Added value of this study
Novelty of this study relates to: a 4th dose boost using RBD rather than whole Spike; a beta variant-directed candidate (as opposed to omicron or ancestral); and MF59 (for Protein-RBD, previously studied only in a small subgroup of another RBD Phase I trial—ABNCoV2) and a new lipid nanoparticle (LNP) for mRNA-RBD.This is one of very few head-to-head placebo-controlled clinical studies of recombinant protein and mRNA COVID vaccines. And also one of the few with a comparison with a licensed ‘gold standard vaccine’ (Moderna). Our results demonstrate strong boosting in a highly immune population, and a remarkable breadth of immune response including against recent omicron sub-variants and against other coronaviruses. No safety signals were observed with either candidate and both exhibited a modest and acceptable reactogenicity profile.