A phase IIB, double-blind, randomized, active controlled, multicentre, non-inferiority trial followed by a phase III, single-arm, open-label trial to assess immunogenicity and safety of a booster vaccination with a recombinant protein RBD fusion dimer candidate (PHH-1V) against SARS-CoV-2 in adults fully vaccinated against Covid-19 (HIPRA-HH-2).