Low power scanning of heart sections showed blue stained thickened visceral pericardium over the right atrium and ventricle at 1 dpi of IV vaccine, which became more prominent at 2 dpi (
Figure 2A). At higher magnification, calcific deposits were seen in these thickened pericardial tissues (
Figure 2D). Multifocal pericardial and myocardial inflammatory cell infiltrates and interstitial oedema were also observed (
Figure 2C). Frequent foci of cardiomyocytes had degenerative changes as evident by the loss of the normal pattern of cross-striation and occasionally sarcoplasmic vacuolation, and necrotic changes as distinguished by the attainment of a homogenous appearance, sarcoplasmic fragmentation, or pyknosis (
Figure 2E). These changes were significantly more frequent in the IV vaccine group at 2 dpi (
Table 1) and more often in the right atrium and right ventricles of the affected animals and especially prominent on their pericardial side. Immunohistochemical staining with anti-CD45 (biomarker for immune cells of lymphoid or myeloid origin) indicated that these were leukocytes, of which many were macrophages or histiocytes positive for CD68. CD3-positive T cells were seen less often (
Figure 2G).The numbers of leukocytes were more than 14 per square millimeter in the affected myocardial foci. These findings suggested that mice given IV mRNA COVID-19 vaccine can develop acute myopericarditis. Similar histopathological changes and severity were found in male mice (
Supplementary Figures 2B, 2C).