inclusion criteria: - efficacy cohort: each subject must meet all of the following criteria to be enrolled into the efficacy cohort: 1. subject is able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments. 2. adult males and females 18 years of age or older on the day of signing the icf (or the legal age of consent in the jurisdiction in which the study is taking place) 3. had document of previous primary vaccination of 2 doses of comirnaty® at least 3 months on the day of signing the icf 4. sars-cov-2 reverse transcription-polymerase chain reaction (rt-pcr)-negative at screening. 5. body mass index of ≥ 18 and ≤ 30 kg/m2 at screening. 6. women of childbearing potential (wocbp) and men whose sexual partners are wocbp must be able and willing to use at least 1 highly effective method of contraception (i.e., including hysterectomy, bilateral salpingectomy, and bilateral oophorectomy, hormonal oral \[in combination with male condoms with spermicide\], transdermal, implant, or injection, barrier \[i.e., condom, diaphragm with spermicide\]; intrauterine device; vasectomized partner \[6 months minimum\], clinically sterile partner; or abstinence) during the study or 6 months after the last study vaccine, whichever later. * a female subject is considered a wocbp after menarche and until she is in a postmenopausal state for 12 consecutive months (without an alternative medical cause) or otherwise permanently sterile. * subjects not of childbearing potential are not required to use any other forms of contraception during the study. non-childbearing potential is defined as subject confirmed: i. surgical sterilization (e.g., bilateral oophorectomy, bilateral salpingectomy, bilateral occlusion by cautery \[essure system® is not acceptable\], hysterectomy, or tubal ligation). ii. postmenopausal (defined as permanent cessation of menstruation for at least 12 consecutive months prior to screening); if postmenopausal status is unclear, pregnancy tests will be performed prior to vaccinations. 7. wocbp must have a negative pregnancy test before vaccination. if menopausal status is unclear, a pregnancy test is required. safety cohort: each subject must meet all of the following criteria to be enrolled into the safety cohort: 1. subject is able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments. 2. adult males and females 18 years of age or older on the day of signing the icf (or the legal age of consent in the jurisdiction in which the study is taking place) in general good health before study participation with no clinically relevant abnormalities (at investigator's discretion) that could interfere with study assessments. subjects may have stable comorbidities (no change in medications, exacerbations, or hospitalizations in the past 3 months). 3. have previously received any full primary series approved by who emergency use authorization at least 3 months prior to enrollment or who have already received one authorized booster vaccination and planned to receive ptx-covid19-b as the 4th shot. 4. sars-cov-2 reverse transcription-polymerase chain reaction (rt-pcr)-negative at screening. 5. body mass index of ≥ 18 and ≤ 30 kg/m2 at screening. 6. women of childbearing potential (wocbp) and men whose sexual partners are wocbp must be able and willing to use at least 1 highly effective method of contraception (i.e., including hysterectomy, bilateral salpingectomy, and bilateral oophorectomy, hormonal oral \[in combination with male condoms with spermicide\], transdermal, implant, or injection, barrier \[i.e., condom, diaphragm with spermicide\]; intrauterine device; vasectomized partner \[6 months minimum\], clinically sterile partner; or abstinence) during the study or 6 months after the last study vaccine, whichever later. * a female subject is considered a wocbp after menarche and until she is in a postmenopausal state for 12 consecutive months (without an alternative medical cause) or otherwise permanently sterile. * subjects not of childbearing potential are not required to use any other forms of contraception during the study. non-childbearing potential is defined as subject confirmed: i. surgical sterilization (e.g., bilateral oophorectomy, bilateral salpingectomy, bilateral occlusion by cautery \[essure system® is not acceptable\], hysterectomy, or tubal ligation). ii. postmenopausal (defined as permanent cessation of menstruation for at least 12 consecutive months prior to screening); if postmenopausal status is unclear, pregnancy tests will be performed prior to vaccinations. 7. wocbp must have a negative pregnancy test before vaccination. if menopausal status is unclear, a pregnancy test is required.

