mRNA vaccines designed to express the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein are potentially advantageous for vaccinating against new variants of SARS-CoV-2. Subjects who have been vaccinated with approved whole-spike proteins are primed to produce non-neutralising antibodies that bind to regions of the spike outside the RBD. Subsequent vaccination with a variant of whole spike protein runs the risk of failing to induce production of useful new neutralising antibodies. An Australian RBD mRNA vaccine is scheduled to be evaluated in a Phase 1 clinical trial over the coming months. The presentation will discuss the design of this mRNA construct and the challenges of formulating the mRNA into lipid nanoparticles.