CureVac probably failed because they used unmodified RNA base pairs. Both Moderna and BioNTech/Pfizer use modified nucleosides such as N1-methyl-pseudouridine. Extracellular unmodified mRNA - which could arise from the lipid nanoparticles which contain the mRNA breaking down before transfection of cells - can trigger a deleterious innate immune reaction against said mRNA. This interferes with the desired immune recognition of the spike protein. The modified mRNA largely evades that undesired immune reaction. That's probably what sunk CureVac's vaccine.