inclusion criteria: - efficacy cohort: each subject must meet all of the following criteria to be enrolled into the efficacy cohort: 1. subject is able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments. 2. adult males and females 18 years of age or older on the day of signing the icf (or the legal age of consent in the jurisdiction in which the study is taking place) 3. had document of previous primary vaccination of 2 doses of comirnaty® at least 3 months on the day of signing the icf 4. sars-cov-2 reverse transcription-polymerase chain reaction (rt-pcr)-negative at screening. 5. body mass index of ≥ 18 and ≤ 30 kg/m2 at screening. 6. women of childbearing potential (wocbp) and men whose sexual partners are wocbp must be able and willing to use at least 1 highly effective method of contraception (i.e., including hysterectomy, bilateral salpingectomy, and bilateral oophorectomy, hormonal oral \[in combination with male condoms with spermicide\], transdermal, implant, or injection, barrier \[i.e., condom, diaphragm with spermicide\]; intrauterine device; vasectomized partner \[6 months minimum\], clinically sterile partner; or abstinence) during the study or 6 months after the last study vaccine, whichever later. * a female subject is considered a wocbp after menarche and until she is in a postmenopausal state for 12 consecutive months (without an alternative medical cause) or otherwise permanently sterile. * subjects not of childbearing potential are not required to use any other forms of contraception during the study. non-childbearing potential is defined as subject confirmed: i. surgical sterilization (e.g., bilateral oophorectomy, bilateral salpingectomy, bilateral occlusion by cautery \[essure system® is not acceptable\], hysterectomy, or tubal ligation). ii. postmenopausal (defined as permanent cessation of menstruation for at least 12 consecutive months prior to screening); if postmenopausal status is unclear, pregnancy tests will be performed prior to vaccinations. 7. wocbp must have a negative pregnancy test before vaccination. if menopausal status is unclear, a pregnancy test is required. safety cohort: each subject must meet all of the following criteria to be enrolled into the safety cohort: 1. subject is able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments. 2. adult males and females 18 years of age or older on the day of signing the icf (or the legal age of consent in the jurisdiction in which the study is taking place) in general good health before study participation with no clinically relevant abnormalities (at investigator's discretion) that could interfere with study assessments. subjects may have stable comorbidities (no change in medications, exacerbations, or hospitalizations in the past 3 months). 3. have previously received any full primary series approved by who emergency use authorization at least 3 months prior to enrollment or who have already received one authorized booster vaccination and planned to receive ptx-covid19-b as the 4th shot. 4. sars-cov-2 reverse transcription-polymerase chain reaction (rt-pcr)-negative at screening. 5. body mass index of ≥ 18 and ≤ 30 kg/m2 at screening. 6. women of childbearing potential (wocbp) and men whose sexual partners are wocbp must be able and willing to use at least 1 highly effective method of contraception (i.e., including hysterectomy, bilateral salpingectomy, and bilateral oophorectomy, hormonal oral \[in combination with male condoms with spermicide\], transdermal, implant, or injection, barrier \[i.e., condom, diaphragm with spermicide\]; intrauterine device; vasectomized partner \[6 months minimum\], clinically sterile partner; or abstinence) during the study or 6 months after the last study vaccine, whichever later. * a female subject is considered a wocbp after menarche and until she is in a postmenopausal state for 12 consecutive months (without an alternative medical cause) or otherwise permanently sterile. * subjects not of childbearing potential are not required to use any other forms of contraception during the study. non-childbearing potential is defined as subject confirmed: i. surgical sterilization (e.g., bilateral oophorectomy, bilateral salpingectomy, bilateral occlusion by cautery \[essure system® is not acceptable\], hysterectomy, or tubal ligation). ii. postmenopausal (defined as permanent cessation of menstruation for at least 12 consecutive months prior to screening); if postmenopausal status is unclear, pregnancy tests will be performed prior to vaccinations. 7. wocbp must have a negative pregnancy test before vaccination. if menopausal status is unclear, a pregnancy test is required.

