Immunohistochemical staining for protein expression of spike receptor binding domain (RBD) in heart sections. A, Representative images of immunohistochemical staining of heart section showed occasional spike RBD positive cardiomyocytes (arrows at 400× magnification) at 1 dpi in IV vaccine group. H&E stain of the same area of the section is on the right. B, Representative images showing spike RBD expression in the histiocytes, and vascular endothelial cells in myocardium (arrows in 400× magnification). C, Representative images of positive control for the staining of spike RBD in SARS-CoV-2 infected mouse bronchiolar epithelium (arrows in left panel), negative expression of spike RBD by immunostaining in the myocardium of IV NS control group (middle), and immunostaining with only biotin conjugated secondary antibody in heart sections of IV vaccine group at 1 dpi (right). Abbreviations: dpi, days post-injection; H&E, hematoxylin and eosin; IV, intravenous; mRNA, messenger RNA; NS, normal saline; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.