inclusion criteria: 1. healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. 2. willing and able to give informed consent prior to study enrollment. 3. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. 4. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. 5. women of childbearing potential (defined as any female participant who is not surgically sterile \[ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy\] or postmenopausal \[defined as amenorrhea at least 12 consecutive months\]) 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 at least 28 days prior to enrollment and through the end of the study. 1. condoms (male or female) with spermicide (if acceptable in-country) 2. diaphragm with spermicide 3. cervical cap with spermicide 4. intrauterine device 5. oral or patch contraceptives 6. norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy 7. abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle 6. participants must be healthy and medically stable, as determined by the investigator (based on a review of health status, vital signs \[to include body temperature\], medical history, and targeted physical examination \[to include body weight\]). participants must have a body mass index (bmi) of 17 to 34 kg/m2, inclusive, at screening. vital signs must be within medically acceptable ranges prior to the first vaccination. 7. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

inclusion criteria: 1. healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. 2. willing and able to give informed consent prior to study enrollment. 3. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. 4. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. 5. women of childbearing potential (defined as any female participant who is not surgically sterile \[ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy\] or postmenopausal \[defined as amenorrhea at least 12 consecutive months\]) 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 at least 28 days prior to enrollment and through the end of the study. 1. condoms (male or female) with spermicide (if acceptable in-country) 2. diaphragm with spermicide 3. cervical cap with spermicide 4. intrauterine device 5. oral or patch contraceptives 6. norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy 7. abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle 6. participants must be healthy and medically stable, as determined by the investigator (based on a review of health status, vital signs \[to include body temperature\], medical history, and targeted physical examination \[to include body weight\]). participants must have a body mass index (bmi) of 17 to 34 kg/m2, inclusive, at screening. vital signs must be within medically acceptable ranges prior to the first vaccination. 7. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

inclusion criteria: healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. willing and able to give informed consent prior to study enrollment. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. condoms (male or female) with spermicide (if acceptable in-country) diaphragm with spermicide cervical cap with spermicide intrauterine device oral or patch contraceptives norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle participants must be healthy and medically stable, as determined by the investigator (based on a review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). participants must have a body mass index (bmi) of 17 to 34 kg/m2, inclusive, at screening. vital signs must be within medically acceptable ranges prior to the first vaccination. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

inclusion criteria: healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. willing and able to give informed consent prior to study enrollment. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. condoms (male or female) with spermicide (if acceptable in-country) diaphragm with spermicide cervical cap with spermicide intrauterine device oral or patch contraceptives norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle participants must be healthy and medically stable, as determined by the investigator (based on a review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). participants must have a body mass index (bmi) of 17 to 34 kg/m2, inclusive, at screening. vital signs must be within medically acceptable ranges prior to the first vaccination. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. willing and able to give informed consent prior to study enrollment. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. condoms (male or female) with spermicide (if acceptable in-country) diaphragm with spermicide cervical cap with spermicide intrauterine device oral or patch contraceptives norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle participants must be healthy and medically stable, as determined by the investigator (based on a review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). participants must have a body mass index (bmi) of 17 to 34 kg/m2, inclusive, at screening. vital signs must be within medically acceptable ranges prior to the first vaccination. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. willing and able to give informed consent prior to study enrollment. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. condoms (male or female) with spermicide (if acceptable in-country) diaphragm with spermicide cervical cap with spermicide intrauterine device oral or patch contraceptives norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle participants must be healthy and medically stable, as determined by the investigator (based on a review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). participants must have a body mass index (bmi) of 17 to 34 kg/m2, inclusive, at screening. vital signs must be within medically acceptable ranges prior to the first vaccination. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

1. healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. 2. willing and able to give informed consent prior to study enrollment. 3. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. 4. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. 5. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. 1. condoms (male or female) with spermicide (if acceptable in-country) 2. diaphragm with spermicide 3. cervical cap with spermicide 4. intrauterine device 5. oral or patch contraceptives 6. norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy 7. abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle 6. participants must be healthy and medically stable, as determined by the investigator (based on a review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). participants must have a body mass index (bmi) of 17 to 34 kg/m2, inclusive, at screening. vital signs must be within medically acceptable ranges prior to the first vaccination. 7. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

1. healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. 2. willing and able to give informed consent prior to study enrollment. 3. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. 4. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. 5. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. 1. condoms (male or female) with spermicide (if acceptable in-country) 2. diaphragm with spermicide 3. cervical cap with spermicide 4. intrauterine device 5. oral or patch contraceptives 6. norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy 7. abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle 6. participants must be healthy and medically stable, as determined by the investigator (based on a review of health status, vital signs [to include body temperature], medical history, and targeted physical examination [to include body weight]). participants must have a body mass index (bmi) of 17 to 34 kg/m2, inclusive, at screening. vital signs must be within medically acceptable ranges prior to the first vaccination. 7. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

1. healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. 2. willing and able to give informed consent prior to study enrollment. 3. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. 4. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. 5. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. 1. condoms (male or female) with spermicide (if acceptable in-country) 2. diaphragm with spermicide 3. cervical cap with spermicide 4. intrauterine device 5. oral or patch contraceptives 6. norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy 7. abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle 6. participants must be healthy and medically stable, as determined by the investigator (based on a 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 first vaccination. 7. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

1. healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. 2. willing and able to give informed consent prior to study enrollment. 3. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. 4. participants must have been baseline seropositive to sars-cov-2 defined as either: • having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 8 weeks prior to enrollment (first study vaccination). or • previously infected with sars cov-2 ≥ 8 weeks prior to enrollment (first study vaccination). note: baseline sars-cov-2 serostatus determination at screening will be based on vaccination documentation (eg, vaccination card or vaccination registry) or participants' report of a previous sars-cov-2 infection. 5. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. 1. condoms (male or female) with spermicide (if acceptable in-country) 2. diaphragm with spermicide 3. cervical cap with spermicide 4. intrauterine device 5. oral or patch contraceptives 6. norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy 7. abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle 6. participants must be healthy and medically stable, as determined by the investigator (based on a 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 first vaccination. 7. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

inclusion criteria: 1. healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. 2. willing and able to give informed consent prior to study enrollment. 3. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. 4. participants must either have been: - baseline seronegative to sars-cov-2 defined as never infected with sars-cov-2 and never having previously received any dose of an authorized or investigational covid-19 vaccine, or - baseline seropositive to sars-cov-2 defined as having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 3 months prior to first study vaccination; or previously infected with sars-cov-2 ≥ 3 months prior to first study vaccination. 5. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. 1. condoms (male or female) with spermicide (if acceptable in-country) 2. diaphragm with spermicide 3. cervical cap with spermicide 4. intrauterine device 5. oral or patch contraceptives 6. norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy 7. abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle 6. participants must be healthy and medically stable, as determined by the investigator (based on a 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 first vaccination. 7. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

inclusion criteria: 1. healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. 2. willing and able to give informed consent prior to study enrollment. 3. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. 4. participants must either have been: - baseline seronegative to sars-cov-2 defined as never infected with sars-cov-2 and never having previously received any dose of an authorized or investigational covid-19 vaccine, or - baseline seropositive to sars-cov-2 defined as having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 3 months prior to first study vaccination; or previously infected with sars-cov-2 ≥ 3 months prior to first study vaccination. 5. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. 1. condoms (male or female) with spermicide (if acceptable in-country) 2. diaphragm with spermicide 3. cervical cap with spermicide 4. intrauterine device 5. oral or patch contraceptives 6. norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy 7. abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle 6. participants must be healthy and medically stable, as determined by the investigator (based on a 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 first vaccination. 7. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

inclusion criteria: healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. willing and able to give informed consent prior to study enrollment. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. participants must either have been: baseline seronegative to sars-cov-2 defined as never infected with sars-cov-2 and never having previously received any dose of an authorized or investigational covid-19 vaccine, or baseline seropositive to sars-cov-2 defined as having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 3 months prior to first study vaccination; or previously infected with sars-cov-2 ≥ 3 months prior to first study vaccination. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. condoms (male or female) with spermicide (if acceptable in-country) diaphragm with spermicide cervical cap with spermicide intrauterine device oral or patch contraceptives norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle participants must be healthy and medically stable, as determined by the investigator (based on a 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 first vaccination. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.

inclusion criteria: healthy, medically stable adult males or females ≥ 50 to ≤ 70 years of age at screening. willing and able to give informed consent prior to study enrollment. able to attend study visits, comply with study requirements, and provide reliable and complete reports of aes. participants must either have been: baseline seronegative to sars-cov-2 defined as never infected with sars-cov-2 and never having previously received any dose of an authorized or investigational covid-19 vaccine, or baseline seropositive to sars-cov-2 defined as having completed a primary vaccination series against sars-cov-2 with an authorized covid-19 vaccine with receipt of second/final dose of authorized vaccine ≥ 3 months prior to first study vaccination; or previously infected with sars-cov-2 ≥ 3 months prior to first study vaccination. women of childbearing potential (defined as any female participant who is not surgically sterile [ie, hysterectomy, bilateral tubal ligation, or bilateral oophorectomy] or postmenopausal [defined as amenorrhea at least 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 at least 28 days prior to enrollment and through the end of the study. condoms (male or female) with spermicide (if acceptable in-country) diaphragm with spermicide cervical cap with spermicide intrauterine device oral or patch contraceptives norplant®, depo-provera®, or other in-country regulatory approved contraceptive method that is designed to protect against pregnancy abstinence, as a form of contraception, is acceptable if in line with the participant's lifestyle participants must be healthy and medically stable, as determined by the investigator (based on a 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 first vaccination. participants must agree to not participate in any other sars-cov-2 or influenza prevention or treatment studies for the duration of the study. note: for participants who become hospitalized with covid-19, participation in investigational treatment studies is permitted.