inclusion criteria: 1. subjects must have read, understood, and signed the informed consent form (icf) prior to participating in the study; subjects must also complete study-related procedures and the subjects must communicate with the study staff at visits and by phone during the study; 2. at the screening visit (visit 1), male and female subjects must be: - study populations #1: 18 to 64 (has not yet had his/her 65th birthday) years of age, inclusive; - study population #2: 65 years of age or older; - study population #3: 18 years of age or older; 3. at screening (visit 1) and vaccination (visit 2), subject must have a body mass index (bmi) of: • study populations #1 and #2: ≥ 18.5 and < 30 kg/m2; 4. subjects are considered by the investigator to be reliable and likely to cooperate with the assessment procedures and be available for the duration of the study; 5. study populations #1: subjects must be in good general health prior to study participation, with no clinically relevant abnormalities that could jeopardize subject safety or interfere with study assessments, as determined by medical history, physical examination, and vital signs. investigator discretion will be permitted with this inclusion criterion; note: subjects with a pre-existing chronic disease will be allowed to participate if the disease is stable and, according to the investigator's judgment that must be documented in the source documents, the condition is unlikely to confound the results of the study or pose additional risk to the subject by participating in the study. stable disease is generally defined as no new onset or exacerbation of pre-existing chronic disease three months prior to vaccination. based on the investigator's judgment, a subject with more recent stabilization of a disease could also be eligible. 6. study populations #1 and #3: female subjects of childbearing potential must have a negative serum pregnancy test result at screening (visit 1) and a negative urine pregnancy test result at vaccination (visit 2): non-childbearing females are defined as: - surgically-sterile (defined as bilateral tubal ligation, hysterectomy or bilateral oophorectomy performed more than one month prior to the first study vaccination); or - post-menopausal (absence of menses for 12 consecutive months and age consistent with natural cessation of ovulation); 7. study populations #1 and #3: female subjects of childbearing potential must use an effective method of contraception for one month prior to vaccination (visit 2) and agree to continue employing highly effective birth control measures for at least one month after the last study vaccination (or in the case of early termination, she must not plan to become pregnant for at least one month after her last study vaccination), with the exception of the following subjects: the following relationship or methods of contraception are considered to be highly effective: - combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation: - oral; - intravaginal; - transdermal; - progestogen-only hormonal contraception associated with inhibition of ovulation: - oral; - injectable; - implantable; - intra-uterine device with or without hormonal release; - credible self-reported history of heterosexual vaginal intercourse abstinence prior to and for at least one month after the last study vaccination. abstinent subjects who are ovulating should be asked what method(s) they would use should their circumstances change, and subjects without a well-defined plan should be excluded; - female partner. 8. study population #2: subjects must be non-institutionalized (e.g. not living in rehabilitation centres or old-age homes; living in an elderly community is acceptable) and have no acute or evolving medical problems prior to study participation and no clinically relevant abnormalities that could jeopardize subject safety or interfere with study assessments, as assessed by the principal investigator or sub-investigator (thereafter referred as investigator) and determined by medical history, physical examination, serology, clinical chemistry and haematology tests, urinalysis, and vital signs. investigator discretion will be permitted with this inclusion criterion. note: subjects with a pre-existing chronic disease will be allowed to participate if the disease is stable and, according to the investigator's judgment that must be documented in the source documents, the condition is unlikely to confound the results of the study or pose additional risk to the subject by participating in the study. stable disease is generally defined as no new onset or exacerbation of pre-existing chronic disease three months prior to vaccination. based on the investigator's judgment and documented in source documentation, a subject with more recent stabilization of a disease could also be eligible; 9. study population #3: subjects must have one or more co-morbid conditions that puts them at higher risk for severe covid-19 disease. these co-morbidities include but are not limited to obesity, documented hypertension, type-1 or type 2 diabetes, chronic obstructive lung diseases (copd), cardiovascular disease, chronic kidney, or be immunocompromised persons (e.g., hiv-infected patients, organ transplant recipients, or patients receiving cancer chemotherapy). investigator discretion will be permitted with this inclusion criterion.

