Abstract
In less than one year since the outbreak of the COVID-19 pandemic, two mRNA-based vaccines, BNT162b2 and mRNA-1273, were granted the first historic authorization for emergency use, while another mRNA vaccine, CVnCoV, progressed to phase 3 clinical testing. The COVID-19 mRNA vaccines represent a new class of vaccine products, which consist of synthetic mRNA strands encoding the SARS-CoV-2 Spike glycoprotein, packaged in lipid nanoparticles to deliver mRNA to cells. This review digs deeper into the scientific breakthroughs of the last decades that laid the foundations for the rapid rise of mRNA vaccines during the COVID-19 pandemic. As well as providing momentum for mRNA vaccines, SARS-CoV-2 represents an ideal case study allowing to compare design-activity differences between the different mRNA vaccine candidates. Therefore, a detailed overview of the composition and (pre)clinical performance of the three most advanced mRNA vaccines is provided and the influence of choices in their structural design on to their immunogenicity and reactogenicity profile is discussed in depth. In addition to the new fundamental insights in the mRNA vaccines' mode of action highlighted here, we also point out which unknowns remain that require further investigation and possibly, optimization in future mRNA vaccine development.