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.

Exclusion Criteria:

- Efficacy Cohort:

Subjects will not be eligible for Efficacy Cohort participation if they meet any of the
exclusion criteria:

1. Individuals who are antigen positive, seropositive or reverse transcriptase
polymerase chain reaction (RT-PCR) positive for SARS-CoV-2.

2. Women currently pregnant, lactating, or planning a pregnancy within 3 months post
boost vaccine.

3. Any medically diagnosed or suspected immunosuppressive or immunodeficient condition
based on medical history and physical examination including known infection with
human immunodeficiency virus (HIV), hepatitis B virus (HBV) or hepatitis C virus
(HCV);

4. Receipt of an organ or bone marrow transplant; or Long-term (>2 weeks) use of oral
or parenteral steroids or high-dose inhaled steroids (>800 μg/day of beclomethasone
dipropionate or equivalent) within 6 months before screening (nasal and topical
steroids are allowed).

5. History of Guillain-Barré Syndrome or any degenerative neurology disorder, or
myocarditis, pericarditis.

6. History of anaphylactic-type reaction to any injected vaccines; or known or
suspected hypersensitivity to 1 or more of the components of the vaccine.

7. Acute illness or fever (axillary temperature ≥ 37.5ºC or oral temperature ≥ 38.0ºC)
within 3 days (72 hours) before study enrollment (enrollment may be delayed for full
recovery if acceptable to the investigator).

8. Congenital or functional absence of spleen, complete or partial removal of spleen in
any case.

9. Donate or loss ≥ 450 ml of blood within 1 month prior to enrollment or expected
blood donation during the study.

10. Individuals currently participating or planning to participate in a study that
involves an experimental agent (vaccine, drug, biologic, device, or medication); or
who have received an experimental agent within 1 month (3 months for
immunoglobulins) before enrollment in this study; or who expect to receive another
experimental agent during participation in this study.

11. Receipt of immunoglobulin or another blood product within the 3 months before
enrollment in this study or those who expect to receive immunoglobulin or another
blood product during this study.

12. Ineligible for the study based on the assessment of investigator.

13. Suspected or known current alcohol or drug dependency within 12 months of screening.

14. Individuals who are using medications with the intention of COVID-19 prophylaxis and
who received or voluntarily plan to receive any other COVID-19 booster vaccine other
than the study vaccines.

15. History of surgery or major trauma within 12 weeks of screening, or surgery planned
during the study.

16. Received influenza vaccination within 14 days prior to first study vaccination, or
any other vaccine within 30 days prior to first study vaccination.

17. Current diagnosis of or treatment for cancer (except basal cell carcinoma of the
skin and cervical carcinoma in situ)

18. Bleeding disorder (e.g., factor deficiency, coagulopathy or platelet disorder), or
prior history of significant bleeding or bruising following IM injections or
venipuncture

19. Continuous use of anticoagulants, such as coumarins and related anticoagulants
(i.e., warfarin) or novel oral anticoagulants (i.e., apixaban, rivaroxaban,
dabigatran and edoxaban)

20. History of heparin-induced thrombocytopenia and thrombosis (HITT or HIT type 2) or
cerebral venous sinus thrombosis, increased risk for cerebrovascular venous and
sinus thrombosis, history of immune thrombocytopenia, or a history of capillary leak
syndrome.

Safety Cohort:

Subjects will not be eligible for Safety Cohort participation if they meet any of the
exclusion criteria:

1. Individuals who are antigen positive or reverse transcriptase polymerase chain
reaction (RTPCR) positive for SARS-CoV-2.

2. Subjects who have vaccinated with 2 or more booster shots and plan to receive
PTXCOVID19-B as the 5th or more shot.

3. Women currently pregnant, lactating, or planning a pregnancy within 3 months post
boost vaccine.

4. Immunocompromised individuals with known or suspected immunodeficiency, as
determined by history and/or laboratory/physical examination.

5. Receipt of an organ or bone marrow transplant; or Long-term (>2 weeks) use of oral
or parenteral steroids or high-dose inhaled steroids (>800 μg/day of beclomethasone
dipropionate or equivalent) within 6 months before screening (nasal and topical
steroids are allowed).

6. History of Guillain-Barré Syndrome or any degenerative neurology disorder, or
myocarditis, pericarditis.

7. History of anaphylactic-type reaction to any injected vaccines; or known or
suspected hypersensitivity to 1 or more of the components of the vaccine.

8. Acute illness or fever (axillary temperature ≥ 37.5ºC or oral temperature ≥ 38.0ºC)
within 3 days (72 hours) before study enrollment (enrollment may be delayed for full
recovery if acceptable to the investigator).

9. Congenital or functional absence of spleen, complete or partial removal of spleen in
any case.

10. Donate or loss ≥ 450 ml of blood within 1 month prior to enrollment or expected
blood donation during the study.

11. Individuals currently participating or planning to participate in a study that
involves an experimental agent (vaccine, drug, biologic, device, or medication); or
who have received an experimental agent within 1 month (3 months for
immunoglobulins) before enrollment in this study; or who expect to receive another
experimental agent during participation in this study.

12. Receipt of immunoglobulin or another blood product within the 3 months before
enrollment in this study or those who expect to receive immunoglobulin or another
blood product during this study.

13. Ineligible for the study based on the assessment of investigator.

14. Suspected or known current alcohol or drug dependency within 12 months of screening.

15. Individuals who are using medications with the intention of COVID-19 prophylaxis and
who voluntarily plan to receive any other COVID-19 booster vaccine other than the
study vaccines.

16. History of surgery or major trauma within 12 weeks of screening, or surgery planned
during the study.

17. Received influenza vaccination within 14 days prior to first study vaccination, or
any other vaccine within 30 days prior to first study vaccination.

18. Current diagnosis of or treatment for cancer (except basal cell carcinoma of the
skin and cervical carcinoma in situ)

19. Bleeding disorder (e.g., factor deficiency, coagulopathy or platelet disorder), or
prior history of significant bleeding or bruising following IM injections or
venipuncture

20. Continuous use of anticoagulants, such as coumarins and related anticoagulants
(i.e., warfarin) or novel oral anticoagulants (i.e., apixaban, rivaroxaban,
dabigatran and edoxaban)

21. History of heparin-induced thrombocytopenia and thrombosis (HITT or HIT type 2) or
cerebral venous sinus thrombosis, increased risk for cerebrovascular venous and
sinus thrombosis, history of immune thrombocytopenia, or a history of capillary leak
syndrome. syndrome.