Safety and Immunogenicity of a Booster Vaccination With an Adapted Vaccine

This is a phase IIb/III, double-blind, randomised, active-controlled, multi-centre, non-inferiority clinical trial that aims to determine safety, tolerability and immunogenicity of a booster vaccination with a recombinant protein receptor binding domain (RBD) fusion homodimer of XBB.1.16 adapted candidate against SARS-CoV-2 developed by HIPRA (PHH-1V81).

Approximately 612 participants who have received a primary scheme with an EU-approved mRNA vaccine (two doses) and at least one booster dose of an EU-approved mRNA vaccine against COVID-19, will be randomly assigned to the following two treatment arms in a PHH-1V81 : Comirnaty (Omicron XBB.1.5) 2:1 ratio:

* PHH-1V81 vaccine arm: Approximately 408 adults will receive a booster dose of PHH-1V81 (HIPRA adapted vaccine). * Comirnaty vaccine arm: Approximately 204 adults will receive a booster dose of Comirnaty Omicron XBB.1.5.

Participants will be stratified before randomisation by age group (approximately 10% of adults ≥60 years old), and by number of doses previously received.

All participants will receive a booster dose of PHH-1V81 or Comirnaty Omicron XBB.1.5 at Day 0 and will be followed for 6 months. All subjects will be closely observed for 15 minutes after vaccination on site.