In short, besides developing an elegant method – the focus of their paper – Luo et al show that LNP-encapsulated mRNA reaches the heart. Once there, it causes local production of spike protein. Cardiac endothelial cell damage ensues, most likely owing to the toxicity of the spike protein itself. This tallies with cardiac harm in a proportion of human vaccinees. Human exposure to spike protein is longer than in Luo’s mice and repeated, owing to the use of modified mRNA and to boosting. It is reasonable to fear that this multiplies the risk of cardiac damage.