Schema for vaccine administration and gross pathology of mouse after vaccination. A, Experimental schema. Groups of mice were injected with COVID-19 mRNA vaccine via intramuscular (IM) or intravenous (IV) route. At 1, 2, 7, and 14 dpi, mice were killed for histopathological analysis. Normal saline (NS) was IV or IM injected in parallel as control. B, Body weight changes of mice after injection. C, Representative images of gross pathology of mouse organs and heart at 1 dpi. Hearts of NS control and IM vaccine groups appeared normal, whereas whitish patches (arrows) were seen on the visceral pericardium of hearts after IV vaccine. D, Representative images of gross pathology of mouse organs including heart at 2 dpi. Large whitish patches (arrows) were seen on the visceral pericardium of mice receiving IV vaccine. Abbreviations: COVID-19, coronavirus disease 2019; dpi, days post-injection; mRNA, messenger RNA.