inclusion criteria: 1. subjects must have read, understood, and signed the informed consent form (icf) prior to participating in the study; subjects must also complete study-related procedures and the subjects must communicate with the study staff at visits and by phone during the study; 2. at the screening visit (visit 1), male and female subjects must be: - study populations #1: 18 to 64 (has not yet had his/her 65th birthday) years of age, inclusive; - study population #2: 65 years of age or older; - study population #3: 18 years of age or older; 3. at screening (visit 1) and vaccination (visit 2), subject must have a body mass index (bmi) of: • study populations #1 and #2: ≥ 18.5 and < 30 kg/m2; 4. subjects are considered by the investigator to be reliable and likely to cooperate with the assessment procedures and be available for the duration of the study; 5. study populations #1: subjects must be in good general health prior to study participation, with no clinically relevant abnormalities that could jeopardize subject safety or interfere with study assessments, as determined by medical history, physical examination, and vital signs. investigator discretion will be permitted with this inclusion criterion; note: subjects with a pre-existing chronic disease will be allowed to participate if the disease is stable and, according to the investigator's judgment that must be documented in the source documents, the condition is unlikely to confound the results of the study or pose additional risk to the subject by participating in the study. stable disease is generally defined as no new onset or exacerbation of pre-existing chronic disease three months prior to vaccination. based on the investigator's judgment, a subject with more recent stabilization of a disease could also be eligible. 6. study populations #1 and #3: female subjects of childbearing potential must have a negative serum pregnancy test result at screening (visit 1) and a negative urine pregnancy test result at vaccination (visit 2): non-childbearing females are defined as: - surgically-sterile (defined as bilateral tubal ligation, hysterectomy or bilateral oophorectomy performed more than one month prior to the first study vaccination); or - post-menopausal (absence of menses for 12 consecutive months and age consistent with natural cessation of ovulation); 7. study populations #1 and #3: female subjects of childbearing potential must use an effective method of contraception for one month prior to vaccination (visit 2) and agree to continue employing highly effective birth control measures for at least one month after the last study vaccination (or in the case of early termination, she must not plan to become pregnant for at least one month after her last study vaccination), with the exception of the following subjects: the following relationship or methods of contraception are considered to be highly effective: - combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation: - oral; - intravaginal; - transdermal; - progestogen-only hormonal contraception associated with inhibition of ovulation: - oral; - injectable; - implantable; - intra-uterine device with or without hormonal release; - credible self-reported history of heterosexual vaginal intercourse abstinence prior to and for at least one month after the last study vaccination. abstinent subjects who are ovulating should be asked what method(s) they would use should their circumstances change, and subjects without a well-defined plan should be excluded; - female partner. 8. study population #2: subjects must be non-institutionalized (e.g. not living in rehabilitation centres or old-age homes; living in an elderly community is acceptable) and have no acute or evolving medical problems prior to study participation and no clinically relevant abnormalities that could jeopardize subject safety or interfere with study assessments, as assessed by the principal investigator or sub-investigator (thereafter referred as investigator) and determined by medical history, physical examination, serology, clinical chemistry and haematology tests, urinalysis, and vital signs. investigator discretion will be permitted with this inclusion criterion. note: subjects with a pre-existing chronic disease will be allowed to participate if the disease is stable and, according to the investigator's judgment that must be documented in the source documents, the condition is unlikely to confound the results of the study or pose additional risk to the subject by participating in the study. stable disease is generally defined as no new onset or exacerbation of pre-existing chronic disease three months prior to vaccination. based on the investigator's judgment and documented in source documentation, a subject with more recent stabilization of a disease could also be eligible; 9. study population #3: subjects must have one or more co-morbid conditions that puts them at higher risk for severe covid-19 disease. these co-morbidities include but are not limited to obesity, documented hypertension, type-1 or type 2 diabetes, chronic obstructive lung diseases (copd), cardiovascular disease, chronic kidney, or be immunocompromised persons (e.g., hiv-infected patients, organ transplant recipients, or patients receiving cancer chemotherapy). investigator discretion will be permitted with this inclusion criterion.