Assessing the Immunogenicity and Safety of a HIPRA's Candidate Booster Vaccination in Adults Fully Vaccinated With Adenovirus Vaccine Against COVID-19.

The study population includes healthy adults aged above 18 years old who have received two doses of the Vaxevria vaccine, and are at least 91 days and less than 365 days after their second dose. Participants will be randomly assigned into two treatment arms. In each arm, volunteers will be randomized in a ratio Test vaccine:Comirnaty of 2:1. Each participant will receive one booster immunisation and will be followed for 6 months to evaluate immunogenicity response and assess the safety of the test vaccine in comparison to Comirnaty.