stable disease, defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the six (6) weeks before enrollment, can be included. capable of giving signed informed consent as described which includes understanding and compliance with the study procedures, requirements and restrictions listed in the informed consent form (icf) and in this protocol. must agree not to enroll in another study of an investigational agent prior to completion of the study.

inclusion criteria: male or female participants 21 years of age, inclusively. individuals who previously received mrna primary series vaccination and a booster dose at least 4 months prior to day 1. documented confirmation of prior mrna series vaccination receipt must be obtained prior to randomization. females and males of childbearing potential must be willing to use effective methods of contraception such as hormones (ocp, oral contraceptive pill), condom or occlusive cap with spermicidal agents, intrauterine device (iud), or intrauterine system (ius) as well as to refrain from donating sperm through 28 days following the last vaccination. participant who is willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. for women of child bearing potential (wocbp): a negative pregnancy test on the day of vaccination (visit 1 and visit 4). participant who is willing and able to operate an electronic diary. healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. note: healthy participants with preexisting stable disease, defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the six (6) weeks before enrollment, can be included. capable of giving signed informed consent as described which includes understanding and compliance with the study procedures, requirements and restrictions listed in the informed consent form (icf) and in this protocol. must agree not to enroll in another study of an investigational agent prior to completion of the study.