Immunogenicity and Safety of a Booster Vaccination With a Recombinant Protein RBD Candidate Vaccine Against SARS-CoV-2 in Healthy Adults Volunteers Fully Vaccinated Followed by an Extension Period to Study a Fourth Dose Administration.

This Phase IIb clinical study aims to compare the immunogenicity and safety of a booster dose of recombinant protein RBD fusion dimer vaccine as a heterologous booster (to subjects who have received the second dose of the Pfizer-BioNTech (Comirnaty) COVID-19 vaccine at least 182 days prior to the booster dose in this study) versus a homologous booster (subjects who received the second dose of the Comirnaty COVID-19 vaccine at least 182 days prior to the booster dose in this study) will receive a third dose of the Comirnaty vaccine). The extension part of the study aims to compare the immunogenicity and safety of a fourth dose of PHH-1V in subjects with a primovaccination with Pfizer-BioNTech (Comirnaty) COVID-19 vaccine plus either a booster dose of Comirnaty or PHH-1V versus those with three vaccinations of Comirnaty.

The study population includes 1075 healthy adults aged above 18 years old who have received two doses of the Comirnaty vaccine, and are at least 182 days and less than 365 days after their second dose will be randomly assigned to 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 1 year to evaluate immunogenicity response and assess the safety of the test vaccine in comparison to Cominarty. The study population of the extension part includes 200 healthy adults abode 18 years old who have received or: 3 doses of Comirnaty vaccine, or 2 doses of Comirnaty + 1 dose of PHH-1V. Each participant will receive one dose of PHH-1V.