The pathogenesis of postvaccination myocarditis remains unclear.[3,15] One theory is that the immune system is activated by the recognition of mRNA as an antigen.[24] The other theory is that antibodies against the spike glycoprotein contained in the vaccine cross-react with α-myosin and transglutaminase to elicit an immune response.[17,25] In this case, chronic thyroiditis and fatty liver were observed, and the possibility that this background contributed to the immunological abnormalities against the vaccine cannot be completely ruled out. This evidence concerns the gene MYH14 and Hashimoto thyroiditis.