inclusion criteria: * have given informed consent by signing the informed consent form (icf) before initiation of any trial-specific procedures. * they must be willing and able to comply with scheduled visits, treatment schedule, laboratory tests, lifestyle restrictions (e.g., to practice social distancing and to follow good practices to reduce their chances of being infected or spreading covid-19), and other requirements of the trial. * they must be able to understand and follow trial-related instructions. * for younger adult cohorts, volunteers must be aged 18 to 55 years, have a body mass index over 19 kg/m\^2 and under 30 kg/m\^2 (i.e., be neither underweight nor obese), and weigh at least 50 kg at visit 0. or for older adult cohorts, volunteers must be aged 56 to 85 years, have a body mass index over 19 kg/m\^2 and under 30 kg/m\^2 (i.e., be neither underweight nor obese), and weigh at least 50 kg at visit 0. * they must be healthy, in the clinical judgment of the investigator, based on medical history, physical examination, 12-lead electrocardiogram (ecg), vital signs (systolic/diastolic blood pressure, pulse rate, body temperature, respiratory rate), and clinical laboratory tests (blood chemistry, hematology, and urine chemistry) at visit 0. * women of childbearing potential (wocbp) must have a negative beta-human chorionic gonadotropin urine test at visit 0 and visit 1. women that are postmenopausal or permanently sterilized will be considered as not having reproductive potential. * wocbp must agree to practice a highly effective form of contraception during the trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. wocbp must agree to require their male partners to use condoms during sexual contact (unless male partners are sterilized or infertile). * wocbp must confirm that they practiced at least one highly effective form of contraception for the 14 days prior to visit 0. * wocbp must agree not to donate eggs (ova, oocytes) for the purposes of assisted reproduction during trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. * men who are sexually active with a wocbp and have not had a vasectomy must agree to practice a highly effective form of contraception with their female partner of childbearing potential during the trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. * men must be willing to refrain from sperm donation, starting after visit 0 and continuously until 60 days after receiving the last immunization. * they must have confirmation of their health insurance coverage prior to visit 0. * they must agree to not be vaccinated during the trial, starting after visit 0 and continuously until 28 days after receiving the last immunization.

inclusion criteria: * have given informed consent by signing the informed consent form (icf) before initiation of any trial-specific procedures. * they must be willing and able to comply with scheduled visits, treatment schedule, laboratory tests, lifestyle restrictions (e.g., to practice social distancing and to follow good practices to reduce their chances of being infected or spreading covid-19), and other requirements of the trial. * they must be able to understand and follow trial-related instructions. * for younger adult cohorts, volunteers must be aged 18 to 55 years, have a body mass index over 19 kg/m\^2 and under 30 kg/m\^2 (i.e., be neither underweight nor obese), and weigh at least 50 kg at visit 0. or for older adult cohorts, volunteers must be aged 56 to 85 years, have a body mass index over 19 kg/m\^2 and under 30 kg/m\^2 (i.e., be neither underweight nor obese), and weigh at least 50 kg at visit 0. * they must be healthy, in the clinical judgment of the investigator, based on medical history, physical examination, 12-lead electrocardiogram (ecg), vital signs (systolic/diastolic blood pressure, pulse rate, body temperature, respiratory rate), and clinical laboratory tests (blood chemistry, hematology, and urine chemistry) at visit 0. * women of childbearing potential (wocbp) must have a negative beta-human chorionic gonadotropin urine test at visit 0 and visit 1. women that are postmenopausal or permanently sterilized will be considered as not having reproductive potential. * wocbp must agree to practice a highly effective form of contraception during the trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. wocbp must agree to require their male partners to use condoms during sexual contact (unless male partners are sterilized or infertile). * wocbp must confirm that they practiced at least one highly effective form of contraception for the 14 days prior to visit 0. * wocbp must agree not to donate eggs (ova, oocytes) for the purposes of assisted reproduction during trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. * men who are sexually active with a wocbp and have not had a vasectomy must agree to practice a highly effective form of contraception with their female partner of childbearing potential during the trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. * men must be willing to refrain from sperm donation, starting after visit 0 and continuously until 60 days after receiving the last immunization. * they must have confirmation of their health insurance coverage prior to visit 0. * they must agree to not be vaccinated during the trial, starting after visit 0 and continuously until 28 days after receiving the last immunization.

