inclusion criteria: 1. adults ≥ 50 years of age at screening. 2. willing and able to give informed consent prior to study enrollment and to comply with study procedures. 3. female participants of childbearing potential (defined as any participant who has experienced menarche and who is not surgically sterile \[ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy\] or postmenopausal \[defined as amenorrhea ≥ 12 consecutive months\]) must agree to be heterosexually inactive from at least 28 days prior to enrollment and through the end of the study or agree to consistently use a medically acceptable method of contraception listed below from ≥ 28 days prior to enrollment and through the end of the study. 4. is medically stable, as determined by the investigator (based on review of health status, vital signs \[to include body temperature\], medical history, and targeted physical examination \[to include body weight\]). vital signs must be within medically acceptable ranges prior to the initial study vaccination. 5. agrees to not participate in any other sars-cov-2 prevention or treatment trials for the duration of the study. 6. have previously received ≥ 3 doses of a covid-19 prototype or bivalent licensed mrnavaccine with the last dose having been given ≥ 90 days previously prior to first study booster.

inclusion criteria: 1. adults ≥ 50 years of age at screening. 2. willing and able to give informed consent prior to study enrollment and to comply with study procedures. 3. female participants of childbearing potential (defined as any participant who has experienced menarche and who is not surgically sterile \[ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy\] or postmenopausal \[defined as amenorrhea ≥ 12 consecutive months\]) must agree to be heterosexually inactive from at least 28 days prior to enrollment and through the end of the study or agree to consistently use a medically acceptable method of contraception listed below from ≥ 28 days prior to enrollment and through the end of the study. 4. is medically stable, as determined by the investigator (based on review of health status, vital signs \[to include body temperature\], medical history, and targeted physical examination \[to include body weight\]). vital signs must be within medically acceptable ranges prior to the initial study vaccination. 5. agrees to not participate in any other sars-cov-2 prevention or treatment trials for the duration of the study. 6. have previously received ≥ 3 doses of a covid-19 prototype or bivalent licensed mrnavaccine with the last dose having been given ≥ 90 days previously prior to first study booster.

inclusion criteria: adults ≥ 50 years of age at screening. willing and able to give informed consent prior to study enrollment and to comply with study procedures. female participants of childbearing potential (defined as any participant who has experienced menarche and who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea ≥ 12 consecutive months]) must agree to be heterosexually inactive from at least 28 days prior to enrollment and through the end of the study or agree to consistently use a medically acceptable method of contraception listed below from ≥ 28 days prior to enrollment and through the end of the study. is medically stable, as determined by the investigator (based on review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). vital signs must be within medically acceptable ranges prior to the initial study vaccination. agrees to not participate in any other sars-cov-2 prevention or treatment trials for the duration of the study. have previously received ≥ 3 doses of a covid-19 prototype or bivalent licensed mrnavaccine with the last dose having been given ≥ 90 days previously prior to first study booster.

inclusion criteria: adults ≥ 50 years of age at screening. willing and able to give informed consent prior to study enrollment and to comply with study procedures. female participants of childbearing potential (defined as any participant who has experienced menarche and who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea ≥ 12 consecutive months]) must agree to be heterosexually inactive from at least 28 days prior to enrollment and through the end of the study or agree to consistently use a medically acceptable method of contraception listed below from ≥ 28 days prior to enrollment and through the end of the study. is medically stable, as determined by the investigator (based on review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). vital signs must be within medically acceptable ranges prior to the initial study vaccination. agrees to not participate in any other sars-cov-2 prevention or treatment trials for the duration of the study. have previously received ≥ 3 doses of a covid-19 prototype or bivalent licensed mrnavaccine with the last dose having been given ≥ 90 days previously prior to first study booster.