This is a Phase III, Randomized, Observer-Blind Study to Evaluate the Safety and Superiority in Immunogenicity of PTX-COVID19-B Administered as Booster Vaccination Compared to Vaxzevria® in Adults Aged 18 Years and Older Who Were Previously Vaccinated with Vaxzevria®. This study will be a competitive enrollment process in all participating countries. Once the target number of subjects is reached, the enrollment in all participating countries in the particular phase will be stopped.