inclusion criteria: - have given informed consent by signing the informed consent form (icf) before initiation of any trial-specific procedures. - they must be willing and able to comply with scheduled visits, treatment schedule, laboratory tests, lifestyle restrictions (e.g., to practice social distancing and to follow good practices to reduce their chances of being infected or spreading covid-19), and other requirements of the trial. - they must be able to understand and follow trial-related instructions. - for younger adult cohorts, volunteers must be aged 18 to 55 years, have a body mass index over 19 kg/m^2 and under 30 kg/m^2 (i.e., be neither underweight nor obese), and weigh at least 50 kg at visit 0. or for older adult cohorts, volunteers must be aged 56 to 85 years, have a body mass index over 19 kg/m^2 and under 30 kg/m^2 (i.e., be neither underweight nor obese), and weigh at least 50 kg at visit 0. - they must be healthy, in the clinical judgment of the investigator, based on medical history, physical examination, 12-lead electrocardiogram (ecg), vital signs (systolic/diastolic blood pressure, pulse rate, body temperature, respiratory rate), and clinical laboratory tests (blood chemistry, hematology, and urine chemistry) at visit 0. - women of childbearing potential (wocbp) must have a negative beta-human chorionic gonadotropin urine test at visit 0 and visit 1. women that are postmenopausal or permanently sterilized will be considered as not having reproductive potential. - wocbp must agree to practice a highly effective form of contraception during the trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. wocbp must agree to require their male partners to use condoms during sexual contact (unless male partners are sterilized or infertile). - wocbp must confirm that they practiced at least one highly effective form of contraception for the 14 days prior to visit 0. - wocbp must agree not to donate eggs (ova, oocytes) for the purposes of assisted reproduction during trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. - men who are sexually active with a wocbp and have not had a vasectomy must agree to practice a highly effective form of contraception with their female partner of childbearing potential during the trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. - men must be willing to refrain from sperm donation, starting after visit 0 and continuously until 60 days after receiving the last immunization. - they must have confirmation of their health insurance coverage prior to visit 0. - they must agree to not be vaccinated during the trial, starting after visit 0 and continuously until 28 days after receiving the last immunization.

inclusion criteria: - have given informed consent by signing the informed consent form (icf) before initiation of any trial-specific procedures. - they must be willing and able to comply with scheduled visits, treatment schedule, laboratory tests, lifestyle restrictions (e.g., to practice social distancing and to follow good practices to reduce their chances of being infected or spreading covid-19), and other requirements of the trial. - they must be able to understand and follow trial-related instructions. - for younger adult cohorts, volunteers must be aged 18 to 55 years, have a body mass index over 19 kg/m^2 and under 30 kg/m^2 (i.e., be neither underweight nor obese), and weigh at least 50 kg at visit 0. or for older adult cohorts, volunteers must be aged 56 to 85 years, have a body mass index over 19 kg/m^2 and under 30 kg/m^2 (i.e., be neither underweight nor obese), and weigh at least 50 kg at visit 0. - they must be healthy, in the clinical judgment of the investigator, based on medical history, physical examination, 12-lead electrocardiogram (ecg), vital signs (systolic/diastolic blood pressure, pulse rate, body temperature, respiratory rate), and clinical laboratory tests (blood chemistry, hematology, and urine chemistry) at visit 0. - women of childbearing potential (wocbp) must have a negative beta-human chorionic gonadotropin urine test at visit 0 and visit 1. women that are postmenopausal or permanently sterilized will be considered as not having reproductive potential. - wocbp must agree to practice a highly effective form of contraception during the trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. wocbp must agree to require their male partners to use condoms during sexual contact (unless male partners are sterilized or infertile). - wocbp must confirm that they practiced at least one highly effective form of contraception for the 14 days prior to visit 0. - wocbp must agree not to donate eggs (ova, oocytes) for the purposes of assisted reproduction during trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. - men who are sexually active with a wocbp and have not had a vasectomy must agree to practice a highly effective form of contraception with their female partner of childbearing potential during the trial, starting after visit 0 and continuously until 60 days after receiving the last immunization. - men must be willing to refrain from sperm donation, starting after visit 0 and continuously until 60 days after receiving the last immunization. - they must have confirmation of their health insurance coverage prior to visit 0. - they must agree to not be vaccinated during the trial, starting after visit 0 and continuously until 28 days after receiving the last immunization.