inclusion criteria: - efficacy cohort: each subject must meet all of the following criteria to be enrolled into the efficacy cohort: subject is able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments. adult males and females 18 years of age or older on the day of signing the icf (or the legal age of consent in the jurisdiction in which the study is taking place) had document of previous primary vaccination of 2 doses of comirnaty® at least 3 months on the day of signing the icf sars-cov-2 reverse transcription-polymerase chain reaction (rt-pcr)-negative at screening. body mass index of ≥ 18 and ≤ 30 kg/m2 at screening. women of childbearing potential (wocbp) and men whose sexual partners are wocbp must be able and willing to use at least 1 highly effective method of contraception (i.e., including hysterectomy, bilateral salpingectomy, and bilateral oophorectomy, hormonal oral [in combination with male condoms with spermicide], transdermal, implant, or injection, barrier [i.e., condom, diaphragm with spermicide]; intrauterine device; vasectomized partner [6 months minimum], clinically sterile partner; or abstinence) during the study or 6 months after the last study vaccine, whichever later. a female subject is considered a wocbp after menarche and until she is in a postmenopausal state for 12 consecutive months (without an alternative medical cause) or otherwise permanently sterile. subjects not of childbearing potential are not required to use any other forms of contraception during the study. non-childbearing potential is defined as subject confirmed: i. surgical sterilization (e.g., bilateral oophorectomy, bilateral salpingectomy, bilateral occlusion by cautery [essure system® is not acceptable], hysterectomy, or tubal ligation). ii. postmenopausal (defined as permanent cessation of menstruation for at least 12 consecutive months prior to screening); if postmenopausal status is unclear, pregnancy tests will be performed prior to vaccinations. wocbp must have a negative pregnancy test before vaccination. if menopausal status is unclear, a pregnancy test is required. safety cohort: each subject must meet all of the following criteria to be enrolled into the safety cohort: subject is able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments. adult males and females 18 years of age or older on the day of signing the icf (or the legal age of consent in the jurisdiction in which the study is taking place) in general good health before study participation with no clinically relevant abnormalities (at investigator's discretion) that could interfere with study assessments. subjects may have stable comorbidities (no change in medications, exacerbations, or hospitalizations in the past 3 months). have previously received any full primary series approved by who emergency use authorization at least 3 months prior to enrollment or who have already received one authorized booster vaccination and planned to receive ptx-covid19-b as the 4th shot. sars-cov-2 reverse transcription-polymerase chain reaction (rt-pcr)-negative at screening. body mass index of ≥ 18 and ≤ 30 kg/m2 at screening. women of childbearing potential (wocbp) and men whose sexual partners are wocbp must be able and willing to use at least 1 highly effective method of contraception (i.e., including hysterectomy, bilateral salpingectomy, and bilateral oophorectomy, hormonal oral [in combination with male condoms with spermicide], transdermal, implant, or injection, barrier [i.e., condom, diaphragm with spermicide]; intrauterine device; vasectomized partner [6 months minimum], clinically sterile partner; or abstinence) during the study or 6 months after the last study vaccine, whichever later. a female subject is considered a wocbp after menarche and until she is in a postmenopausal state for 12 consecutive months (without an alternative medical cause) or otherwise permanently sterile. subjects not of childbearing potential are not required to use any other forms of contraception during the study. non-childbearing potential is defined as subject confirmed: i. surgical sterilization (e.g., bilateral oophorectomy, bilateral salpingectomy, bilateral occlusion by cautery [essure system® is not acceptable], hysterectomy, or tubal ligation). ii. postmenopausal (defined as permanent cessation of menstruation for at least 12 consecutive months prior to screening); if postmenopausal status is unclear, pregnancy tests will be performed prior to vaccinations. wocbp must have a negative pregnancy test before vaccination. if menopausal status is unclear, a pregnancy test is required.

