The purpose of this clinical trial is to Evaluate the Safety and Immunogenicity of PTX-COVID19-B Administered as Booster Vaccination in Previously Vaccinated Adults Aged 18 Years and Older.

This study is seeking participants who are:

Adult males and females 18 years of age or older; In efficacy cohort : Subjects who were previously vaccinated with 2 doses of Comirnaty® administered at least 3 months prior to the booster dose..

All participants in this efficacy cohort will receive 1 of the 2 study vaccines: PTX-COVID19-B or Comirnaty®.

All participants in efficacy cohort will receive a single 40 microgram dose PTX-COVID19-B of the study vaccine or one dose of Comirnaty® at the first study clinic and will return to the study clinic 6 more times. At each clinic visit, a blood sample will be taken. They study is about 6 months long for each participant.

In safety cohort: Subjects who have previously received any primary series approved by WHO Emergency Use Authorization at least 3 months prior to enrollment or subjects who have already received one authorized booster vaccination and planned to receive PTX-COVID19-B as the 4th shot will be enrolled.

All participants in this safety cohort will receive 1 dose vaccines: PTX-COVID19-B.

All participants in safety cohort will receive a single 40 microgram dose of the study vaccine at the first study clinic and will return to the study clinic 5 more times. At each clinic visit, a blood sample will be taken. They study is about 6 months long for each participant.