inclusion criteria: - efficacy cohort: each subject must meet all of the following criteria to be enrolled into the efficacy cohort: subject is able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments. adult males and females 18 years of age or older on the day of signing the icf (or the legal age of consent in the jurisdiction in which the study is taking place) had document of previous primary vaccination of 2 doses of comirnaty® at least 3 months on the day of signing the icf sars-cov-2 reverse transcription-polymerase chain reaction (rt-pcr)-negative at screening. body mass index of ≥ 18 and ≤ 30 kg/m2 at screening. women of childbearing potential (wocbp) and men whose sexual partners are wocbp must be able and willing to use at least 1 highly effective method of contraception (i.e., including hysterectomy, bilateral salpingectomy, and bilateral oophorectomy, hormonal oral [in combination with male condoms with spermicide], transdermal, implant, or injection, barrier [i.e., condom, diaphragm with spermicide]; intrauterine device; vasectomized partner [6 months minimum], clinically sterile partner; or abstinence) during the study or 6 months after the last study vaccine, whichever later. a female subject is considered a wocbp after menarche and until she is in a postmenopausal state for 12 consecutive months (without an alternative medical cause) or otherwise permanently sterile. subjects not of childbearing potential are not required to use any other forms of contraception during the study. non-childbearing potential is defined as subject confirmed: i. surgical sterilization (e.g., bilateral oophorectomy, bilateral salpingectomy, bilateral occlusion by cautery [essure system® is not acceptable], hysterectomy, or tubal ligation). ii. postmenopausal (defined as permanent cessation of menstruation for at least 12 consecutive months prior to screening); if postmenopausal status is unclear, pregnancy tests will be performed prior to vaccinations. wocbp must have a negative pregnancy test before vaccination. if menopausal status is unclear, a pregnancy test is required. safety cohort: each subject must meet all of the following criteria to be enrolled into the safety cohort: subject is able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments. adult males and females 18 years of age or older on the day of signing the icf (or the legal age of consent in the jurisdiction in which the study is taking place) in general good health before study participation with no clinically relevant abnormalities (at investigator's discretion) that could interfere with study assessments. subjects may have stable comorbidities (no change in medications, exacerbations, or hospitalizations in the past 3 months). have previously received any full primary series approved by who emergency use authorization at least 3 months prior to enrollment or who have already received one authorized booster vaccination and planned to receive ptx-covid19-b as the 4th shot. sars-cov-2 reverse transcription-polymerase chain reaction (rt-pcr)-negative at screening. body mass index of ≥ 18 and ≤ 30 kg/m2 at screening. women of childbearing potential (wocbp) and men whose sexual partners are wocbp must be able and willing to use at least 1 highly effective method of contraception (i.e., including hysterectomy, bilateral salpingectomy, and bilateral oophorectomy, hormonal oral [in combination with male condoms with spermicide], transdermal, implant, or injection, barrier [i.e., condom, diaphragm with spermicide]; intrauterine device; vasectomized partner [6 months minimum], clinically sterile partner; or abstinence) during the study or 6 months after the last study vaccine, whichever later. a female subject is considered a wocbp after menarche and until she is in a postmenopausal state for 12 consecutive months (without an alternative medical cause) or otherwise permanently sterile. subjects not of childbearing potential are not required to use any other forms of contraception during the study. non-childbearing potential is defined as subject confirmed: i. surgical sterilization (e.g., bilateral oophorectomy, bilateral salpingectomy, bilateral occlusion by cautery [essure system® is not acceptable], hysterectomy, or tubal ligation). ii. postmenopausal (defined as permanent cessation of menstruation for at least 12 consecutive months prior to screening); if postmenopausal status is unclear, pregnancy tests will be performed prior to vaccinations. wocbp must have a negative pregnancy test before vaccination. if menopausal status is unclear, a pregnancy test is required.